Mercer 1990

7/24/2019 Mercer 1990

http://slidepdf.com/reader/full/mercer-1990 1/57

Journal of Contaminant Hydrology,

6 (1990) 10%163

Elsevier Science Publishers B.V., Amsterdam

R e v i e w P a p e r

A R E V I E W O F IM M I SC I BL E F L U I D S I N T H E S U B S U R F A C E :

P R O P E R T I E S M O D E L S C H A R A C T E R I Z A T IO N A N D

R E M E D I A T I O N

107

JAMES W. MERCER and ROBERT M. COHEN

GeoTrans, Inc., Herndon, VA 22070 U.S.A.)

(Received F eb ruary 5, 1990; revised and accepted May 8, 1990)

B S T R C T

Mercer, J.W. and Cohen, R.M., 1990. A review of immiscible fluids in the subsurface: Properties,

models, characterization and remediation.

J. Contain. Hydrol.,

6: 107-163.

In the past few years~ as hazardous waste sites have been studied more often and in more detail,

immiscible fluids have been encountered in the subsurface with greater frequency. These

nonaqueous phase liquids (NAPL's) behave differently than dissolved solutes in the subsurface.

This behavior depends on fluid properties such as interfacial tension, viscosity and density. In

addition, mass transfer produces vapor transport in the vadose zone and solute transport in

groundwater. Mass transfer depends on properties associated with volatilization and aqueous

solubility. As a consequence, characterization techniques as well as remediation efforts must be

modified at sites where NAPL's are present. Although considerable research is necessary before

NAPL problems are well understood, sufficient work has been performed to permit a review of

NAPL properties and behavior in the subsurface.

INTRODU TION

N o n a q u e o u s p h a s e l i q u i d s ( N A P L ' s ) h a v e b e e n d i s c o v e r e d a t n u m e r o u s

ha za rd ou s w as te s i t es (e .g ., Fa us t , 1985; M erc er e t a l . , 1985; C oh en e t a l ., 1987) .

I n a d d i t io n , N A P L o f t e n is i d e n ti f ie d w i t h c o n t a m i n a t i o n p r o b l e m s a s s o c ia t e d

w i t h u n d e r g r o u n d s t o r a g e t a n k s . A c c o r d i n g t o V i l la u m e (1 9 8 4 ) , t y p i c a l

c h e m i c a l a n d i n d u s t r i a l p r o c e ss e s t h a t m a y i n v o l v e N A P L i n c l u d e t r a n s f o r m e r

o i l c o n t a i n i n g p o l y c h l o r i n a t e d b i p h e n y l s ( R o b e r t s e t a l ., 1 98 2; S c h w a r t z e t a l .,

1 98 2), t r i c h l o r o e t h e n e a n d r e l a t e d c h l o r i n a t e d h y d r o c a r b o n s ( P a lo m b o a n d

Jacob s , 1982 ; Ca rpe n t e r , 1984), coa l t a r s f rom ma nu fac tu re d g as p l an t s (D .C .

W i l son and S t eve ns , 1981; Ya z i c ig i l and S end l e in , 1981; La fo r na ra e t a l . , 1982 ;

U n i t e s a n d H o u s e m a n , 1 9 82 ; V i l l a u m e , 1 9 82 , 1 9 8 4; W . R . A d a m s a n d A t w e l l,

1983; A na s to s e t a l . , 1983; Th om pso n e t a l . , 1983; V i l lau m e e t a l . , 1983a, b) , s t ee l

i n d u s t r y c o k i n g o p e r a t i o n s ( C o a t e s e t a l ., 1 98 2), w o o d t r e a t i n g o p e r a t i o n s ( H u l t

and S ch oen berg , 1981; Ram sey e t a l . , 1981 ; Eh r l i c h e t a l . , 1982 ; H icko k e t a l .,

1982; P e re i r a e t a l ., 1983) , and pe t ro l eu m p rod uc t s (Ho lze r , 1976 ; P . L . Ha l and

Q u a m , 1 9 7 6 ; P f a n n k u c h , 1 9 83 ) . A s a n e x a m p l e o f t h e s iz e o f t h e p o t e n t i a l

0169-7722/90/ 03.50 © ].990 - - Elsev ier Science Publ ishers B.V.

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1 08 J W M E R C E R A N D R M C O H E N

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c o n c e n t r a t i o n o f c h e m i c a l i n w a t e r ( to o l m - 3 )

g r a v i t a t i o n a l a c c e l e r a t i o n c o n s t a n t

n o n - a d v e c t i v e f lu x o f s p e c i e s i i n t h e ~ p h a s e

c a p i l l a r y h e a d

H e n r y s l a w c o n s t a n t ( a t m . m a m o 1 -1 )

p a r t i t i o n c o e f fi c ie n t o f s p e c i e s i b e t w e e n t h e ~ a n d f l p h a s e s

o c t a n o l / w a t e r p a r t i t i o n c o e f f i c i e n t

o r g a n i c c a r b o n / w a t e r p a r t i t i o n c o e f fi c ie n t ( m l g - l )

i n t r i n s i c p e r m e a b i l i t y

g a s p h a s e p e r m e a b i l it y

v o l u m e t r i c m a s s e x c h a n g e c o e f f i c i e n t

r e l a t i v e p e r m e a b i l i t y o f a i r

r e l a t i v e p e r m e a b i l it y o f N A P L

r e l a t i v e p e r m e a b i li t y o f N A P L i n a n a i r - N A P L s y s t e m

r e l a t i v e p e rm e a b i l i ty o f N A P L i n a w a t e r - N A P L s y s t e m

r e l a t i v e p e r m e a b i l it y o f N A P L a t t h e r e s i d u a l s a t u r a t i o n o f w a t e r

r e l a t i v e p e r m e a b i l i t y o f w a t e r

m a s s o f N A P L s o u r c e

m a s s e x c h a n g e d

p o r o s i t y

p a r t i a l p r e s s u r e o f c h e m i c a l i n g a s p h a s e ( a t m . )

v a p o r p r e s s u r e o f a s o l u t i o n

v a p o r p r e s s u r e o f t h e p u r e s o l v e n t

c a p i l l a r y p r e s s u r e

N A P L p r e s su r e

w a t e r p r e s s u r e

s t r e n g t h o f a h y d r o c a r b o n s o u r c e

v o l u m e t r i c r e t e n t i o n c a p a c i t y ( l it e r s N A P L / m a o f m e d i u m )

e x t e r n a l s u p p l y o f s p e c i es i t o t h e ~ p h a s e

p o r e r a d i u s

m a s s e x c h a n g e o f s p ec i es i d u e t o i n t e r p h a s e d i f fu s i on a n d / o r p h a s e c h a n g e

s a t u r a t i o n

r e s i d u a l s a t u r a t i o n

t i m e

v o l u m e o f N A P L s o u r c e

v o l um e o f N A P L

v o l u m e o f p o r e s p a c e

m a s s a v e r a g e v e l o c i t y o f t h e ~ p h a s e

m a s s f r a c t i o n o f s p e ci e s i i n t h e • p h a s e

m o l e f r a c t i o n o f t h e s o l v e n t

c o m p r e s s i b i l i ty o f ~ p h a s e

f r a c t i o n o f v o l u m e o c c u p i e d b y t h e ~ p h a s e

c o n t a c t a n g l e

a b s o l u t e v i s c o s i t y

i n t r i n s i c m a s s d e n s i t y o f t h e ~ p h a s e

N A P L d e n s i t y

w a t e r d e n s i t y

i n t e r f a c i a l t e n s i o n b e t w e e n N A P L a n d w a t e r

i n t e r fa c i a l t e n s i o n b e t w e e n N A P L a n d s o l id

i n t e r f a c i a l t e n s i o n b e t w e e n w a t e r a n d s o l i d

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A R E V I EW O F I M M I S C I B L E F L U I D S I N T H E S U B S U R F A C E 1 0 9

pr ob le m , U .S .E .P .A . ( 1979 ) e s t im a te s t ha t i n 1974 ~ 310 ,200 t* o f w a s t e so lve n t s

a l o n e w e r e p r o d u c e d b y d e g r e a s i n g o p e r a t i o n s . I n a d d i t io n , t h e r e a r e a n

e s t i m a t e d 7 96 ,0 00 i n d i v i d u a l m o t o r f u e l s t o r a g e t a n k s i n t h e U . S . A . ( U .S . E .P . A ,

1986a).

A l t h o u g h c h e m i c a l p r o p e r t i e s a n d s i t e c o n d i t i o n s v a r y f r o m s i t e t o s i t e , t h e

b a s ic p r i n c i p l e s g o v e r n i n g t h e f a t e a n d t r a n s p o r t o f N A P L s a r e t h e s a m e .

T h e s e p r i n c ip l e s m a y b e u s e d t o u n d e r s t a n d t h e c o n t a m i n a t i o n p r o b l e m a n d to

e v a l u a t e r e m e d i a t i o n . U n f o r t u n a t e l y , d e v e l o p m e n t o f s ta t e- o f- th e - ar t

t e c h n o l o g y f o r d e a l i n g w i t h N A P L p r o b l e m s l a g s b e h i n d t h e t e c h n o l o g y

d e v e l o p ed fo r m a n y o t h e r g r o u n d w a t e r c o n t a m i n a t i o n p r o b le m s . F o r e x am p l e ,

s e v e r a l m o d e l s a r e a v a i l a b l e t o s i m u l a t e t h e f lo w o f N A P L ; h o w e v e r , o b t a i n i n g

c he m ic a l - spe c i fi c a nd s i t e- spe c if i c da t a i s d i ff ic u lt . C ons e qu e n t ly , f o r mo s t s i te s ,

m o d e l s o n l y m a y b e u s e d i n a c o n c e p t u a l i z a t i o n m o d e .

N A P L m i g r a t i o n i n t h e s u b s u r f a c e i s a f f e c t e d b y ( F e e n s t r a a n d C h e r r y ,

1988): (1 ) vo lum e o f N A P L r e l e a se d ; ( 2) a r e a o f in f i l t r a t i on ; (3 ) t im e du r a t io n o f

r e l e a se ; ( 4) p r op e r t i e s o f t he N A P L; ( 5) p r o pe r t i e s o f t he m e d ia ; a nd ( 6)

s u b s u r f a c e f lo w c o n d i ti o n s . T h e c r o s s - s e c t i o n a l s c h e m a t i c i n F i g . l a d e p i c t s t h e

d i s t r i b u t i o n o f o r g a n i c c h e m i c a l s i n m u l t i p l e p h a s e s r e s u l t i n g f r o m a re l e a s e o f

l i g h t e r - t h a n - w a t e r n o n a q u e o u s p h a s e li q u id ( L N A P L ) . W h e n i n t r o d u c e d i n t o

t h e s u b s u rf a c e , g r a v i t y c a u s e s t h e N A P L t o m i g r a t e d o w n w a r d t h r o u g h t h e

v a d o s e z o n e a s a d i s t i n c t l i q u i d . T h i s v e r t i c a l m i g r a t i o n a l s o i s a c c o m p a n i e d t o

s o m e e x t e n t b y l a t e r a l s p r e a d i n g d u e t o t h e e f f ec t o f c a p i l l a r y f o r c e s ( S c hw i l le ,

1 98 8) a n d d u e t o m e d i u m s p a t i a l v a r i a b i l i t y ( e. g. , l a y e r i n g ) w h i c h i s n o t s h o w n

i n F ig . l a . A s t h e N A P L p r o g r e s s e s d o w n w a r d t h r o u g h t h e v a d o s e z on e, it

l e a v e s r e s i d u a l l i q u i d ( r e s i d u a l s a t u r a t i o n ) t r a p p e d i n t h e p o r e s p a c e s . T h i s

e n t r a p m e n t i s d u e t o s u r f a c e t e n s i o n e ff ec ts . I n a d d i t i o n t o m i g r a t i o n o f N A P L ,

s o m e o f t h e i m m i s c ib l e f lu i d m a y v o l a t il i z e a n d f o r m a v a p o r e x t e n d i n g b e y o n d

t h e N A P L .

I f t h e r e l e a s e i s s u ff ic i en t ly l a r g e , s o m e o f t h e N A P L w i l l e v e n t u a l l y r e a c h

t h e s a t u r a t e d z o ne . H e r e L N A P L w i l l s p r e a d l a t e r a l l y a l o n g t h e c a p i ll a r y

f ri n g e. I t m a y a l s o de p r e ss n a t u r a l g r o u n d w a t e r l ev e ls . T h e L N A P L d i s tr ib u -

t i o n d e p e n d s o n L N A P L , w a t e r a n d a i r p r e s s u r e s a n d t h e p o r e s i ze d i s t r ib u t i o n .

F ig . l a i s m o r e t y p i c a l o f a h o m o g e n e o u s , p e r m e a b l e m e d i um . I n h e t e r o g e n e o u s

m e d i a, t h e L N A P L d i s t r i b u t i o n w i l l b e m o r e c o m p l ex . A s t h e N A P L e n c o u n t e r s

f lo w i n g w a t e r , s o l u b l e c o m p o n e n t s m a y d is s o l v e t o f o r m a s o l u t e p l u m e t h a t c a n

m i g r a t e d u e t o h y d r a u l i c g r a d i e n t s . E x a m p l e s a n d d e s c r i p t i o n s o f d i s s o lv e d

c h e m i c a l p l u m e s a r e p r o v i d e d i n M a c k a y a n d C h e r r y ( 1 9 8 9 ) .

D e n s e r - t h a n - w a t e r n o n a q u e o u s p h a s e l iq u i d ( I ) N A P L ) w il l d i s p la c e w a t e r

a n d c o n t i n u e i t s m i g r a t i o n u n d e r p r e s s u r e a n d g r a v i t y f o r c e s ( F i g . l b ) .

P r e f e r e n t i a l s p r e a d i n g w i l l o c c u r w h e r e D N A P L e n c o u n t e r s r e l a t i v e l y

p e r m e a b l e l a y e r s , f r a c t u r e s , o r o t h e r p a t h w a y s t h a t p r e s e n t l e s s c a p i l l a r y

r e s i s t a n c e t o e n t r y t h a n u n d e r l y i n g l e s s p e r m e a b l e s t r a t a . G i v e n s u f f i c i e n t

v o l u m e , D N A P L w i l l c o n t i n u e i ts d o w n w a r d m i g r a t i o n u n t i l i t e n c o u n t e r s a

l t = 1 m e t r i c t o n n e = 1 0 3 k g .

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11 J . W . M E R C E R A N D R . M . C O H E N

'

R O U N D S U R F A C E

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Z O N E

~;'i=~i ~:; 't :~:~; -::~=~ -~ G A S Z O N E e v a p o r a t i o n e n v e l o p e )

C A P I L L A R Y F R I N G E ~ ~ " :~:~ ,I=~ ~ -

: ~ : N : . ~ / . ~ . ~ . : . ~ i~ . ~

. ~ > . ~ . . ; ..~ . : ~ , , ~ . - . . . - , ~ . ~, . ; , '~.

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s o l u b l e c o r r k o o n e n t ~ )

( b ) ~ . _ ~ D N A P L

D N A P L R E L E A S E

A I R O R W A T E R -

F I L L E D P O R E S P A C E

R E S I D U A L D N A P L

, . ~ \ \ \ ~ . . . , , ' ~ ' - ' - ' - ~ , "

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, 4 , G R O U N I ~ W A T E R ' . " ¢ . ' " : ' .~ : '. . " ; ; . ~ : ' ; / L O W E R

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.=," ::: •, -~"" " ".:'::::i:~.:,:~ ' : ' . ' : : : : : : : 1 . . .

S T R A T A

D N A P L

W A T E R - F I L L E

P O R E S P A C E

F i g . 1 . a . L N A P L inf i l tra t ion schem at ic modi fi ed from Pinder and Abr io la , 1986 .

b . D N A P L

inf i l tra t ion schem at ic modi fi ed from Feens tr a and Cherry ,

1988 .

b a r r i er la y e r u p o n w h i c h i t m a y c o n t i n u e t o f lo w u n d e r p r e s s u r e a n d g r a v i t y

f o r c es . A s i n t h e v a d o s e z o n e s o m e o f t h e N A P L w i l l b e h e l d i n th e p o r e s p a c e

w i t h i n t h e s a t u r a t e d z o n e . T h i s r e s i d u a l N A P L w i l l s e r v e a s a c h e m i c a l s o u r c e

t o t h e f lo w i n g g r o u n d w a t e r d e p e n d i n g o n t h e a q u e o u s s o l u b i l it y o f t h e o r g a n i c

c o m p o u n d s . I n t h e v a d o s e z o n e i n f i lt r a t i n g r a i n w a t e r m a y d i s s o l v e o r g a n i c

v a p o r s o r t h e r es id u a l N A P L a n d t r a n s p o r t t h e s e o r g a n i c c o m p o n e n t s t o t h e

s a t u r a t e d r e g i o n .

A s i n d ic a t e d m a n y o f t h e p r o c e s s e s a s s o c i a t e d w i t h N A P L m o v e m e n t a r e

u n d e r s t o o d c o n c e p t u a l l y . I n a d d i t io n c o n s i d e r a b l e e f fo r t r e c e n t l y h a s b e e n

d e v o t e d t o N A P L s t u d i e s . T h e p u r p o s e o f t h i s p a pe r t h e r e f o r e i s t o r e v i e w t h e

7/24/2019 Mercer 1990


REVIEW OF IMMISCIBLE FLUIDS IN THE SUBSURF CE il l

w o r k p e r f o r m e d o n N A P L s . T o e s t a b l is h a b a s i c f o u n d a t i o n , p r o p e r t ie s

g e n e r a l l y a s s o c i a t e d w i t h N A P L s a r e d is c u s se d a n d t a b u l a t e d v a l u e s p r o v id e d .

A b r ie f r e v i e w o f t h e m a t h e m a t i c a l e q u a t i o n s t h a t d e s c ri b e N A P L f lo w is

p r e s e n t e d . B e c a u s e o f t h e i m p o r t a n c e o f d a t a a c q u i s i t i o n , a d i s c u s s i o n i s g i v e n

c o n c e r n i n g f ie ld t e c h n i q u e s u s e d to c h a r a c t e r i z e N A P L m o v e m e n t . F i n a l ly ,

t e c h n o l o g i e s c o n s i d e re d a n d u s e d t o r e m e d i a t e N A P L c o n t a m i n a t i o n a r e

p r e s e nt e d . T h r o u g h o u t t h e d is c u s s io n , t h e i m p o r t a n c e o f N A P L p r o p e r t ie s is

e m p h a s i z e d .

PROPERTIES OF FLU ID AND MED IA

O r g a n i c c o n t a m i n a n t s c a n r e a c h t h e w a t e r t a b le e i t h e r t h r o u g h v a p o r

t r a n s p o r t , d i s s o l v e d in w a t e r , o r a s N A P L . T h e s u b s u r f a c e t r a n s p o r t o f N A P L

i s g o v e r n e d b y v a r i o u s f a c t o r s , s o m e o f w h i c h a r e d i f f e r e n t f r o m t h o s e f o r

d i s s o l v e d ( o r m i s c i b l e ) c o n t a m i n a n t s . I n th i s s e c t i o n , p r o p e r t i e s a s s o c i a t e d

w i t h N A P L f lo w a r e d is c u s s e d . S e e t h e N o t a t i o n f o r s y m b o l s u s ed i n t h i s p a p e r .

Sa tura t ion

T h e s a t u r a t i o n , s , o f a f lu i d is t h e v o l u m e f r a c t i o n o f t h e t o t a l v o i d v o l u m e

o c c u p i e d b y t h a t f l ui d. S a t u r a t i o n s v a r y f ro m z e r o t o on e a n d t h e s a t u r a t i o n s

o f a l l f lu i d s s u m t o o n e . S a t u r a t i o n i s i m p o r t a n t b e c a u s e o t h e r p r o p e r t i e s , s u c h

a s c a p i l l a r y p r e s s u r e a n d r e l a t i v e p e r m e a b i l i t y , a r e r e p r e s e n t e d a s f u n c ti o n s o f

s a t u r a t i o n .

M e a s u r i n g s a t u r a t i o n i s d i ff ic u lt . F e r r a n d e t a l. (1989) p r e s e n t a d u a l - g a m m a

(~37Cs-241 A m ) t e c h n i q u e f o r l a b o r a t o r y d e t e r m i n a t i o n o f t h r e e - f l u i d s a t u r a t i o n

p r o fi l es in p o r o u s m e d i a . T h e y p r o v i d e p o r o s i t y a n d s a t u r a t i o n p r o fi le s f o r

s a n d s c o n t a i n i n g a i r , w a t e r , a n d t r ic h l o ~ o e t h e n e o r t e t r a c h l o r o e t h e n e . D r i l l i n g

a n d s a m p l i n g , p u m p i n g t e s t s , a n d b o r e h o l e g e o p h y s i c a l l o g g i n g c a n b e u s e d t o

f a c i l i t a t e f ie ld e s t i m a t e s o f s a t u r a t i o n . T h e s e e s t i m a t e s a r e q u a l i t a t i v e a d d t h e

m e t h o d s u s e d a r e l a r g e ly u n d o c u m e n t e d .

nterracia l tens ion

L i q u i d i n t e r f a c i a l t e n s i o n i s e q u a l t o t h e f r ee s t .r f a c e e n e r g y a t t h e i n t e r f a c e

f o r m e d b e t w e e n t w o i m m i s c i b l e o r n e a r l y i m m i s c i b l e l i q u i d s ( V i l l a u m e , 1985).

I t r e s u l ts f r o m t h e d i ff e re n c e b e t w e e n t h e m u t u a l a t t r a c t i o n o f l i k e m o l e c u le s

w i t h i n e a c h f lu i d a n d t h e a t t r a c t i o n e f d i s s i m i l a r m o l e c u l e s a c r o s s t h e f lu id

i n t e r f a c e ( S c h o w a l t e r , 1979). L i q u i d i n t e r f a c i a l t e n s i o n i s d i r e c t l y r e l a t e d t o t h e

c a p i l l a r y p r e s su r e a c r o s s a n N A P L - w a t e r i n t e r f a c e a n d i s a f a c t o r c o n t r o l l in g

w e t t a b i l i t y .

T h e i n t e r f a c i a l t e n s i o n b e t w e e n a l i q u i d a n d i t s o w n v a p o r i s c a l l e d v a p o r

t e n s i o n o r s u r f a c e t e n s i o n . S u r f a c e t e n s i o n i s r e s p o n s i b l e f o r c a p i l l a r y e f f ec ts .

T h e m a g n i t u d e o f t h e l i qu i d i n t e r fa c i a l t e n s i o n i s a l w a y s l e ss t h a n t h e l a r g e r

o f t h e s u r f a c e t e n s i o n s f o r t h e p u r e l i q u id s . T h i s i s d u e t o t h e m u t u a l a t t r a c t i o n

o f u n l i k e m o l e c u l e s a t t h e i m m i s c i b l e l i q u id i ~ lt er fa c e .

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2

J W MERCER AND R M COHEN

M e a s u r e d i n u n i t s o f e n e r g y p e r u n i t a r e a , i n t e r f a c i a l t e n s i o n d e c r e a s e s w i t h

i n c r e a s i n g t e m p e r a t u r e ( ~ 5 .5 1 0- ~ d y n c m - ~ ° C -~ f o r c r u d e o i l - w a t e r s y s t e m s )

a n d m a y b e a f f e c t e d b y p H , s u r f a c e - a c t i v e a g e n t s a n d g a s i n s o l u t i o n

( S c h o w a l t e r , 1 9 79 ). I n t e r f a c i a l t e n s i o n s f o r N A P L - w a t e r r a n g e f r om z e r o, f o r

c o m p l e t e l y m i s c ib l e l iq u i d s , t o 72 d y n c m - 1 , t h e s u r f a c e t e n s i o n o f w a t e r a t

25 ° C ( L y m a n e t a l ., 1 9 82 ). V a l u e s o f i n t e r f a c i a l a n d s u r f a c e t e n s i o n s f o r N A P L -

f o r m i n g c h e m i c a l s a r e l is t e d in A p p e n d i x B a n d g e n e r a l l y r a n g e b e t w e e n 15 a n d

50 d y n c m 1. I n g e n e r a l , l a r g e r s u r f a c e t e n s i o n s r e s u l t i n h i g h e r c a p i l l a r y

p r e s s u r e , w h i c h m a y p r o d u c e l a r g e r r e s i d u a l s a t u r a t i o n s ( s e e s u b s e q u e n t

sec t ion ) .

et tabi l i ty

W e t t a b i l i t y d e s c r i b e s t h e p r e f e r e n t i a l s p r e a d i n g o f o n e f lu i d o v e r s o l id

s u r f a c e s i n a tw o - f l ui d s y s t e m ; i t d e p e n d s o n i n t e r f a c i a l t e n s i o n . W h e r e a s t h e

w e t t i n g f l u id w i ll te n d t o c o a t t h e s u r f a c e o f g r a i n s a n d o c c u p y s m a l l e r s p a c e s

( i . e . p o r e t h r o a t s ) i n p o r o u s m e d i a , t h e n o n w e t t i n g f l u i d w i l l t e n d t o b e c o n -

s t r i c t e d t o t h e l a rg e s t o p en i n g s . W . G. A n d e r s o n (19 8 6a , b , c , 1 9 8 7a , b , c ) r e ce n t l y

p r e p a r e d a c o m p r e h e n s i v e l i t e r a t u r e r e v i e w o n w e t t a b i l i t y , i t s m e a s u r e m e n t ,

a n d e f f e c t s o n c a p i l l a r y p r e s s u r e , r e l a t i v e p e r m e a b i l i t y , r e s i d u a l N A P L

s a t u r a t i o n a n d e n h a n c e d N A P L r e c o v e r y .

A m e a s u r e o f w e t t a b i l i t y is t h e c o n t a c t a n g l e a t t h e f l u i d - s o l id i n t e r f a c e ( F i g.

2 ) . F o r t w o f l u i d s , s u c h a s N A P L a n d w a t e r , i n c o n t a c t w i t h a s o l i d , Y o u n g ' s

e q u a t i o n d e s c r ib e s t h e c o n t a c t a n g l e o f t h e i n t er f a c e :

co s ~b = (a ~ s aw~)/aNw (1)

w h e r e a Ns i s th e i n t e r r a c i a l t e n s i o n b e t w e e n N A P L a n d s o l id ; aws i s t h e i n -

t e r r a c i a l t e n s i o n b e t w e e n w a t e r a n d s o l id ; aN ~ i s t h e i n t e r r a c i a l t e n s i o n

b e t w e e n N A P L a n d w a t e r ; a n d ~ i s t L e c o n t a c t a n g l e m e a s u r e d i n t o th e w a t e r

( i n d e g r e e s ) . T h e c o n t a c t a n g l e i n d i c a t e s w h e t h e r t h e p o r o u s m e d i u m w i l l b e

p r e f e r e n t i a l l y w e t t e d b y N A P L o r w a t e r a n d m a y v a r y b e t w e e n 0 a n d 1 80° . If

~b < 70° , t he sys te m i s w ate r -w et ; i f ~b > 110° , i t i s N A P L - w e t ; a n d i f

= 70° - 110°, i t is c o n s i d e r e d n e u t r a l ( W . G . A n d e r s o n , 1 98 6a ). M e t h o d s f o r

m e a s u r i n g c o n t a c t a n g l e s a r e d e s c r i b e d b y G o u l d (1 96 4), W . G . A n d e r s o n ( 19 86 b)

an d Ho n a rp o u r e t a l . ( 1 9 8 6 ) .

W i t h t h e e x c e p t i o n o f m e r c u r y , l iq u i d s (N A P L o r w a t e r ), r a t h e r t h a n a i r ,

p r e f e r e n t i a l l y w e t s o l i d s u r f a c e s i n t h e v a d o s e z o n e . W e t t a b i l i t y r e l a t i o n s i n

N A P L - w a t e r s y s t e m s a r e a f f e c t e d b y s e v e r a l f a c t o r s i n c l u d i n g m e d i u m

m i n e r a l o g y , N A P L c h e m i s t ry , w a t e r c h e m i s t r y , t h e p r e s e n c e o f s u r f a c t a n t s o r

o r g a n i c m a t t e r , a n d m e d i u m s a t u r a t i o n h i s t o r y . W i t h th e e x c e p t i o n o f o r g a n i c

m a t t e r ( s u c h a s c o a l , p e a t a n d h u m u s ) , g r a p h i t e , s u l f u r , t a l c a n d t a l c - l i k e

s i li c a te s , a n d m a n y s u lf id e s, m o s t n a t u r a l p o r o u s m e d i a a r e s t r o n g l y w a t e r - w e t

i f n o t c o n t a m i n a t e d b y N A P L ( W .G . A n d e r s o n , 1 98 6a ). A l t h o u g h w a t e r i s o f te n

t h e w e t t i n g f l u i d i n N A P L - w a t e r s y s t e m s a n d h a s b e e n c o n s i d e r e d a p e r f e c t

w e t t i n g a g e n t i n c e r t a i n p e t r o l e u m r e s e r v o i r s ( S m i t h , 1 9 6 6 ; B e r g , 1 9 7 5 ;

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R E V I E W O F IM M I S C I BL E F L U ID S I N T H E S U B S U R F C E

W T ER W ET

q < ~ 7 0 °

r///~///A

113

NAPL WET

0 > 1 1 0 °

On ~ N

o Y / . ,, // / // /L// /A

Fig 2 W et tab i l i ty con f igu ra tions

S c h o w a l t e r , 1 9 7 9 ; C o r ey , 1 9 8 6 ) , o t h e r r e s e a r c h e r s h a v e d o c u m e n t e d t h a t

p e t r o l e u m r e s e r v o i r s , p a r t i c u l a r l y l i m e s t o n e a n d d o l o m i te , m a y b e p a r t ia l l y o r

p r e f e r e n t i a l l y w e t by o il ( N u t t i ng , 1934 ; B e nn e r a n d B a r t e l l , 1941 ; Le a c h e t al .,

1962; Cra ig , 1971; Tr e ib e r e t a l . , 1972; Sa la th ie l , 1973).

N A P L w e t t i n g u s u a l l y i n c r e a s e s d u e t o a d s o r p t i o n a n d / o r d e p o s i t io n o n

m i n e r a l s u r f a c e s o f o r g a n i c m a t t e r a n d s u r f a c t a n t s d e r iv e d f ro m N A P L o r

w ~, te r ( T r e ibe r e t a l ., 1972 ; S c h ow a l t e r , 1979 ; JB F S c ie n t i f i c C or p . , 1981;

T h o m a s , 1 9 8 2; H o n a r p o u r e t a l ., 1 9 86 ). N A P L w e t t i n g h a s b e e n s h o w n t o

i n c r e a s e w i t h a g i n g d u r i n g c o n t a c t a n g l e s t u d i e s ( C r a i g , 1 9 71 ; J B F S c i e nt i fi c

C o r p. , 1 9 8 1 ), p r e s u m a b l y d u e t o m i n e r a l s u r f a c e c h e m i s t r y m o d i f i ca t i o n s

i n d u c e d b y N A P L p r e s e n c e . S i m i l a r l y , a h y s t e r e s i s e f fe c t h a s b e e n d o c u m e n t e d

i n w h i c h t h e c o n t a c t a n g l e i s l e s s w h e n N A P L a d v a n c e s o v e r a n i n i t i a l l y

w a t e r - s a t u r a t e d m e d i u m t h a n w h e n N A P L is r e c e d i n g fr om a n N A P L - c o n t a m i -

na t e d me d ium ( V i l l a ume , 1985 ) .

G i v e n t h e h e t e r o g e n e o u s n a t u r e o f s u b s u r f a c e m e d i a a n d t h e f a c to r s t h a t

i n f l u e n c e w e t t a b i l i t y , s o m e i n v e s t i g a t o r s h a v e c o n c l u d e d t h a t t h e w e t t i n g o f

p o r o u s m e d i a b y N A P L c a n b e h e t e r o g e n e o u s , o r fr a c ti o n a l , r a t h e r t h a n

u n i f o r m ( W . G . A n d e r s o n , 1 98 6a; H o n a r p o u r e t a l ., 1 9 8 6 ). U n f o r t u n a t e l y , f ew

w e t t a b i li W s t u d i e s h a v e b e e n c o n d u c t e d o n n o n - p e t r o le u m N A P L s. R e s u l ts o f

c o n t a c t a n g l e e x p e r i m e n t s u s i n g s e v e r a l D N A P L s a n d v a r io u s s u b s t r a t e s a r e

p r ov id e d in Ta b le 1 ( A r thu r D . L i t t l e , I nc . , 1981).

C a p i l l a r y p r e s s u r e

C a p i l l a r y p r e s s u r e i s a p r o p e r t y t h a t c a u s e s p o r o u s m e d i a t o d r a w i n t h e

w e t t i n g f lu i d a n d r e p e l t h e n o n w e t t i n g f lu id ( B e a r , 1 9 7 2). I f c a p i l l a r y p r e s s u r e

7/24/2019 Mercer 1990


114 J W MERCER AND R M COHEN

TABLE 1

Results of contact angle experiments conducted using DNAPL's by Arthur D. Little, Inc. (1981)

Immiscible Fluid Substrate Medium Cont act

ngle

°)

Tetrach loroet hene clay APL 23-48

Tet rach loro ethene clay air 153--168

1 2 4-Trichlorobenzene clay APL 28--38

1 2 4

-Trichlorobenzene clay air 153

Hexachlorobu tadiene clay water 32-48

Hexachlorocyclopentadiene clay wate r 32-41

2 6.Dichlorotoluene clay water 30-38

4-Chlorobenzotrifluoride clay wate r 30-52

Carbon tetrach loride clay wate r 27-31

Chlorobenzene clay wate r 27-34

Chloroform clay wat er 29-31

S-Area DNAPL clay APL 21-54

S-Area DNAPL clay wat er 20-37

S-Area DNAPL clay air 170-171

S-Area DNAPL fine sand and silt wat er 30-40

S-Area DNAP L clayey till (30-40 clay) wat er 20-37

S-Area DNAPL Ottawa fine to coarse sand water 33-50

Tetrach loroet hene Ottawa fine to coarse sand water 33-45

Tetrach loroet hene Lockport Dolomite wate r 16-21

Tetrach loroet hene Lockport Dolomite air 171

S-Area DNAPL Lockport Dolomite wate r 16-19

S-Area DNAPL Lockport Dolomite air 164-169

S-Area DNAPL NAPL-contaminat ed fine sand APL 45-105

S-Area DNAPL soils with vegetat ive matte r wate r 50-122

S-Area DNAPL paper wat er 31

S-Area DNAPL wood wat er 34-37

S-Area DNAPL cotton clot~ wat er 31-33

S-Area DNAPL stainl ess s,~, : wat er 131-154

S-Area DNAPL clay wat er (SA) 25-54

S-Area DNAPL with solvents clay wat er 15-45

Adsorbed S-Area (New York, U.S.A.) chemicals were detected on some of the clay samples. APL

refers to aqueous phase liquids (water containing dissolved chemicals). S-Area DNAPL is com-

prised primari ly of tetrachlorobenzene , trichlorobenzenes, tetra chloro ethene, hexachlorocy-

clopentadiene, and octac hlorocyclopentene. SA refers to surface-active age nts (Tide ® and Alco-

nox ®) which were added to the water.

is assumed positive, it is defined as the difference between the nonw etti ng fluid

pressure and the wetting fluid pressure. For a wat er- NAPL system with wate r

being the wetting phase, capillary pressure, Pc, is defined as:

Pc - P N - Pw 2)

where PN is the NAPL pressure; and Pw is the water pressure.

Capillary pressure is related, to interfacial tension, contact angle, and pore

size by Bear, 1979):

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A REVIEW OF IMMIS CIBLE FLUIDS IN THE SUBSU RFACE

115

L i q u i d In t e r f a c i a l T e n s i o n :

1 5 D y n e s / C r n

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r . . . ~ ' ~ _ - - C A - 0 ~ . gr ee s

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. .

: : ~ . . . .

c A . o ~ . ~ .

° F f

o o01 O Ol o 1 1 o ool

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L i q u id I n te r f a c i a l T e n s io n : 3 0 D y n e s / C r n

Hc cm

w a t e r

1 0 0 0 L

~ - . . . : .- ~ , ~ I _ _ r ~ . 0 = ,~ ,e e s

L i q u id In t e r f a c i a l T e n s io n : 6 0 D y n e s / C m

i q u i d I n t e r r a c ia l T e n s i o n : 4 5 D y n e s / C m

Hc cm water) Hc cm

w a t e r

IO~DO~ , . 'L •

000 IE ~

I I E

L . - : . . ~ . . . I - ~ ° ° ' ~ I F - . . - - : . - . ~ i - - ~ - o ~ , ~ . .. .

, o o ~ - - . : ~ I . .. . c A . , 0 o , ~ , . , , o o i ~ - - . : : . - ~ . . . . . ~ . . . ~ . , o 0 , 0 , ~ ,

. . . . J I

o.ool

o.o l o.1 1 o.ool o.ol o.1 1

o re

R a d i u s ( m m ) P o r e R a d i u s ( m r n )

O.Ol o.1 1

ore R a d i u s ( ra m )

Fig 3 Capillary pressure as a function of liqmd interfacial tension and contact angle C A ) .

P c = 2 a c o s ¢ /r 3)

w h e r e r is t h e r a d i u s o f t h e w a t e r - f i l le d p o r e t h a t t h e N A P L m u s t e n t e r ; a n d a

i s t h e i n t e r f a c i a l t e n s i o n b e t w e e n N A P L a n d w a t e r w i t h t h e s u b s c r i p ts

d r o p p e d . E q . 3 i s v a l i d o n l y f o r i n t e r f a c e s t h a t f o r m s u b s e c t i o n s o f a s p h e r e .

C a p i l l a r y p r e s s u r e i n c r e a s e s a s r a n d ¢ d e c r e a s e a n d a s a i n c re a s e s . C a p i l l a ry

p r e s s u r e a f f e c t s t h e s h a p e o f a s p i ll i n t h e v a d o s e z o n e . W h i l e t h e s h a p e o f a sp i ll

is in f l u e n c e d b y m a n y f a c t o r s , t h e h o r i z o n t a l c o m p o n e n t is d u e l a r g e l y t o

c a p i l l a r i t y d e P a s t r o v i c h e t al ., 19 79 ) a n d m e d i u m s p a t i a l v a r i a b i l i t y .

T h e c a p i l l a r y p r e s s u r e t h a t m u s t b e o v e r c o m e fo r a n o n w e t t i n g N A P L t o

e n t e r w a t e r - s a t u r a t e d m e d i a is k n o w n a s t h e t h r e s h o l d o r d i s p la c e m e n t e n t r y

p r e s s u r e. B e c a u s e c a p i l l a r y f o r ce s c a n r e s t r i c t t h e m o v e m e n t o f N A P L i n t o

w a t e r - s a t u r a t e d m e d i a , l a y e r s w i t h s m a l l r c a n s e r v e a s c a p i l l a r y b a r ri e r s . T h a t

i~, b e f o re a n N A P L c a n p e n e t r a t e i n t o a w a t e r - s a t u r a t e d p o r o u s m e d i u m , t h e

N A P L p r e s s u r e h e a d m u s t e x c e e d t h e r e s i s ta n c e o f t h e c a p i l l a r y f o r c es e .g .,

S c h w i l le , 1 98 8). T h e t h r e s h o l d e n t r y p r e s s u r e f o r N A P L m i g r a t i o n c a n b e

e s t i m a t e d a s a n e q u i v a l e n t h e a d o f w e t t i n g f l u id w a t e r ) b y t h e f o l lo w i n g

e q u a t i o n :

7/24/2019 Mercer 1990


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0 ~ I

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7/24/2019 Mercer 1990


8


hc = (2a cos

¢)/ r p~ g)

(4)

w h e r e hc i s t h e c a p i l l a r y r i s e o f th e w e t t i n g f lu id ; Pw i s th e d e n s i t y o f w a t e r ; a n d

g i s t he g r a v i t a t i on a l c on s t a n t . Th i s e q ua t ion i s so lve d f o r a r a n ge o f a -, ~b- a nd

r - v a l u e s i n F ig . 3 . S e v e r a l r e l a t e d u s e f u l t h r e s h o l d e n t r y p r e s s u r e r e l a t i o n s h i p s

a r e g i v e n i n T a b l e 2 . E q u a t i o n s 3 , 4 , a n d t h o s e i n T a b l e 2 a r e a p p r o x i m a t i o n s

f o r i n t e r fa c e s i n p o r o u s m e d ia . T o d e t e r m i n e e n t r y p r e s s u r e m o r e a c c u r a t e l y ,

i t m u s t b e m e a s u r e d .

L a b o r a t o r y e x p e r i m e n t s s h o w t h a t c a p i l l a r y p r e s s u r e c a n b e r e p r e s e n t e d a s

a f u n c t i o n o f s a t u r a t i o n ( e.g ., T h o m a s , 1 9 8 2 ). C h a n g e s i n c a p i l l a r y p r e s s u r e

w i t h s a t u r a t i o n d e p e n d o n w h e t h e r t h e m e d i u m i s u n d e r g o i n g w e t t i n g

( i m b i b i ti o n ) o r d r a i n i n g o f t h e w e t t i n g f l ui d . C a p i l l a r y h y s t e r e s i s i s r e l a t e d t o

t r a p p e d n o n w e t t i n g f l u i d a n d t o t h e d i f f e r e n c e s i n t h e c o n t a c t a n g l e s w i t h

w e t t i n g a n d d r a i n i n g , a n d c a u s e s d i f f e r e n t w e t t i n g a n d d r y i n g c u r v e s t o b e

f o ll o w e d d e p e n d i n g o n t h e p r e v i o u s w e t t i n g / d r y i n g h i s t o r y . D u r i n g d r a i n a g e ,

t h e l a r g e r p o r e s d r a i n q u i c k l y w h i l e t h e s m a l l e r p o re s d r a i n r e l u c t a n t l y , i f a t

a l l . T h i s c a p i l l a r y r e t e n t i o n e x p l a i n s w h y c a p i l l a r y p r e s s u r e c o r r e s p o n d s t o

h i g h e r s a t u r a t i o n s o n t h e d r~ i n a g e c u r v e . O n w e t t i n g , t h e s m a l l e r p o r e s f il l f i rs t

a n d t h e l a r g e r o n e s a r e l e a s t l i k e l y t o f i l l . T h i s c o n d i t i o n l e a d s t o a l o w e r

c a p i l l a r y p r e s s u r e c u r v e w i t h s a t u r a t i o n ( T h o m a s , 1 9 8 2 ) .

L e n h a r d a n d P a r k e r (1 98 7b ) m e a s u r e d s a t u r a t i o n - c a p i l l a r y p r e s s u r e

r e l a t i o n s f o r b e n z e n e , o -x y l e ne , p - c y m e n e a n d b e n z y l a l c o h o l i n a s a n d y p o r o u s

m e d i u m . A s p a r t o f t h i s w o r k , t h e y e v a l u a t e d a s c a l i n g p r o c e d u r e ( P a r k e r e t al .,

1987) appl ied to

s-Pc

r e l a ti o n s o f t w o -p h a se ~ i r - w a t er , a i r - N A P L a n d N A P L -

w a t e r p o r o u s m e d i a s y s t e m s . R e l a t i v e l y g o o d f i t s w e r e o b t a i n e d b y f i t t i n g t h e

e x p e r i m e n t a l d a t a t o m u l t if l u i d v e r s i o n s o f t h e B r o o k s a n d C o r e y ( 19 64 ) a n d

v a n G e n u c h t e n (1 98 0) r e t e n t i o n f u n c t i o n s . T h e y c o n c l u d e d t h a t u s i n g s c a l i n g

f a c t o r s f ro m in t e r f a c i a l t e n s i o n d a t a p e r m i t s p r e d i c t i o n o f s-Pc r e l a t i o n s f o r

a n y t w o - p h a s e f lu id s y s t e m i n a p o r o u s m e d i u m f r o m m e a s u r e m e n t o f a s i n g l e

t w o - p h a s e s y s t e m a n d a p p r o p r i a t e i n t e r f a c i a l t e n s i o n d a t a . B a s e d o n t h e

a s s u m p t i o n s o f L e v e r e t t (1 94 1)~ t h r e e - p h a s e s y s t e m b e h a v i o r m a y a l s o b e

p r e d ic t e d .

Residual saturation

R e s i d u a l s a t u r a t i o n (S r) o f N A P L i s t h e s a t u r a t i o n

VNAPL/Vvoids)

a t w h i c h

N A P L b e c o m e s d i s c o n t i n u o u s a n d i s i m m o b i l i z e d b y c a p i l l a r y f o r c e s u n d e r

a m b i e n t g r o u n d w a t e r f lo w c o n d it io n s . T h e p h y s i c s o f o i l e n t r a p m e n t a n d d e-

v e l o p m e n t o f m e t h o d s t o r e d u c e re s i d u a l s a t u r a t i o n b y e n h a n c e d o il r e c o v e r y

a r e o f o b v i o u s im p o r t a n c e t o t h e p e t r o l e u m i n d u s t r y ( M e l r o s e a n d B r a n d e r ,

1974; C ha tz i s e t a l ., 1983 ; M c C a f f e r y a nd B a ty c ky , 1983 ; W .G . A nde r son , 1987c ;

M oh a n ty e t a l ., 1987 ; C h a tz i s e t a l ., 1988 ; M or r o w e t a l . , 1988 ; W a ng , 1988 ).

S i m i l a rl y , r e s id u a l s a t u r a t i o n o f N A P L h a s i m p o r t a n t c o n s e q u e n c e s i n t h e

r e m e d i a t i o n o f s u b s u r f a c e c o n t a m i n a t i o n . R e s i d u a l N A P L s m a y p r o v i d e a

l a s t in g s o u r c e o f s i g n i fi c a n t g r o u n d w a t e r c o n t a m i n a t i o n b e c a u s e d r i n k i n g

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R E V I E W O F I M M I S C IB L E F L UI D S I N T H E S U B S U R F C E 9

water stand ards for many NAPL s are orders of magnitude less than their

solubility limits. Combined with practical limitations on residual NAPL

recovery (J.L. Wilson and Conrad, 1984), NAPL di ssolution necessitates

perpetual hydraulic containment at some contamination sites (Cohen et al.,

1987; Mackay and Cherry, 1989).

Residual saturation results from capillary forces and depends on several

factors , including: (1) the medium pore size dis tribution; (2) wet tab ilit y; (3) fluid

viscosity ratio and density ratio; (4) interracial surface tension; (5) gravity]

buoyancy forces; and (6) hydraulic gradients. Residual saturation for the

wetting fluid is conceptually different from that for the nonwetting fluid. The

nonwetting fluid is discontinuous at residual saturation, whereas the wetting

fluid is not. Field-scale values of sr are difficult to measure or estimate

accurately and are subject to considerable error.

In the vadose zone, NAPL is retained as wetting pendular rings and as

nonwet ting blobs in the presence of water (e.g., Cary et al., 1989). The ability

of the vadose zone to trap NAPL is sometimes measured and reported as the

volumetric retention capacity:

R = sr × (porosity) × 1000

where R is liters of residual NAPL per cubic meter of medium (de Pastrovich

et al., 1979; Schwille, 1984; J.L. Wilson and Conrad , 1984). Residual sa turat ion

measurements involving various NAPL s and media compiled in Table 3

indica te th at st-values typicall y range from 0.10 to 0.20 in the vadose zone. In

general, residual saturation and retention capacity values in the vadose zone

increase with decreasing int rinsic per_m_eability~ effective porosity and

mois ture conten t (Fussell et al., 1981; Schwille, 1984; Hoag and Marley, 1986;

M.R. Anderson, 1958).

Usually more NAPL is immobilized in the saturated zone than in the vadose

zone because: (1) the fluid density ratio (NAPL/air vs. NAPL/ wate r above and

below the w ate r table, respectively) favors gr eate r drainage in the vadose zone;

(2) as the nonwet ting ~tuid in m~st satura ted media, NAPL is trapped in the

larger pores; and (3) as the wetti ng fluid (with respect to air) in th e vadose zone,

NAPL tends to spread into adjacent pores and leave a lower residual content

behind, a process that is inhibited in the sa tura ted zone where NAPL is usually

the non wetting fluid (M.R. Anderson, 1988). Values of sr in s atu rat ed media

generally range from 0.15 to 0.50 (Table 3).

On the pore scale, residu al NAPL below the water tab le is immobilized as a

resu lt of snap-off and bypass ing mechanisms (Chatzis et al., 1983). Snap-off is

prevalent in high aspect ratio pores wherc the pore body is much larger than

the pore throat, resulting in single droplets or blobs of residual NAPL.

Bypassing occurs when wetting fluid flow disconnects the nonwetting fluid,

causing NAPL ganglia to be trapped in clusters of large pores surrounded by

smaller pores. Residual saturation is thought to depend on soil pore size

distribution in the vadose zone, but depends more on pore aspect ratio in the

sat ura ted zone (J.L. Wilson and Conrad, 1984). Residual sa tur at ion tends to

7/24/2019 Mercer 1990


12 J W M E R C E R A N D R M C O H E N

0

o

q~

~

II II II II II II II II II II II II II II H H II II II II II II II II II II II II II II II II II

0 ~ ~ ~ 0

~ ~ ~ ~ ~ ~ o

r l l ~ ~ r

~

~ .~

7/24/2019 Mercer 1990


R E V I E W O F I M M I S C I B L E F L U I D S I N T H E S U B S U R F C E 2

~ ~ ~ ~ . ~ ~ .

II II II II II II II II II II II II II II II II II II II II II II II II II II II II II

~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

°

m ;~ mo o m m ~ ~ ~ ~ m

~J

o

o o o o o o o o o o o o o - - -~ -~

~ ~ ~ ~ ~ ~ ~

~ N N N N N ~ N N N a . a . ~ ~ ~ m ~

Z

o o

c o o ~ o 0

° ~

° ~

I I I I ~

o°

i i t

~

o o

. - i

H

H

7/24/2019 Mercer 1990



increase with increasing pore aspect ratios and pore size heterogeneity

(Chatzis et al., 1983), and with decreasing porosity, p robab ly due to reduced

pore connectivity and a ~lecrease in mobile nonwetting fluid in smaller pore

thro ats (J.L. Wilson and Conrad, 1984). Residual s atura tio n is reduced in

near-neutra l wettability media because the capill ary forces tha t t rap NAPL are

minimized (W.G. Anderson, 1987c).

Some residual NAPL at contamination sites can be mobilized by increasing

the prevailing hydraulic gradient or reducing interfacial tension. Dissolution

and volatilization cause the residual saturation to decrease with time. These

topics are discussed in more detail in subsequent sections.

elative permeab ility

When more than one fluid exists in a porous medium, the flowing fluids

compete for pore space. The net result is that the mobility is reduced for each

fluid. The reduction can be quantified by mult iplyi ng the in trins ic permeability

by a dimensionless ratio, know n as relative permeability. Relative permeability

is the ratio of the effective permeability of a fluid at a fixed saturation to the

intrinsic permeability. Relative permeability varies from zero to one. Like

capillary pressure, relative permeability can be represented as a function of

saturation and also can exhibit hysteresis. An example of relative permeabili-

ties in a water-oil system is shown in Fig. 4a. At residual saturation, the

respective relative permeability becomes zero; that is, flow ceases to occur.

Setting the relative permeability of the wetting fluid to zero at residual

saturation is an approximation, whereas the relative permeability of the

nonwetting fluid is zero. Similar curves are obtained for air-NAPL systems.

Extensive reviews of two- and three-phase re lati ve permeability data, measure-

ment methods, and contro lling factors are presented by Saraf and McCafferty

(1982) and Honarpour et al. (1986).

If water, NAPL and air are flowing simultaneous ly at a point, three-phase

relative permeabilities are required to describe the movement of each. The

functional dependence of relative permeabilities is based on experiments

(Corey et al., 1956; Snell, 1962). Unf ortuna tely, given the expense and difficulty

of measurement, actu al site-specific data and the functional form of three-phase

relative permeability are generally not available, part icul arly for NAPL s

other than petroleum. As a result, theoretical models have been developed to

characte rize three-phase rel ativ e permeability (Stone, 1970, 1973; Dietrich and

Bonder, 1976; Fayers and Matthews, 1984; Pa rke r et al., 1987; Delshad and

Pope, 1989). For example, Stone (1973) has proposed a model for e stimat ing

three-fluid relative permeabilities based on data for two-fluid relative per-

meabilities. In this approach, relative permeability data for NAPL are obtained

in both water-NAPL and air-NAPL systems. The relative permeability of

NAPL in the three-fluid system is determined from the following equation:

k r n = k * r n w [ k rn . /k* rn w ~- k rw ) k rn a /k* rn w + k r a ) - k r w + k r a ) ] 5 )

7/24/2019 Mercer 1990


A REVIEW OF IMM ISCIBLE FLUIDS IN THE SUBSURFACE 23

a ) 1 0 0

8O

7 0

a.

> :

~ - 6 0

u i

1-

u i

o .

uJ

I -

m

n

5 0 -

4 0

3 0 -

2 0 -

1 0 -

I

O 1

I I I I I I I I I

N A P L

I I 1

2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0

W A T E R S A T U R A T I O N

p e r c e n t

1 0 0

S a = l

b )

S w = 1 S n = 1

S a = A I R S A T U R A T I O N

S w = W A T E R S A T U R A 1 1 O N

S n = N O N A Q U E O U S P H A S E

S A T U R A T I O N

Fi g . 4 . a . W a t e r - N A PL r e l a t iv e p e r m e a b i l it y m o d if ie d f r o m H o n a r p o u r a n d Ma h m o o d , 1 9 8 8) .

b . T e r n a r y d i a g r a m s h o w i n g t h e r e l a t i v e p e r m e a b i li t y o f t h e n o n a q u e o u s p h a s e a s a f u n c t i o n o f

pha se sa tu rat ion s Fau s t , 1985) .

w h e r e k * * i s t h e r e l a t iv e p e r m e a b i l i t y o f N A P L a t t h e r e si d u a l s a t u r a t i o n o f

w a t e r i n t h e w a t e r - N A P L s y s t e m ; k r ,, i s t h e r e l a t i v e p e r m e a b i l i ty o f N A P L i n

t h e w a t e r - - N A P L s y s t e m ( a f u n c t i o n o f w a t e r s a t u r a t io n ) ; a n d k ~a i s t h e r e l a t iv e

p e r m e a b i l i ty o f N A P L i n t h e a i r - N A P L s y s t e m ( a f u n c t i o n o f a i r s a t u r a ti o n ) .

T h i s e q u a t i o n m a y b e u s e d t o c o n s t r u c t a t e r n a r y d i a g r a m , a s p r e s e n t e d i n Fi g.

4 b . D e l s h a d a n d P o p e ( 1 9 8 9 ) r e c e n t l y e v a l u a t e d s e v e n d i f f e r e n t t h r e e - p h a s e

r e l a t i v e p e r m e a b i l i ty m o d e l s b y c o m p a r i n g p r e d i c t e d p e r m e a b i l i ti e s w i t h th r e e

s e t s o f e x p e r i m e n t a l d a t a.

R e l a t i v e p e r m e a b i l i ty d a t a a r e g e n e r a l l y n o t a v a i l a b l e f o r N A P L s f o u n d in

h a z a r d o u s w a s t e s i t e s . H o w e v e r , L i n e t a l . ( 1 9 8 2 ) m a d e l a b o r a t o r y m e a s u r e -

m e n t s o f p r e s s u r e - s a t u r a t i o n r e l a t i o n s fo r w a t e r - a i r a n d t r i c h l o r o e t h e n e

( T C E ) - a ir s y s t e m s i n h o m o g e n e o u s s a n d co l u m n s . T h e s e d a t a w e r e c o n v e r t ed

t o t w o - fl u id s a t u r a t i o n - r e l a t i v e p e r m e a b i l it y d a t a b y A b r i o l a ( 1 98 3) u s i n g

M u a l e m s (1 97 6 ) t h e o r y . O t h e r p r e s s u r e - s a t u r a t i ° n d a t a f o r t e t r a c h l o r o e t h e n e

a r e p r o v i d e d i n K u e p e r e t a l . ( 1 9 8 9 ) .

olubility

T h e a q u e o u s s o lu b i l i t y o f a c h e m i c a l i s t h e m a x i m u m c o n c e n t r a t i o n o f t h e

c h e m i c a l t h a t w i l l d i s s o l v e i n p u r e w a t e r a t a p a r t i c u l a r t e m p e r a t u r e . R e s u l t s

o f l a b o r a t o r y d i s s o l u t i o n e x p e r i m e n t s ( M . R . A n d e r s o n , 1 9 88 ; S c h w i l l e , 1 98 8)

s h o w t h a t c o n c e n t r a t i o n s a p p r o x i m a t e l y e q u a l t o t h e a q u e o u s s o l u b i l i t y o f t h e

c o m p o u n d a r e o b t a in e d i n w a t e r f l ow i n g a t 1 0 - 1 0 0 c m d a y

1

t h r o u g h N A P L -

7/24/2019 Mercer 1990


24


contaminated sands. According to Mackay et al. (1985), however, organic

compounds are commonly found in groundwater at concent rations of < 10 of

NAPL solubility limits, even when NAPL is known or suspected to be present.

This discrepancy between laboratory and field measurements is probably

caused by diffusional limitations of dissolution in conjunc tion with heteroge-

neous field conditions, such as non-uniform groundwater flow, variable NAPL

distribution, and mixing of stratified groundwater in a well (Mackay et al.,

1985; Feenstra and Cherry, 1988). These processes favor the creation of large

plumes of groundwater with low chemical concentrations that may greatly

exceed acceptable levels. Another factor to consider is that dissolved chemical

concentrations will also be less than aqueous solubilities reported for pure

chemicals where the NAPL is composed of multiple liquids. For this case, the

actual aqueous solubility of a particular component of the multi-liquid NAPL

can be approximated by multiplying the mole fraction of the chemical in the

NAPL by its pure form aqueous solubility (Banerjee, 1984).

NAPL's vary widely in their aqueous solubility (Appendix A). Nonpolar

hydrophobic compounds are less soluble than polar hydrophilic compounds.

Solubilities may be measured experimentally or estimated based on empirical

relationships developed between solubility and other chemical properties such

as partition coefficients and molecular structure. For example, Kenaga and

Goring (1980) and Lyman et al. (1982) present numerous regression equations

that correlate aqueous solubility with Kow (octanol/water) and Koc (organic

carbon/water) partition coefficients for different chemical groups. Kow and Koc

data for NAPL's are given in Appendix A. Nirmalakhandan and Speece (1988)

developed a predictive equation for aqueous solubility based on correlations

between the solubilities and molecular structures of 200 environmentally

relevant chemicals. Organic concentrations in water can also be calculated

from an equilibrium relat ionship based on Raoult 's law and Henry's law (Cor-

apcioglu and Baehr, 1987).

Several factors influence solubility, including temperature, cosolvents,

salinity and dissolved organic matter. Although the aqueous solubility of most

organic chemicals increases with temperature, the direction and magnitude of

the temperature-solubility relationship are variable (Lyman et al., 1982).

Similarly, the effect of cosolvents (multiple organic compounds) on chemical

solubility depends on the specific mix of compounds and concentrations.

Banerjee (1984) and Groves (1988) describe methods for predicting the solubili-

ties of organic chemical mixtures in water based on activity coefficient

equations. The aqueous solubility of organic chemicals generally declines with

increasing sal inity (Eganhouse and Calder, 1976; Rossi and Thomas, 1981).

However, dissolved organic matter, such as naturally occurring fulvic and

humic acids, has been shown to enhance the solubility of hydrophobic organic

compounds in water (Lyman et al., 1982; Chiou et al., 1986).

If NAPL has a range of intermixed components of varying individual solu-

bilities, the more soluble components will dissolve more rapidly and leave

behind a less soluble residue (Senn and Johnson, 1987)~ For a gasoline, this

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R E V I E W O F I M M I S C I B L E F L U ID S I N T H E S U B S U R F C E

T A B L E

125

Mass -exch ange coef f ic i en ts* fo r var ious hy drocarb ons

Hydrocarbons

M ass-exch ange coeff icients, k m

( 1 0 - 3 m g m - 2 s - ~ )

G a s o l i n e t a r oi l

Fue l o i l, d i ese l , ke rosene

Lub e o i ls , heavy fue l o i l

100

10

1

* W o r k i n g G r o u p W a t e r a n d P e t r o l e u m (1 97 0).

process is referred to as wea thering, where the light end compounds are lost to

dissolution and volatilization. Thus, the ratios of chemicals in the NAPL and

dissolved plume change with time.

Subsurface NAPL trapped as ganglia at residual saturation and NAPL

contained in pools, such as LNA PL floating above the wate r table or DNAPL

in depressions along the base of an aquifer, present long-term sources of

groundwater contamination. Factors controlling NAPL dissolution into

groundwater and its eventual depletion include the aqueous solubility of

NAPL chemicals, groundwater velocity, NAPL-water contact area and the

molecula r diffusivity of the NAPL chemicals in wate r (Pfannkuch, 1984; M.R.

Anderson, 1988; Feenst ra and Cher ry, 1988; Hunt et al., 1988a; Schwille , 1988).

Various experimental, theoreti cal and conceptual analyses of these factors and

mass exchange across the NAPL-water interface are discussed below.

Pfannkuch (1984) reviewed literature related to the mass exchange of

petroleum hydrocarbons to groundwater. Laboratory studies of LNAPL

transfer to w ater were conducted by Hoffmann (1969, 1970), Work ing Group

"Water and Pet role um" (1970), van der Waarden et al. (1971), Zilliox et al.

(1973, 1974), and Fried et al. (1979). Schwil le (1988) and M.R. Anderson (1988)

performed experiments to analyze the mass tran sfer of DNAPL chemicals into

groundwater. Estima tes of mass-exchange coefficients (kin), which repre sent the

mass of NAPL dissolved into groundw ater per uni t contact area per unit time,

are given in Table 4. The NAPL-water contact area is difficult to determine

where residual NAPL is trapped as globules in the satura ted zone. To describe

mass exchange in these cases, Ziiliox et al. (1973) utilize the volumetric

exchange coefficient (kmv) which equa ls dm/dt)/Vwhere m and V are th e mass

and volume of the source, respectively.

Experimental data indicate that mass-exchange co,~fficients generally

increase with greun dwat er ve nc~ty~ except a t low velocities where the

exchange r ate is controlled by molecula r diffusion, and decrease with time as

the NAPL ages (Zilliox et al., 1973, 1978; Pfannkuch, 1984). However,

Pfannkuch (1984) notes that fluctuations of the water table rejuvenate the

dissolution process (presumably by changing the exposed NAPL surface area)

and effectively increase k~-values for LNAPL trapped below the water table.

The mass-exchange ra te (m, T- ~), or strength of the dissolved cont amin atio n

source, can be expressed simply as the p roduct of the mass-exchange coefficient

7/24/2019 Mercer 1990


26


(m~ L-2T - 1) and some measure of the con tac t area (L2). The con tac t area of a

given mass of residual NAPL ganglia is more difficult to estimate, but much

greater than that of an equivalent mass of pooled NAPL. Consequently, dis-

solution of residual NAPL produces higher co ncent rations of NAPL chemicals

in groundwater and depletes the NAPL source more quickly than dissolution

of an NAPL pool of equivalentmass.

Several relationships have been derived for residual and pooled NAPL

geometries to predict dissolved chemical concentrations in groundwater and

the time requi red for NAPL source depletion (Azbel, 1981; M.R. Anderson, 1988;

Hunt et al., 1988a). Given current limitations characterizing subsurface NAPL

distributions and mass-exchange processes, these models are primarily useful

as a conceptual tool to evaluate the long-term contamination potential

associated with subsurface NAPL. For example, the time required to

completely dissolve an NAPL source given an existing or induced inter stitial

groundwater velocity, 0, can be estimated as:

t = m / O n Cw A )

where A is the cross-sectional area co ntain ing NAPL thr ough which ground-

water flow exits with a dissolved NAPL con cen trat ion, Cw. Considering limits

to solubility and groundwater velocities, it is apparent that dissolution is an

ineffective removal process for significant quantities of many NAPL s.

V o l a t i l i z a t i o n

Volatilization refers to the transfer of matter from liquid and soil to the

gaseous phase. Thus, chemicals in the soil gas may indicate the presence of

NAPL or dissolved chemicals. Chemical properties affecting volatilization are

vapor pressure (Appendix A) and solubility in water. Other factors affecting

volatilization rate include: concent ration in the soil, soil moisture content, soil

air movement, sorptive and diffusion characte ristics of the soil, temperature,

and bulk properties of the soil such as organ ic carbo n content, porosity, density

and clay conten t (Lyman et al., I982). For example, Zytne r et al. (1989) observed

that the gre ater the organic carbon content, the higher the tetrach loroethen e

(PCE) sorption on soil and, consequently, the slower the volatili zation rate for

both aqueous and pure PCE. On the other hand, volatilization increases with

soil air movement. Volatilization losses in the subsurface from NAPL are

expected where NAPL exists close to the ground surface or in dry pervious

sandy soils, or where NAPL has a very high vapor pressure (Feenstra and

Cherry, 1988).

Once a chemical volatilizes and becomes part of the gas phase, it is trans-

ported and ultimately condenses, is sorbed onto soil particles, degraded, or is

released to the atmosphere. In the case of flammable organics, volati liza tion in

soil can result in a fire or explosion hazard (Fussell et al., 1981). The vapor can

also adsorb to the soil. Using kerosene experiments, Acher et al. (1989) found

that adsorption of vapor decreased with increasing soil moisture content.

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A R E V I E W O F I M M I S C I B L E F L U Ig S I N T H E S U B S U R F A C E 2 7

Estimating volatilization from soil involves two steps: (1) estimating the

organic partitioning between water and air, and NAPL and air, and (2)

estimating the vapor tr ansp ort from the soil. Henr y s law is commonly used to

determine the partitioning between water and air, whereas the partitioning

between an NAPL and air is described by Raoult s law. Vapor transport in the

soil is usually described by the diffusion equation and several approximate

methods have been developed where the main tr ans port mechanism is macro-

scopic diffusion (Lyman e t al., 1982). More complicat ed models a re also

ava ilable (e.g., Fal ta et al., 1989; Sleep and Sykes , 1989; Ju ry et al., 1990).

Because a chemical can volatilize from either a dissolved state or from

NAPL, both conditions need to be considered to characterize th e tot al amount

of the chemical that is volatilized. In general, local equilibrium is assumed

between the air and othe r fluids. Henry s law relate s the conc entr ation of a

dissolved chemical in water to the partial pressure of the chemical in the gas:

P = KH Cw (6)

where P is the pa rti al pressure of the chemical in the gas phase (atm.); Cw is the

con centr ati on of the chemical in wa ter (molm-3); and KH is Henr y s law

con stant (atm. m 3mol-1). Henry s law is obeyed for spa ringly soluble, non-elec-

trolytes where the gas phase is considered ideal (Noggle, 1985). Henry s law

constants for NAPL compounds are given in Appendix A. The tendency of a

chemical to volatilize increases with an increase in Henry s law constant.

Raoult s law has been used to quantify the ideal reference state for the

TABLE 5

Vapor c onc ent rat ion and total gas densit y data for selected NAP L's at 25 ° C (from Fal ta et al., 1989)

Chemical

Molecular Vapor Saturat ed vapor T~tal gas

weight, M pressure concentr ation density

(gmo1-1 ) (kP a ~a 25°C) (k gm -~) (k gm -~)

Tri ch lo roe thene 131.4 9.9 0.52 1.58

Tol uene 92.1 3.8 0.14 1.27

Benzene 78.1 12.7 0.40 .42

Chloroform 119.4 25.6 1.23 2.11

Te tr ac hloroe then e 165.8 2.5 0.17 1.31

1 1 1

-Trich loroethene 133.4 16.5 0.89 1.87

Ethy lbenzene 106.2 1.3 0.06 1.22

Xylene 106.2 1.2 0.05 1.21

Dic hloro met hane 84.9 58.4 2.00 2.50

1,2 -Dichlor oet hene 96.9 43.5 1.70 2.37

1,2 -Di chloroeth ane 99.0 10.9 0.44 1.48

Chlorobenz ene 112.~ 1.6 0.07 1.23

1,1 -Dichloroeth ~me 99.0 30.1 1.20 2.03

Te tr achl orom etha ne 153.8 15.1 0.94 1.93

Ai r at 1 atm., 25 ° C 28.6 101.3 n.a. 1.17

n.a. = not applicable.

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128

J.W. MERCER AND R.M. COHEN

equilibrium between NAPL and air (Corapcioglu and Baehr, 1987). Raoult’s

law relates the ideal vapor pressure and the relative concentration of a

chemical in solution to its vapor pressure over the solution:

PA

X P

where PA is the vapor pressure of the solution; XA is the mole fraction of the

solvent; and

i

is the vapor pressure of the pure solvent.

Volatilization represents a source to vapor transport. Recent studies have

examined the gas-phase advection resulting from gas pressure and gas density

gradients (Falta et al., 1989; Sleep and Sykes, 1989; Mendoza and McAlary,

1990; Hughes et al., 1990; Mendoza and Frind, 1990). Density-driven gas flow

can be an important transport mechanism in the vadose zone that may result

in contamination of the underlying groundwater and significant dissipation of

residual NAPL.

Density-driven gas flow is a function of the gas-phase permeability, the

gas-phase retardation coefficient, and the total gas density which depends on

the NAPL molecular weight and saturated vapor pressure (Falta et al., 1989).

Saturated vapor concentrations and total gas densities calculated for some

common NAPL’s using the ideal gas law and Dalton’s law of partial pressures,

respectively, are given in Table 5. Density-driven gas flow will likely be signifi-

cant where the total gas density exceeds the ambient gas density by >

10

and

the gas-phase permeability exceeds l*lO- ‘Im2 in homogeneous media (Falta et

al., 1989). Dense gas emanating from NAPL in the vadose zone will generally

sink to the water table where it and gas that has volatilized from the saturated

zone will spread outward. Gas migration patterns will be strongly influenced

by subsurface heterogeneities.

Density is the mass per unit volume of a substance. It often is presented in

terms of specific gravity, which is the ratio of a substance’s density to that of

some standard substance, usually water. Density varies as a function of

different parameters, most notably temperature.

According to Mackay et al. (1985), density differences of * 1 influence

fluid movement in the subsurface. In many situations, NAPL densities differ

from water by 10-50 . The specific gravities of gasoline and other petroleum

distillates may be as low as 0.7 (Appendix B). Halogenated hydrocarbons

generally are more dense than water. Chlorinated aliphatic compounds

containing one and two carbon atoms have specific gravities from

1.2

to

1.5

(Mackay et al., 1985). The densities of selected NAPL’s are given in Appendix

B.

LNAPL’s and DNAPL’s can be referred to as floaters and sinkers, respective-

ly. After a spill or release, LNAPL’s in sufficient quantity, migrate in response

to pressure and elevation head gradients. Thus, knowing the shape of the

water-table surface may help locate LNAPL, which will tend to move toward

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REVIEW OF IMMISCIBLE FLUIDS IN THE SUBSURF CE

29

w a t e r -t a b l e l ow s . D o w n w a r d D N A P L m o v e m e n t b e l o w t h e w a t e r t a b l e w i ll be

i m p e de d b y c a p il l a ry a n d / o r p e r m e a b i l it y b a r r i e r s u p o n w h i c h D N A P L m a y

f lo w d o w n d i p c o u n t e r t o t a e h y d r a u l i c g r a d i e n t ( F a u s t, 1 9 85 ). P r e f e r e n t i a l

D N A P L s p r e a d i n g w i l l o c c u r a l o n g p a t h w a y s o f l e a s t c a p i l l a ry a n d p e rm e a b i l-

i ty r e s is t a n c e . A l t h o u g h s i t e s t r a t i g r a p h y a n d s t r u c t u r a l g e o l o g y c a n b e h e l p fu l

i n l o c a t i n g D N A P L , l i t t l e s u c c e s s h a s b e e n a c h i e v e d a t s o m e s i t e s i n e v e n

l o c a t i n g s u b s u r f a c e D N A P L s o u r c e s ( M a c k a y a n d C h e r r y , 1 9 8 9 ) , p a r t i c u l a r l y

i n f r a c t u r e d m e d i a .

iscosity

V i s c o s i t y i s t h e i n t e r n a l f r i c t i o n w i t h i n a f l u i d t h a t c a u s e s i t t o r e s i s t f l o w .

F o l l o w i n g a s p i ll , a p r o d u c t o f lo w v i s c o s i t y w i l l p e n e t r a t e m o r e r a p i d l y i n t o

t h e s oi l t h a n a p r o d u c t w i t h h i g h e r v i s co s it y . N A P L v i s c o s it y c a n c h a n g e w i t h

t i m e . F o r e x a m p l e , f r e s h c r u d e o i l s w i t h v o l a t i l e c o m p o n e n t s b e c o m e i n c r e a s -

i n g l y v i sc o u s a s t h e y e v a p o r a t e . A b s o l u t e v i s c o s i ty d a t a f o r s e l e ct e d N A P L s

a r e p r o v i d e d i n A p p e n d i x B .

A l s o s i g n i f ic a n t i s t h e N A P L - w a t e r v i s co s i t y r at i o , w h i c h i s p a ~ o f a te i m

u s e d in t h e p e t r o l e u m i n d u s t r y k n o w n a s t h e m o b i l it y r at i o . I n a w a t e r f lo od ,

t h e m o b i l i t y r a t i o i s d e f in e d a s t h e m o b i l i t y o f t h e d i s p l a c i n g f l u i d ( r e l a t iv e

p e r m e a b i l i t y / v i s c o s i t y f o r w a t e r ) d i v i d e d b y t h e m o b i l i t y o f t h e d i s p l a c e d f lu i d

( r e l a t i v e p e r m e a b i l i t y / v i s c o s i t y f o r N A P L ) . M o b i l i t y r a t i o s o f > 1 f a v o r t h e

f lo w o f w a t e r w h e r e a s t h o s e < 1 f a v o r t h e f lo w o f N A P L . I n t h e d i s p l a c em e n t

p r o ce s s, l o w e r m o b i l i ty r a t i o s f a v o r N A P L r e c o v e r y .

W h e n a f lu i d i n a p o r o u s m e d i u m i s d i s p l a c e d b y a n o t h e r f lu i d, t h e i n t e r f a c e

b e t w e e n t h e m m a y b e c o m e u n s t a b l e . T h i s p h e n o m e n o n , k n o w n a s f i n g e r i n g ,

r e s u l t s , i n p a r t , b e c a u s e o f t h e v i s c o s i t y d i f f e r e n c e b e t w e e n t h e t w o f lu i ds (e .g .,

C h o u k e e t a l . , 1 9 5 9 ; H o m s y , 1 9 8 7 ). W h e r e v i s c o u s f i n g e r i n g s t a r t s i s a l s o

i n f lu e n c e d b y h e t e r o g e n e i t i e s . T h e s e f a c to r s a r e d i s cu s s e d b y K u e p e r a n d F r i n d

(1 98 8). A s a r e s u l t o f v i s c o u s f i n g e r i n g , N A P L m a y n o t o c c u p y t h e c o m p l e t e

c r o s s - s e c t i o n a l a r e a t h r o u g h w h i c h i t p a s s e s , t h u s a l l o w i n g w a t e r t o f l o w

t h r o u g h a n d i n c r e a s e d i s s o lu t i o n . I n a d d i t io n , f o r a g iv e n v o l u m e o f N A P L ,

v i s c o u s f i n g e r i n g w i l l a l l o w t h e N A P L t o p e n e t r a t e d e e p e r t h a n i t w o u l d

w i t h o u t v i s c o u s f i n g e r i n g .

M T H E M T I C L D E SC R IP T IO N O F N F L F L O W

P e t r o l e u m r e s e r v o i r c o d e s f o r s i m u l a t i n g t h e f lo w o f i m m i s c i b le f lu i d s h a v e

e x i s t e d f o r m or e t ha n 20 ye a r s ( e. g ., C r i c h low , 1977, o r P e a c e m a n , 1977 ); bu t ,

w i t h f e w e x c e p t i o n s , i t o n l y h a s b e e n i n t h e l a s t f e w y e a r s t h a t t h e s e t e c h n i q u e s

h a v e b e e n u s e d t o e x a m i n e o i l a n d c h e m i c a l s p i l l p r o b l e m s . C o d e s u s e d t o

e x a m i n e N A P L f l o w a r e r e v i e w e d i n P i n d e r a n d A b r i o l a (1 98 6) a n d A b r i o l a

(1988).

E a r l y r e c o g n i t i o n o f N A P L m o v e m e n t in s h a l l o w g r o u n d w a t e r a s a tw o -f lu id

f lo w p h e n o m e n o n i s a t t r i b u t e d t o v a n D a m (1 96 7). L a t e r , s e v e r a l m o d e l s w e r e

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developed to describe mathematically the flow of NAPL in the subsurface

(Mull, 1969, 1971, 1978; Holzer, 1976; Schiegg, 1977; Dracos, 1978; Hochmuth

and Sunada , 1985). Common to each of these is the a ssumpt ion of negligible

capillarity (piston-like flow).

Brutsaert ._~973) presents an early code used to examine multif luid well flow

that account : . r capi llar ity. The model is radi al and based on a finite-differ-

ence approximation. Later, Guswa (1985) developed a one-dimensional

(vertical) finite-difference, two-fluid flow simu lato r. Faust (1985) extended this

work to accommodate two dimensions as well as a static air phase, a necessary

step to simulate NAPL flow in the vadose zone. A model similar to Faust's

(1985) model, which did not consider an ai r phase, was applied to the Hyde Park

Landfill, Niagara Falls, New York, U.S.A., by Osborne and Sykes (1986).

Abriola and Pinder (1985a, b) developed a two-dimensional model that also

considers volatili zation and dissolution. A similar model is presented in Baehr

and Corapciog lu (1987) and Corapc ioglu and B aehr (1987). Subsequen tly,

Lenhard and Parker (1987a) and Parker and Lenhard (1987) incorporated

hysteretic constitutive relations. More recently, a three-dimensional model

that extends Faust's (1985) model is described in Faust et al. (1989).

Because of previous reviews on NAPL models, a detailed review is not

provided. The basic governing equations are presented for completeness, along

with constitutive relationships that concern many of the properties discussed

in the preceding section. This discussion follows closely that presented by

Abriola (1988).

Mass balance equations

The equation development begins with the mass-balance equation for

species i in phase ~, where ~ stands for soil, air, water and NAPL, or a subset

of these. A species is defined as a specific chemical that is presen t in one or more

phases or is considered as a group of chemicals with average characteristics.

The mass-balance equation is written as (Abriola, 1988):

c o + V - v - V . = +

8 )

where v~ is the mass average velocity of the ~ phase; o~ is the mass fr action of

species i in the ~ phase; ~ is the f raction of volume occupied by the • phase; p~

is the intrinsic mass density of the ~ phase; J~ is the non-advective flux of a

species i in th e ~ phase; S~ represent s the exchange of mass of species i due to

interphase diffusion and/or phase change; R~ represents an external supply of

species i to the ~ phase; and V is the differential operator.

The first term in eq. 8 accounts for the ac cumu lati on of the mass of species

i in phase ~. The second term accounts for the movement of mass due to

advection of the phase. Motion due to non-advective effects (such as diffusion

and dispersion) is accounted for by the third term. The first term on the

right-hand side of eq. 8 is a source/sink term due to phase changes. R7

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R E V I E W O F I M M I S C I B L E F L U I D S I N T H E S U B S U R F C E

3

r e p r e s e n t s t h e d e s t r u c t i o n o r c r e a t i o n o f t h e s p e c ie s d u e t o c h e m i c a l o r

b i o l o g i c a l t r a n s f o r m a t i o n s . E q . 8 i s s u b j e c t t o t h e f o l l o w i n g c o n s t r a i n t s :

to ~ = 1 a n d ~ e ~ = 1 9 a ) , 9 b )

i

w h i c h f o l l o w f r o m t h e d e f i n i ti o n o f m a s s a n d v o h a m e f r a c t i o n . A l s o , w h e n m a s s

i s l o s t b y o n e p h a s e d u e t o i n t e r p h a s e e x c h a n g e , a n e q u a l a m o u n t o f m a s s is

g a i n e d b y a n o t h e r p h a s e, o r:

S~ = 0 (9c)

B a s e d o n e q . 8 , a m a s s - b a l a n c e r e l a t i o n s h i p f o r a s p e ci fi c p h a s e m a y b e

d e v e l o p e d b y s u m m i n g o v e r a l l s p e c i e s p r e s e n t i n t h e p h a s e o r a l t e r n a t i v e l y

s u m m i n g o v e r a l l p h a s e s .

I m m i s c i b l e f l o w e q u a t i o n s

A s e t o f e q u a t i o n s c a n b e d e v e l o p e d f r o m e q. 8 w h e r e i t i s a s s u m e d t h a t t h e r e

i s n o m a s s e x c h a n g e b e t w e e n p h a s e s a n d n o c h e m i c a l r e a c t i o n o r b io l o g ic a l

t r a n s f o r m a t i o n . E q . 8 i s s u m m e d o v e r a l l s p e c i e s t o y i e l d ( A b r i o l a , 1 98 8):

~ ( p ~ ) + V - ( p e ~ v ~) = 0 (10)

w h e r e u s e h a s b e e n m a d e o f c o n s t r a i n t ( 9 a). T h e n o n - a d v e c t i v e f l ux t e r m s ,

w h i c h d e a l w i t h r e l a t i v e m o t i o n o f t h e s p e c i e s w i t h i n a p h a s e , a l s o s u m t o z e ro .

E q . 10 h a s b e e n u s e d t o m o d e l t h e m i g r a t i o n o f p u r e N A P L s , t h a t is , N A P L s

w i t h o n e c h e m i c a l o r p h y s ic a l p r o p e r t i e s t h a t a r e s p a t i a l l y in v a r i a n t .

I n g e n e r a l , o n e e q u a t i o n i s w r i t t e n f o r e a c h o f t h e f o u r p h a s e s : s o i l (s ), a i r

( a), w a t e r (w ) a n d N A P L ( N ). I f t h e p o r o u s m e d i u m i s r i g i d ( p o r o s i ty i s i n v a r i a n t

i n ti m e ) , t h e s o i l e q u a t i o n i s n o t n e e d e d . S i m i l a r l y , i f t h e g a s p h a s e r e m a i n s a t

a t m o s p h e r i c p r e s s u r e , t h e g a s e q u a t i o n a J s o c a n b e e l i m i n a t e d t o yi e ld :

n - ~ ( s ~ p ~ ) + V ( p ~ s ~ n v ~ ) = O , • - :

H e r e n i s p o r o s i t y a n d s= i s t h e s a t u r a t i o n o f t h e ~ p h a s e ( ~

c o m p r e s s i b i l i t i e s a r e n e g l e c t e d , t h e n :

n - ~ ( s~ ) + V ( s ~ n v ~) = O , ~ = w , N

(11)

= n s ~ ) .

I f f l u id

12)

C o m p o s i t i o n a l e q u a t i o n s

F o r t h e i n t e r p h a s e t r a n s f e r o f m a s s (i .e . t h e f o r m a t i o n o f a d i ss o l v e d p l u m e

o r t h e t r a n s p o r t o f o r g a n i c v a p o r s ) , b a l a n c e e q u a t i o n s f or e a c h s p e ci e s a re

w r i t t e n . T h e s p e c i e s -b a l a n c e e q u a t i o n s a r e o b t a i n e d b y s u m m i n g e q . 8 o v e r al l

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1 3 2 J .W . M E R C E R A N D R . M. C O H E N

phases to yield (Abricla, 1983):

~ - a , s , w , N 13)

~ L t l _I

where const raint (9c) has been incorporated. For non react ive components, the

right-band side of eq. 13 is zero. The number of equations that are required

depends upon the number of species. If the soil matrix is rigid, the soil species

equa tion may be eliminated. Solving the governing eqs. 13 yields the distribu-

tion of each fluid and the composition of the fluids in space and time.

C o n s t i t u t i v e r e l a t i o n s

Assuming it is valid, Dar cy s law may be substit uted into a system of mass-

balance equations (such as eq. 11 or eq. 13) to yield the equations govern ing the

multiphase flow of fluids in a porous medium. As an example, consider NAPL

flow in a rigid matrix (Faust et al., 1989):

n - ~ s ~ o ~) - V- O5 (VP~ - o ~ g ) = 0 (14)

P~

Eq. 14 is formulated in the unknowns P~, which are continuous in space.

Porosity n and intrinsic permeability k are assumed known properties of the

matrix. Generally, viscosity (p~), a weak function of pressure, is assumed

constant. Relative permeability is generally considered a function of

saturation. For the incompressible fluid case, p~ is a constant. In general,

however, density will depend on the fluid pressure, P~. Thus, fluid density may

be expanded in terms of fluid pressure by incorpor ating fl~, the compressibilit y

of the ~ phase. For slightly compressible fluids, fl~ is essentially constant.

Saturation is considered a known function of capillary pressure, which can

exhibit hysteretic behavior.

The compositional model equations (13) have additional considerations due

to the dependence of properties on composition. For example, viscosity and

density may be functions of composition. There are also two additional

equa tion terms, those a ccoun ting for non-advective flux and chemical reaction~

which must be evalua ted. The non-advec tive flux term is commonly assumed to

have a Fickian form. This is the form used in most existing solute transport

models and accounts for both molecular diffusion and hydrodynamic

dispersion effects (Bear, 1979). There is no general func tiona l form that may be

specified for the reaction terms that appear in eqs. 13. Simple decay rates can

be directly substituted in eqs. 13 or a system of chemical reaction equilibrium

or rate equations can be solved to determine these term~

Finally, for a composi tional model, expressions m e needed to re late mass

fractions of a given species within all the phases. Generally, the assumption of

local equi librium is used to develop these i z~lations (e.g., for sorption, see

Valocchi, 1985). Local equ ilibrium implies that witLin some rela tive ly short

time scale, contiguous phases reach a thermodynamic equilibrium. Thus, the

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REVIEW OF IMMISCI BLE FLUIDS IN THE SUBSURF CE

33

m a s s f r a c t i o n o f a s p e c ie s in o n e p h a s e c a n b e r e l a t e d t o t h e m a s s f r a c t i o n s o f

t h e s a m e s p e c i es i n o t h e r p h a s e s v i a p a r t i t i o n e x p r e s s i o n s o f t h e f o l l o w i n g fo r m

(Abriola , 1988) :

o~ = K~ ~oJ~ (15)

w h e r e K ~ ~ i s c a l l e d t h e p a r t i t i o n c o e ff ic i en t o f s p e c i e s i b e t w e e n t h e ~ a n d fl

p h a s e s . I n g e n e r a l , p a r t i t i o n c o e ff i ci e n ts a r e f u n c t i o n s o f p h a s e c o m p o s i t i o n s

a n d p r e s s u r e s . T h e s e c o e f fi c ie n t s m a y b e d e t e r m i n e d f r o m H e n r y s l a w

c o n s t a n t s ( A p p e n d i x A ) a n d s o l u b i l i t y r e l a t i o n s . S e e , f o r e x a m p l e , B a e h r ( 19 87 ),

B a e h r a n d C o r a p c i o g l u ( 1 9 8 7 ) , C o r a p c i o g l u a n d B a e h r ( 1 9 8 7 ) .

A l t h o u g h N A P L m o d e ls c an , i n t h e o r y , s i m u l a t e a v a r i e t y o f p r o b le m s , t h e

d a t a r e q u i r e d f o r a p p l i c a ti o n s a r e g e n e r a l l y la c k i n g . H e t e r o g e n e i t y i n f lu e n c es

N A P L f l ow a n d s o l u t e t r a n s p o r t ; h o w e v e r , s p a t i a l v a r i a b i l i t y o f p o r e si ze

( e f f e c t i n g d i s p l a c e m e n t p r e s s u r e s ) a n d i n t r i n s i c p e r m e a b i l i t y a r e r a r e l y s u f f i -

c i e n t l y d e f in e d t o p e r m i t a c c u r a t e p r e d i c t io n . A s p o i n t e d o u t i n t h e s e c t i o n s o n

s o l u b i l i t y a n d v o l a t i l i z a t i o n , t h e t h e o r e t i c a l d e s c r i p t i o n o f m a s s t r a n s f e r i n

p o r o u s m e d i a h a s n o t b e e n a d e q u a t e l y d e v e l o p ed , a n d t h e r e f o r e a d d s a d d i t io n a l

u n c e r t a i n t y t o a n y m o d e l i n g a p p r o a c h . C u r r e n t l y , e x i s t in g N A P L m o d e l s c a n

b e u se d p r i m a r i l y i n a c o n c e p t u a l i z a t i o n m o d e .

FIELD TECH NIQUES USED TO CHARACTERIZENAPL FLOW

A s i m p o r t a n t a s t h e a b i l i t y t o t h e o r e t i c a l l y d e s c r ib e N A P L f lo w is t h e a b i l it y

t o c h a r a c t e r i z e i t i n t h e f i e l d . N A P L s o u r c e l o c a t i o n s a n d v o l u m e s a r e o f t e n

p u u r l y d e f i n e d . M e d i u m h e t e r o g e n e i t i e s p r o m o t e c o m p l e x a n d u n p r e d i c t a b l e

s p r e a d i n g o f N A P L . P r e f e r e n t i a l N A P L m i g r a t i o n i n t h e s a t u r a t e d z o n e o c c u rs

t h r o u g h p e r m e a b l e p a t h w a y s ( s oi l a n d r o c k f r a c t u r e s , r o o t h o l e s, s a n d y l a y e rs ,

e t c. ) ~ u e t o c a p i l l a r y p r e s s u r e a ri d h y d r a u l i c e f fe c ts . G u i d a n c e f o r c o n d u c t i n g

f ie ld i nv e s t i ga t ion s a t N A P L ~ i t es is g ive n by the A .P . I. (1980, 1989 ) a nd

Vi l laume (1985) .

C h a r a c t e r i z i n g t h e p r e s e n c e, c o m p o s i ti o n a n d p r o p e r t ie s o f m o b i l e a n d

r e s i d u a l N A P L i s f u n d a m e n t a l t o d e t e r m i n i n g t h e n a t u r e , e x t e n t a n d r a t e o f

c h e m i c a l m i g r a t i o n d u r i n g a s i t e a s s es s m e n t . D a t a c o l l e ct i o n t y p i c a ll y o c c u r s

i n p h a s e s u s i n g m a n y o f t h e s a m e t e c h n i q u e s e m p l o y e d a t c o n t a m i n a t i o n s it e s

w h e r e N A P L i s n o t p r es e n t . H o w e v e r , s p ec i a l p r e c a u t i o n s a n d c o n s i d e r a t i o n s

m u s t b e re c o g n i z e d w h e r e N A P L i s p r e s en t . F i e l d t e c h n i q u e s u s e d t o c h a r ac -

t e r i z e N A P L t r a n s p o r t d i s c u s s e d h e r e i n c l u d e s o i l g a s a n a l y s i s , s o i l a n d r o c k

s a m p l i n g , w e l l m e a s u r e m e n t s , a n d f l u i d s a m p l i n g .

o i l gas ana l ys i s

F o r v o l a t i l e N A P L s , s oi l g a s a n a l y s i s m a y b e a s c r e e n i n g t o o l f o r l o c a ti n g

p o t e n t i a l s o u r c e s o f c o n t a m i n a t i o n a n d f o r s i t in g m o n i t o r i n g w e l ls . V o l a ti l e

o r g a n i c c o m p o u n d s ( V O C s ) i n c l u d e f u e l c h e m i c a l s s u c h a s b e n z e n e , t o l u e n e ,

e t h y l b e n z e n e a n d x y l e n e s , a n d s o l v e n t s s u c h a s t r i c h l o r o e t h e n e , t e t r a -

7/24/2019 Mercer 1990



c h l o r o e t he n e , d i c h l o r oe t h e n e , t r i c h l or o e t h a n e, a n d F r e o n ® s . T h e t r a n s f e r o f

V O C s f r o m g r o u n d w a t e r o r N A P L v i a v ol a ti li za t io n h a s a l r e ad y b e e n

d i sc u ss e d. A l t h o u g h t h i s p r o ce s s m a y b e e n h a n c e d b y w a t e r -t a bl e f lu c t ua t io n s

( L a p p a l a a n d T h o m p s o n , 1 98 3) , e q u il i b ri u m v a p o r c o n c e n t r a t i o n s a r e p r o b a b l y

n e v e r a t t ai n e d i n t h e f i el d d u e t o r a pi d v a p o r d i f fu s i on a t t h e w a t e r t a b l e

( M a r r i n a n d T h o m p s o n , 1 98 4) .

O n c e V O C s e n te r t h e s o i l g a s , t h e y d i f f u s e i n r e s p o n s e t o t he c h e m i c a l

c o n c e n t r a t i o n g r a d i e n t . V o l at i li z a ti o n o f c h e m i c a l s w i t h h i g h m o l e c u l a r

w e i g h t s a n d s a t ur a t ed v a p o r c o n c e n t r a t i o n s (T a b le 5) c a n i n d u c e d e n si t y -

d r i v e n g a s t r a n s p o r t i n m e d i a w i t h h i g h g a s p h a s e p e r m e a b i l i t y (i.e. g > 1-

1 0 - n m 2 i n u n i f o r m m e d i a ) ( F a l t a e t al., 1 9 8 9; S l e e p a n d S y k e s , 1 9 8 9; M e n d o z a

a n d F r i n d , 1 9 90 ; M e n d o z a a n d M c A l a r y , 1 99 0) . D e n si t y -d r i ve n g a s f l o w c a u s e s

V O C s t o s i nk a n d m o v e o u t w a r d a b o v e t h e w a t e r ta bl e. A l t h o u g h a d ve c ti v e

p r o c e s s es d u e t o d e n s i t y e ff e ct s r h i g h v a p o r p r e s s u r e g r a d i e n ts m a y i n f l u e n ce

V O C m i g ra t i on , g a s e o u s d i f fu s i on is c o n s i d e r e d t h e p r e d o m i n a n t t r a ns p o rt

m e c h a n i s m a t m o s t s it es ( M a r r i n a n d K e r fo o t , 1 98 8) . A t s t e a d y s ta te , v e rt i ca l

solut e flu x is propo rtion al to the air-filled oro sity, the V O C diff usion coeffi-

c ie nt , a n d t h e g a s - p h a s e c o n c e n t r a t i o n g r a d i e n t.

S u b s u r f a c e g e o l o g i c h e t e r o g en e i t i e s , s o i l p o r o s i t y , m o i s t u r e c o n d i t i o ns ,

V O C c o n c e n t r a t i o n s i n g r o u n d w a t e r a n d s o r pt i o n e q ui l ib r ia c a n s i gn if ic an tl y

a ff ec t V O C g r a di e n ts i n s o il g a s ( M a r r i n a n d T h o m p s o n , 1 9 87 ) . F o r e x a m p l e ,

f al se n e g a t i v e i n t er p r et a t io n s m a y r e s ul t f r o m t h e p r e s e n c e o f v a p o r b a rr i e rs

( p e r c h e d g r o u n d w a t e r , c l a y l e n s e s , o r i r r i g a t e d s oi l s) b e l o w t h e g a s p r o b e

i n ta k e. T h u s , s a m p l e l o ca t i on s a n d d e p t h s i n f l u e n c e t h e m e a s u r e d v a p o r c o n -

centrations.

S e v e r a l c h e m i c a l c h ar a ct e ri s ti c s i n d i c a t e w h e t h e r a m e a s u r a b l e v a p o r c o n -

c e n t r a t io n c a n b e d e t e c t e d ( D e v i t t e t a l ., 1 9 8 7 ; M a r r i n , 1 9 8 8 ) . I d e al l y,

c o m p o u n d s s u c h a s V O C s m o n i t o r e d u s i n g s o il g a s a n a ly s i s w il l: (1) b e s u b je c t

to little etardation in gr ou nd wa te r; (2) partition significantly fr om wa te r to

s o il g a s ( H e n r y s l a w c , ) n st a n t > 0 . 0 0 0 5 a t m . m 3 m o l - 1 ) ; ( 3) h a v e s uf fi ci en t

v a p o r p r e s s u r e t o d i ff u se ~ ig ni fi ca nt ly p w a r d i n t h e v a d o s e z o n e ( 1. 0 m m H g

@ 20 °C); (4) be persistent; a nd (5) be susceptible to detection a nd quantitation

by affordable analytical techniques.

G r a b a n d p a s si v e s a m p l i n g t e c h n i q u e s a r e u s e d t o c o ll e c t s o il g a s . D u r i n g

s t at ic g r a b s a m p l i n g, s a m p l e s a r e c o l l e c t e d f r o m a q u i e s c e n t s o il g a s s a m p l e . I n

d y n a m i c g r a b s a m p l i n g , s a m p l e s a r e co l l ec t ed f r o m m o v i n g s o il g a s a s it is

p u m p e d t h r o u g h a h o l l o w p r o be . G r a b s a m p l e s c a n b e a n a l y z e d o n s it e d u r i n g

t h e s a m p l i n g e p i so d e o r s h i p p e d t o a l a b o r at o r y. P a s s i v e s a m p l i n g p r ov i d es a n

i n t eg r a te d m e a s u r e o f V O C c o n c e n t r a t i o n s o v e r t i m e . C h a r c o a l s o r be n t s t o

t r a p s o l ut e s t h a t d i ff u se t h r o u g h t h e so il g a s e s a r e b u r i e d u p t o o n e m o n t h a n d

t h e n r e t r i e v e d f o r a n a l y s i s . S o i l g a s s a m p l i n g i s d i s c u s s ed i n m o r e d e t a i l i n

De vi tt et al. (1987).

A l t h o u g h s oi l g a s a n al y si s h as b e e n u s e d s uc c es s fu l ly a t m a n y c o n t a m i n a -

t i o n si te s, it c a n p r o v i d e m i s l e a d i n g r e s u l t s i f s u b s u r f a c e c o n d i t i on s a r e n o t

u n d e r s t o o d a d eq u a te l y. T h u s , i n f o r m a t i o n r e g a r d i n g t h e l o c a ti o n, e x t en t a n d

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REVIEW OF IMMISCIB LE FLUIDS IN THE SUBSURF CE

35

composition of VOC s in soil and g roundw ater generate d by a soil gas survey

must be confirmed by analysis of soil and fluid samples taken at depth.

r i l l i ng i nves t i ga t i ons

Borings and monitoring wells are the primary means for evaluating the

distribution of subsurface NAPL. Selection of drilling locations and depths

should be based on available information regarding site hydrogeologic and

contaminant conditions. Care must be exercised to hmit the potential for

fugitive vapor and parti culat e emissions, damage to subsurface utilities, small

flash fires or explosions, and cross-con tamina tion. In part icula r, it is importan t

to p revent d ownw ard migration of mobile perched Db~:APL or NAPL-contami-

nated soils th at may resul t from drilling thro ugh a ba~:rier layer. One approach

is to work in from the edge of the plume to characteri.~e the site prio r to drilling

at the source of contaminants.

Drilling a t NAPL-con taminated sites occurs mo~;t frequen tly using hollow-

stem augers with split-spoon samplers in soils and wireline-coring or rotary

drill rigs in bedrock. In addition to logging soil characteristics such as grain

size distribution, consistency and moisture state, it is important to note visual

observations and olfactory evidence (where consisten t with site safety plans)

of NAPL or other chemicals. Inner sections of cores should be examined to

prevent misinterpreting core surfaces that may be contaminated during the

sampling process. The presence and density of NAPL relative to wat er can be

easily identified in the field by shak ing free liquid or NAPL-contaminated soil

in a jar with water and watching for phase separation. Free liquids or NAPL

extracted using centrifugation or other methods can be tested for fluid

properties such as density, interfacial tension and viscosity. Physical or

chemical extraction processes can also be used to estimate NAPL saturation,

and analyses can be made to determine NAPL composition.

Monitoring well design considerations for DNAPL and LNAPL sites are

discussed by V illaume (1985} and the A.P.I. (1989), respectively. Well screens in

LNAPL-mon itoring wells should be long enough to ensure t hat the entire free

LNAPL layer will be within the screened interva l during seasonal water-table

fluctuations. Misleading LNAPL measu rements will result from a well that is

screened thro ugh perched LNAPL in the vadose zone to a deeper water table.

At DNAPL sites, a well that is fully screened from above the DNAPL-water

inmrface to the cop of the underlying stratigraphic barrier is usually appro-

priate. Sandpucks should be coarser than surrounding media to ensure the

movement of NAPL to the well. NAPL can chemically attack well casing,

screen and pumps. The compatibility of NAPL with well materials including

grout and Bentonit e @ seals should be considered.

NAPL

t h i cknes s and e l eva t i on measuremen t s

If NAPL is at residual satu rat ion or under negative pressure, it will not flow

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into a monitoring well. Under such circumstances, soil or core samples

showing NAPL or high aqueous concentrations may be the only indication, of

NAPL presence. If the NAPL is mobile, then product thickness, elevation and

composition need to be determined. Therefore, fluid levels must be measured

and NAPL samples obtained.

Measurements of NAPL elevation and thickness in monitoring wells are

typically made using hydrocarbon-detection paste on a steel tape, an interface

probe, or a transparen t, bottom-loading bailer. For all methods, care should be

exercised to minimize disturbance of the fluid column during static level

measurements. Hydrocarbon-detection paste applied to a measuring tape

reveals the top of floating hydrocarbons as a wet line and the top of water as

a dist inct color change. This method is accura te to ~ ___0.01 ft. (~ ± 3 ram), but

cannot be used to measure DNAPL.

Interface probes employ optical and conduct ivity sensors to distinguish the

air-fluid and NAPL -wa ter interfaces, respectively. These probes are expensive

(typically US $1500-3000) and can be used to measure both LNAPL and DNAPL

to within 0.01-0.10ft. (~ 0.3-3cm) but may produce spurious results in the

presence of conductive NAPL, emulsified NAPL, or viscous NAPL that coats

the sensors. Standard electric-line water-level probes can detect interfaces

between non-conductive NAPL and water, but may fail to identify the interface

between LNAPL and air.

NAPL can also be measured and sampled using a transparent bottom-load-

ing bailer. For LNAPL, the bailer should be long enough so that its top is in

air when its check valve is in water. NAPL rise due to displacement by the

bailer may result in slight overestimat ion of product thickness in the well. This

influence can be minimized by allowing time for well equilibration after

lowering the bailer.

Various other means are available to estimate NAPL thickness and

elevation in wells. For example, measurements can be made by taking small

fluid samples from specific levels using mechanical discrete-depth (Kemmerer

type) samplers or by measu ring DNAPL that coats a weighted stri ng following

its retr ieva l from a well. The cost of measu ring device decontaminat ion should

be cgnsidered when selecting a measurement method, part icu lar ly given uncer-

tainties involved in interpreting NAPL thickness and elevation data.

n t e r p r e t in g w e l l m e a s u r e m e n t s

LNAPL elevations measured in wells that are screened across the air-fluid

interface are generally assumed to indicate the surface of the LNAPL pool

or panca ke in the surrounding formation, although this may not always be

the case (Hampton, 1988). Water-tab le elevations can be calculat ed by adding

the product of the density and thickness of LNAPL measured in a well to the

LNAPL-water interface elevation.

Measured LNAPL thickness in wells (hw) typically exceeds corresponding

LNAPL-saturated formation thickness (h0 by a factor between 2 and 10. This

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A REVIEW OF IMMISCIBLE FLUIDS IN THE SUBSURFACE

37

o c c u r s b e c a u s e L N A P L p e r c h e d a b o v e t h e w a t e r - s a t u r a t e d z o n e w il l flo w i n to

t h e w e l l a n d d e p r e s s t h e w e l l w a t e r l e v e l . G e n e r a l l y , t h e

h w / h f

r a t i o i n c r e a s e s

w i t h d e c r e a s i n g f o r m a t i o n g r a i n s i z e , i n c r e a s i n g c a p i l l a r y f r i n g e h e i g h t , a n d

i n c r e a s i n g L N A P L d e n s i t y .

M u c h t h e o r e t i c a l a n d e x p e r i m e n t a l r e s e a r c h h a s b e e n c o n d u c t e d t o q u a n -

t i t a t i v e l y r e l a t e hw t o h f v a n D a m , 1 9 6 7 ; Z i l li o x a n d M u n t z e r , 1 9 7 5 ; d e

Pa s t ro v i c h e t a l . , 1 9 7 9 ; B l a k e an d H a l l , 1 9 8 4 ; R .A . H a l l e t a l ., 1 9 8 4 ; Sch i eg g ,

1 98 5; A b d u l e t a l ., 1 9 89 ; F i ed l e r , 1 9 89 ; F a r r e t a l . , 1 9 9 0 ; K e m b l o w s k i a n d Ch i an g ,

1 99 0; L e n h a r d a n d P a r k e r , 1 99 0). A r e l i a b l e p r e d i c t i v e r e l a t i o n s h i p w o u l d

f a c i li t a te a n a l y s i s o f L N A P L p l u m e g e o m e t r y a n d r e m e d i a l o p t io n s . H a m p t o n

1 98 8) a n d H a m p t o n a n d M i l l e r 1 98 8) e x a m i n e d th i s p r o b l e m b y c o n d u c t i n g

l a b o r a t o r y e x p e r i m e n t s a n d c o m p a r i n g r e s u l t s t o v a r i o u s e q u a t i o n s p r o p o s e d

f o r e s t i m a t i n g h f f r o m hw . T h e y c o n c l u d e t h a t a l l t h e e q u a t i o n s a n d m e t h o d s

i n v e s t i g a t e d l a c k p r e d i c t i v e c a p a b i l i t i e s , b u t s u g g e s t u s i n g a s i m p l e e q u a t i o n

p ro p o s ed b y d e Pas t r o v i ch e t a l . 1 97 9) :

h ~ / h f ~ P n / P w -

Pn)

f o r m a k i n g c r u d e e s t im a t e s o f L N A P L - s a t u r a t e d f o r m a t i o n t h i c k n e s s .

M a n y f a c t o r s c o n f o u n d d e v e l o p m e n t o f a r e l i a b l e p r e d i c ti v e e q u a t i o n

b e t w een h w an d h r. Ca p i l l a ry f r i n g e h e i g h t s a r e s o i l- s p ec if i c an d h y s t e r e t i c w i t h

f l u i d - l e v e l f l u c t u a t i o n s . I n c r e a s i n g t h i c k n e s s e s o f L N A P L c a n c o l l a p s e t h e

w a t e r c a p i l l a r y f r in g e , c a u s i n g t h e h w / h f r a t i o t o d e c r e a s e u n p r e d i c ta b l y . T h e

t h i c k n e s s o f L N A P L - s a t u r a t e d s o i l i s a m b i g u o u s b e c a u s e t h i s l a y e r i s n o t

c o m p l e t e l y s a t u r a t e d w i t h p r o d u c t H a m p t o n a n d M i l l e r , 1 9 88 ). E n t r a p v a e n t

,1 . ~ w at er

- o l . . 1 - . -

a n d m o b i l i z a t i o n o f L N A P L a s ~ ,,= ~ , ~ r ls e ~ a n d f a ll s, r e sp e c ti v el y ~ a n d

t r a n s i e n t p r e f e r e n t i a l v e r t i c a l f lo w o f l iq u i d s t h r o u g h w e l l s i n t i g h t f o r m a t i o n s

m a y h e l p e x p l a i n t h e i n v e r s e r e l a t i o n s h i p b e t w e e n h ~ a n d c h a n g e s i n t h e

L N A P L - w a t e r i n t e rf a c e e l e v a t i o n K e m b l o w s k i a n d C h i a n g , 1 9 90 ). W e l l c on -

s t r u c t i o n d e t a i l s c a n a l s o a f f e c t t h e h w / h f r a t i o , p a r t i c u l a r l y d u r i n g p e r i o d s o f

f l u i d l e v e l c h a n g e . A s a r e s u l t , m o b i l e L N A P L v o l u m e s c a n n o t b e r e l i a b l y

e s t i m a t e d b a s e d o n w e l l t h i c k n e s s m e a s u r e m e n t s o r b a i l i n g t e s t s A . P .I ., :[980;

H am p t o n an d M i l l e r , 1 9 8 8 ; A b d u l e t a l ., 1 9 8 9 ; T e s t a an d Pac zk o w s k i , 1 98 9).

K e m b l o w s k i a n d C h i a n g 1 99 0) s u g g e s t t h a t a b o r e h o l e g e o p h y s i c a l m e t h o d ,

s u c h a s a d i e l e c t r ic l o g , m a y b e t h e m o s t p r o m i s i n g a p p r o a c h t o d e t e r m i n i n g t h e

d i s t r ib u t i o n o f L N A P L b e l o w g r o u n d .

L e s s r e s e a r c h h a s f o c us e d o n i n t e r p r e t i n g D N A P L t h i c k n e s s a n d e l e v a t io n

m e a s u r e m e n t s in w e ll s. D N A P L t h i c k n e s s a n d e l e v a t i o n m e a s u r e m e n t s c a n b e

m a d e u s i n g w e l ls t h a t a r e s c r e e n e d d o w n w a r d f ro m a b o ve t h e w a t e r - D N A P L

i n t e rf a c e a n d c o m p l e t e d t o t h e t o p o f t h e s t r a t i g r a p h i c b a r r i e r u n d e r t h e

D N A P L . T h e e l e v a ti u ~ , o f D N A P L i n a w e ll , h o w e v e r , m a y e x c e e d t h a t i n t h e

f o r m a t i o n b y a l e n g t h e q u i v a l e n t t o t h e c a p i l l a r y p r e s s u r e e x e r t e d a t t h e t o p o f

t h e D N A P L p o o l T .V . A d a m s a n d H a m p t o n , 1 9 90 ). I f t h e w e l l s c r e e n o r c a s i n g

e x t e n d s in t o t h e b a r r i e r l a y e r , t h e m e a s u r e d D N A P L t h i c k n e s s w i ll e xc e e d t h a t

i n t h e f o r m a t i o n b y t h e l e n g t h o f w e l l b e lo w t h e b a r r i e r l a y e r s u r fa c e . B o t h t h e

s u r fa c e a n d t h i c k n e s s o f m e a s u r e d D N A P L a r e l i k e l y to b e e r r o n e o u s i n a w e ll

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138 J W MERCER AND R M COHEN

that connects a DNAPL pool above a stratigraphic barrier to a deeper

permeable formation. As noted previously (p. 135), such a well will cause

DNAPL to short-circuit the barrie r layer and con tamina te the lower permeable

formation. As a result of these factors, DNAPL presence in wells should be

evaluated in conjunction with evidence of NAPL presence obtained during

drilling.

S a m p l i n g

NAPL

f rom we l l s

NAPL samples should be taken after measuring the static elevations and

thicknesses of immiscible fluids in a well. A bottom-loading bailer or

mechanical discrete-depth sampler is often adequate for collecting NAPL

samples. These and o ther methods can be used to selectively sample NAPL from

multiple specific depths within a well. Well purging is generally not recom-

mended because it will cause mixing of the multiphase fluids (Villaume, 1985).

However , pumping or bail ing tests can be used to examinee t_be mobil ity of

NAPL in the formation and the feasibility of NAPL recovery by pumping.

Advantages and disadvantages of many common sampling methods are

summarized by the A.P.I. (1989). The cost to decon tamina te or replace NAPL-

contaminated equipment is usually a major consideration in selecting a

sampling method.

REMEDIAT ION OF NAPL CONTAMINATION

Before methods for the in situ removal of NAPL from the subsurface can be

successfully implemented, the location and nat ure of NAPL must be known. If

NAPL contami nation is shallow, then excava tion may be an economical alter-

native for remediation. If the NAPL is immobile, hydraulic containment is

possible while purging and treating contaminated groundwater from the

dissolved plume. Purge wells and collector trenches effectively remove mobile

LNAPL; recovery wells remove mobile DNAPL less effectively. Partial

residual NAPL recovery may be possible only by using methods such as mobili-

zation, enhanced oil recovery techniques, or vacuum extraction. These NAPL

remediation methods are discussed below.

roduct recovery

Experience in the recovery of mobile NAPL is currently limited and pertains

almost exclusively to the recovery of lenses of LNAPL s, most notably

petroleum products floating on the surface of the wa ter table. Blake and Lewis

(1983) provide a summary of the special considerat ions ar, d procedures

involved in the recovery of LNAPL s. Oth er references describing recovery

techniques include de Pastrovich et al. (1979), A.P.I. (1980, 1989), and Fussell

et al. (1981). Examples of DNAPL recovery are prc~cided by Villaume et al.

(1983a, b) for coal tar from gravel, and by Ferry et al. (1986) for DNAPL from

fractured rock.

7/24/2019 Mercer 1990



39

W h e r e t h e w a t e r t a b l e is s h a l lo w , o p e n o r p e r m e a b l e t r e n c h e s e q u i p p ed w i t h

p u m p s c a n b e u s e d . T h e t r e n c h c a n b e e q u i p p e d w i t h p u m p s d e s i g n e d o n l y to

s k i m t h e L N A P L a s it c o l l e c ts i n th e t r e n c h . A n a l t e r n a t i v e a p p r o a c h i s t o

p u m p g r o u n d w a t e r f ro m t h e t r e n c h t o re d u c e w a t e r l e v el s a n d c r e a t e g r a d i e n ts

t h a t w i l l b r i n g t h e L N A P L i n t o t h e t r e n c h a t g r e a t e r r a t e s an d a g a i n s t n a t u r a l

g r a d i e n t s .

W e l l s a l s o m a y b e u s e d t o c o l l e c t m o b i l e L N A P L . T h e g e n e r a l a p p r o a c h i s

t o u s e t h e g r a d i e n t s e s t a b l i s h e d i n t h e c o n e o f d e p r e s s i o n s u r r o u n d i n g a

p u m p i n g w e l l t o c o l l e c t t h e L N A P L . A s t h e L N A P L c o l l e c t s i n t h e c o n e o f

d e p r e s s io n , i t c a n b e r e c o v e r e d w i t h a p u m p . T h i s m a y b e a c c o m p l i s h e d u s i n g

o n e w e l l to e s t a b l i s h t h e c o n e o f d e p r e s s i o n a n d a s ec o n d , n e a r b y w e l l t o

r e c o v e r t h e L N A P L a s i t c o l l e c t s . A l t e r n a t i v e l y , a s i n g l e w e l l c a n b e u s e d t o

b o t h e s t a b l i s h t h e c o n e o f d e p r e s s i o n a n d r e c o v e r t h e L N A P L .

A s i n g le p u m p c a n b e u s e d to c o l le c t t h e g r o u n d w a t e r a n d L N A P L . H o w e v e r ,

t h e p u m p m u s t b e p o s i t io n e d n e a r t h e w a t e r - L N A P L i n t e rf a c e to e n s u r e th a t

b o t h g r o u n d w a t e r a n d L N A P L a r e d r a w n i n t o t h e w e l l. A fl o a t i s u s e d t o

r e g u l a t e a n d m a i n t a i n t h e p r o p e r d r a w d o w n i n th e w e ll . A l t e r n a t iv e l y ,

s e p a r a t e p u m p s c a n b e u s ed t o d r a w d o w n t h e w a t e r t a b le a n d c o l le c t t h e

L N A P L . T h e s e c a n b e l o c a t e d i n t h e s a m e o r s e p a r a t e w e l l s . T h e s e p u m p i n g

a r r a n g e m e n t s a l so r e q u i r e sp e c i a l a u t o m a t i c c o n t r o l s t o r e g u l a t e t h e p u m p s

a n d d r a w d o w n s .

S c r e e n s m u s t b e p o s i t i o n e d t o a l l o w L N A P L i n t a k e . C o n s e q u e n t l y , t h e

s c r e en m u s t e x t e n d a b o v e th e l e v e l t o w h i c h g r o u n d w a t e r w i ll b e d r a w n d o w n

a n d i n t o t h e a r e a i n w h i c h L N A P L w i ll c o l le c t . O t h e r w i s e , L N A P L w i l l c o l le c t

a b o v e t h e s c r e e n a n d n o t b e a b l e t o e n t e r t h e w e l l . T o e n h a n c e r e c o v e r y ,

i n j e c t io n w e l l s m a y b e u s e d i n c o m b i n a t i o n w i t h t h e r e c o v e r y w e l l s to f lu s h

c o n t a m i n a n t s f r om t h e v a d o s e zo n e a n d i n c r e a se t h e h y d r a u l i c g r a d i e n t t o w a r d

t h e r e c o v e r y w e l ls .

L a r g e d r a w d o w n s w i ll in c r e a s e t h e flo w r a t e o f L N A P L t o w a r d t h e w e l l, t h u s

i n c r e a s i n g t h e s p e e d o f r e c o v e r y . U n f o r t u n a t e l y , t h e y a l so w i ll c r e a t e l a r g e r

d e w a t e r e d a r e a s i n t o w h i c h t h e L N A P L w i l l c o l l ec t . T h e s e a r e a s w i ll h a v e b e e n

p r e v i o u s l y u n c o n t a m i n a t e d b y L N A P L . S u b s t a n t i a l a m o u n t s o f L N A P L w i ll b e

r e t a i n e d i n t h e a q u i f e r m a t r i x a t r e si d u a l s a t u r a t i o n , e v e n a f t e r w a t e r l e ve l s

a r e a l l o w e d t o r e c o v e r ( B l a k e a n d L e w i s , 1 9 8 3 ) . T h u s , r e c o v e r y s y s t e m s s h o u l d

m a i n t a i n r e c o v e r y w h i l e m i n i m i z i n g d r a w d o w n .

P r o c e d u r e s f o r m o b i le D N A P L c l e a n u p a r e e x p e r i m e n t a l a n d a r e

d o c u m e n t e d p o o r l y c o m p a r e d t o p r o c e d u r e s f o r L N A P L c l e a n u p . R e m o v i n g

D N A P L f r o m t h e s u b s u r f a c e i s e x t r e m e l y d if fi cu lt . P r o b l e m s a s s o c i a t e d w i t h

D N A P L r e c o v e r y a r e d e s c ri b e d b y F e e n s t r a a n d C h e r r y (19 88) a n d M a c k a y a n d

C h e r r y ( 1 9 8 9 ) .

T h e p r o p e r t i e s t h a t c o n t r o l D N A P L m o v e m e n t a r e l a r g el y r es p o n s ib l e fo r

t h e d i ff ic u lt ie s a s so c i a t e d w i t h t h e i r c l e a n u p . D N A P L m o v e m e n t i s i n f lu e n c e d

n o t o n l y b y p r e s s u r e g r a d i e n t s b u t a l s o b y g r a v i ty . T h u s , t o m o v e D N A P L s

t o w a r d a w e l l , g r a d i e n t s m u s t b e c r e a t e d t h a t w i l l o v e r c o m e g r a v i t y . I n ca s e s

w h e r e D N A P L s a r e m i g r a t i n g a l o n g a n i n c l i n e d c o n f i ni n g b e d, t h e r e q u i r ed

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140


10

J o 1

O Ol

0.001

lO-e

1 I I I

= 1 3 x 10 3

0 = 1 0 ( d y n e c m 1 )

- \

N ~= 1 0 5 ~ ~ k

I I

1 0 7 l O S l O S 1 0 - 4 1 0 - 3 1 0 -2

I 0 I

I G R A V E L

C L E A N S A N [ }

s . Y s o - - k c m 2 1

F i g . 5 . H y d r a u l i c g r a d i e n t , J , n e c e s s a r y t o i n i t i a t e b l o b m o b i l i z a t i o n a t N ~ ) i n s o i ls o f v a r i o u s

p e r m e a b i l i t i e s , f o r h y d r o c a r b o n s o f v a r i o u s i n t e r f a c i a l t e n s i o n s , a . T h e u p p e r c u r v e r e p r e s e n t s t h e

g r a d i e n t n e c e s s a r y f o r c o m p l e t e r e m o v a l o f a l l h y d r o c a r b o n /V ~* ) , w i t h a = 1 0 d y n c m ~. N o t e t h a t

1 0 - s c m 2 = 1 d a r c y f r o m J .L . W i l s o n a n d Co n r a d , 1 98 4 ).

gradients may be large. For this reason, it is best to collect DNAPL s from low

points along the bedrock or at some downslope position where the pool is

migrating.

Mobile DNAPL may be recovered using the same basic techniques used in

other recove ry programs (e.g., Ferry et al., 1986). If DNAPL is located near the

surface, drains may be capable of recovering these contamin ants. Otherwise,

wells are required. In both cases, recovery requires pump intakes placed far

into the DNAPL to ensure that as much DNAPL as possible is collected.

Pumping rates should be used that discourage mixing and the formation of

emulsions at the interface between the DNAPL and groundwater. In most

cases, the groundwater above the pool of DNAPL will be contaminated and

also will require cleanup. This can be accomplished using separate wells or a

single well screen over the entire aquifer equipped with two pumps.

ob i l i z a t i o n

During mobilization, increased hydraulic gradients, resulting in increased

groundwater velocity, cause residual blobs of NAPL to move (J.L. Wilson and

Conrad, 1984). The capil lar y number, No, the ra tio of capil lary to viscous forces,

provides a measure of the propensity for NAPL trapping and mobilization. It

is defined as the product of intrinsic permeability, water density, gra vitat ional

7/24/2019 Mercer 1990


REVIEW OF IMMISCIB LE FLUIDS IN THE SUBSURF CE 4

acceleration constant and hydraulic gradient divided by the interfacial

tension. The critical value, N*, of the capillary number is defined as the value

at which motion of some of the NAPL blobs is initiated. Through experiments

(Chatzis e t al., 1983, 1988; Morrow et al., 1988), J.L. Wilson and Conrad (1984)

observed a stron g corre lation between displacement of residual NAPL and the

capillary number, when the hydraulic gradient was greater than that

producing the critical value of the capillary number. The hydraulic gradient

necessary to initiate blob mobilization for various permeabilities and in-

terfacial tensions is shown in Fig. 5. As may be seen, in very permeable media

(e.g., gravel or coarse sand), it is theoretically possible to obtain sufficient

hydr auli c grad ien ts to remove all NAPL blobs. In soils of medium permeability

(e.g., fine to medium sand), some of the re sidual can be hydraul ica lly removed.

In less permeable media, removal is not possible.

n h a n c e d o i l r e c o v e r y (EOR) m e t h o d s

Once mobile NAPL is collected, immobile NAPL remains at residual

saturation. Residual saturation decreases with decreasing capillary pressure;

therefore, enhanced oil recovery (EOR) methods have been suggested for

residual NAPL removal because they reduce the interfaciaI tension and/or

NAPL viscosity b flooding with one of the following: (1) hot wate r or steam,

(2) carbon dioxide, (3) surfactant, (4) alcohol, (5) alkaline, or (6) polymers. For

furt her info rmat ion on these topics, see Shah (1981). If EOR is used to mobilize

NAPL at residual saturation, NAPL flow must be controlled carefully.

Otherwise, previously clean portions of the subsurface may become contami-

nated during remediation.

Thermal methods include hot water flooding and steam flooding (Sale and

Piontek , 1988). These methods decrease residual NAPL by increasing contami-

nant solubility and achieving a more favorable mobility ratio. Contaminant

solubility is increased because water solubility of many organics increases

with increasi ng water temperature. NAPL viscosity decreases with increasing

tempe ratur e causing a corresponding decrease in the mobility ratio. Therefore,

NAPL recovery increascs with decrea s ng viscosity. Steam flooding also

resul ts in vapor iza tion of NAPL, which may allow recovery. A limit ation in the

use of thermal methods is that DNAPL may be converted to LNAPL. Therefore,

NAPL initia lly limited in extent may move throu gh previously uncontaminat-

ed portions of the subsurface. Costs may also be high due to heat loss and the

need to heat large volumes of subsurface materials. Hunt et al. (1988a, b)

examined steam floodinb theory and performed laboratory experiments to

eva lua te its suitabili ty for DNAPL recovery. I Io~ ~ver, the effectivenes~ of ~bis

technology in environmental applications is unknown.

Carbon dioxide flooding is an EOR technique th at also produces a decreased

mobility ratio. Carbon dioxide is injected under pressure and the viscosity of

the NAPL decreases as carbon dioxide dissolves into the NAPL. Because of the

high pressures, this method is applicable only at relatively large depths in a

7/24/2019 Mercer 1990



c o n f in e d l a y e r . T h e e f f e c t iv e n e s s o f t h i s t e c h n i q u e i n e n v i r o n m e n t a l a p p l ic a -

t i o n s i s n o t k n o w n (Sa l e an d P i o n t ek , 1 9 8 8 ) .

S o i l f l u s h in g w i t h s u r f a c t a n t s o l u t i o n s ( s u r f a c t a n t f l o o d in g ) t o e x t r a c t

h y d r o p h o b i c o r g a n i c c o n t a m i n a n t s a p p e a r s p r o m i s i n g ( S a l e a n d P i o n t e k , 1 98 8).

A q u e o u s s u r f a c t a n t s o l u t io n s a r e s u p e r i o r t o w a t e r a l o n e i n e x t r a c t i n g h y d r o -

p h o b i c c o n t a m i n a n t s ( E l l i s e t a l. , 19 85 ). S u r f a c t a n t a d d i t i o n i m p r o v e s b o t h t h e

d e t e r g e n c y o f w a t e r a n d t h e m o b i li ty o f N A P L i n w a t e r . I m p r o v i n g t h e

d e t e r g e n c y o f w a t e r c a u s e s p r e f e r e n t i a l w e t t i n g , i n c r e a s e d N A P L s o l u b il iz a -

t i o n , a n d e n h a n c e d N A P L e m u l s i f i c a t i o n . A d d i n g s u r f a c t a n t s a l s o m a y

i n c r e a s e r e s i d u a l N A P L m o b i l it y in w a t e r b y lo w e r i n g t h e i n t e rf a c i a l t e n s i o n

b e t w e e n w a t e r a n d N A P L , w h i c h f a c il i ta t e s t h e d i s t o r ti o n o f s p h e r i c a l N A P L

d r o p l e t s a s t h e y p a s s t h r o u g h t h e m e d i a ( S a l a g e r e t a l. , 1 9 79 ) . E x a m p l e s o f

D N A P L a n d L N A P L s o i l w a s h i n g u s i n g s u r f a c t a n t s a r e g i v e n i n S a l e e t a l .

(1988) and Tuck e t a l . (1988) , respec t ive ly .

I n a l c o h o l f l o o di n g (T a b e r , 1 98 1) , a l c o h o l a n d N A P L d i s s o l v e e a c h o t h e r ,

t h e r e b y m o b i l iz i n g t h e r e s i d u a l . P r o b l e m s w i t h t h i s m e t h o d i n c l u d e c o s t, p h a s e -

b eh av i o r d i ff icu lt ie s , an d l ack o f f ie ld ex p e r i en ce ev e n i n E O R ( J .L . W i l s o n an d

Conrad , 1984) .

W h e n i n c o n t a c t w i t h c e r t a i n h y d r o c a r b o n m i x tu r e s , a l k a l i n e a g e n t s (e .g .,

s o d i u m c a r b o n a t e ) c a n r e a c t t o f o r m s u r f a c t a n t s v i a a s a p o n i f i c a t i o n r e a c t i o n

( S a le a n d P i o n t ek , 1 98 8) . T h e s e s u r f a c t a n t s a r e c r e a t e d a t t h e w a t e r - N A P L

i n t e r f a c e , e f f e c t i v e l y r e d u c i n g t h e i n t e r f a c i a l t e n s i o n . C o m b i n i n g a l k a l i n e

a g e n t s a n d s u r f a c t a n t s m a y b e a c o st - ef f e ct iv e w a y o f r e d u c i n g i n t e r f a c i a l

t e n s i o n a n d e n h a n c i n g N A P L r e c o v e r y ( S u r k a lo , 1 9 90 ) . A s i n s u r f a c t a n t

f l o o d i n g , d e c r e a s e d i n t e r f a c i a l t e n s i o n r e s u l t i n g f r o m a l k a l i n e a g e n t s i s n o t

l i k e l y t o b e e f f e c t i v e u n l e s s f a v o r a b l e m o b i l i t y r a t i o s a l s o a r e a c h i e v e d .

P o t e n t i a l p r o b le m s a s s o c i a te d w i th a l k a l i n e a g e n t s i n c l u d e p r e c i p i ta t i o n a n d

r e s u l t a n t a q u i f e r p lu g g i n g , d i s p e r sa l a n d e x p a n s i o n o f c l a y s , a n d l e a c h i n g o f

t r a c e m e t a l s .

A n o t h e r E O R m e t h o d t h a t m a y h a v e e n v i r o n m e n t a l a p p l i c a t io n s is p o l y m e r

f l o o d i n g . A d d i n g p o l y m e r i n c r e a s e s t h e e f f e c t i v e n e s s o f a w a t e r f l o o d b y

i n c r e a s i n g t h e v i s c o s i t y o f t h e f lo od , t h u s l o w e r i n g t h e m o b i l i t y r a t i o ( S a l e a n d

P i o n t e k , 1 98 8) . T h i s m e t h o d i s e x p e n s i v e a n d h a s n o t b e e n e v a l u a t e d e x t e n s i v e -

l y f o r u s e i n e n v i r o n m e n t a l a p p l i c a t i o n s .

A s i n d i c a t e d , E O R t e c h n i q u e s a r e l a r g e l y u n t e s t e d f o r e n v i r o n m e n t a l

p r o b l e m s . T h e r e a r e t e c h n i c a l p r o b l e m s , e s p e c i a l l y w h e n d e a l i n g w i t h s h a l l o w

w a t e r - t a b l e a q u i f e r s w h e r e i n j e c t i o n p r e s s u r e i s l i m i t e d b y a q u i f e r t h i c k n e s s .

L a b o r a t o r y s t u d i e s a n d s m a l l - sc a l e f ie ld e x p e r i m e n t s a r e l i k e l y t o y i e l d o v e r -

o p t i m i s t i c e x p e c t a t i o n s ( M a c k a y a n d C h e r r y , 1 9 89 ). I n t h e f ie ld , u n e x p e c t e d

s p a t ia l v a r i a b i l it y i n p o r o u s m e d i u m p a r a m e t e r s c a n d e t r i m e n t a l l y a f fe c t t h e

o u t c o m e o f a n E O R a p p l i c a t i o n . I n a d d i t i o n t o t h e t e c h n i c a l p r o b l e m s , h i g h

c o s t s a n d r e g u l a t o r y r e s t r i c t i o n s a r e a s s o c i a t e d w i t h i n j e c t i o n . F i n a l l y ,

p r i m a r y re c o v e r y t y p i c a l l y r e m o v e s ~ 3 0 -4 0 o f t h e N A P L . S e c o n d a r y a n d

t e r t i a r y r e c o v e r y , s u c h a s E O R , if s u c c e s s fu l , m a y r e m o v e o n l y a n a d d i t i o n a l

3 0- 50 . T h u s , a s m u c h a s 1 0 -4 0 o f t h e N A P L c a n r e m a i n i n t h e s u b s u r f a c e

a f t e r s u c c e s s f u l a p p l i c a t i o n o f E O R m e t h o d s .

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43

acuum extraction

I f N A P L i s v o l a t il e a n d n e a r o r a b o v e t h e w a t e r t a b l e, v a c u u m e x t r a c t i o n i s

a p o t e n t i a l t e c h n i q u e f o r r e m o v i n g r e s i d u a l s a t u r a t i o n . D u r i n g v a c u u m

e x t r a c t i o n , a i r i s f o r c e d t h r o u g h s o i l s c o n t a m i n a t e d w i t h v o l a t i l e o r g a n i c

c o m p o u n d s . T h e r e s u l t i n g v a p o r s g e n e r a l l y a r e c o l l e c t e d a n d t r e a t e d . S i m p l e

t e c h n i q u e s h a v e b e e n d e v e l o p e d t o c o n t ro l s u b s u r f a c e h y d r o c a r b o n v a p o r s a n d

a r e d i s c u s s e d i n D u n l a p (1 98 4), M a r l e y a n d H o a g (1 98 4), a n d O C o n n o r e t a l.

(1 98 4). I n g e n e r a l , t w o p r i n c i p a l t y p e s o f v a p o r m a n a g e m e n t s y s t e m s a r e

a v a i l a b l e : p o s i t i v e d i f f e r e n t i a l p r e s s u r e s y s t e m s a n d n e g a t i v e d i f f e r e n t i a l

p r e s s u r e s y s t e m s . P o s i t i v e d i f f e r e n t i a l p r e s s u r e s y s t e m s i n d u c e v a p o r f l o w

a w a y f r o m t h e c o n t r o l p o i n t s w h i l e n e g a t i v e d i f f e r e n t i a l p r e s s u r e s y s t e m s

i n d u c e v a p o r f lo w t o w a r d t h e c o n t r o l p o in t s. T h e v a p o r m a n a g e m e n t m e t h o d s

m a y b e e i t h e r p a s s i v e o r a c t i v e . P a s s i v e m e t h o d s u s e n a t u r a l l y o c c u r r i n g

d i f f e re n c e s i n g a s p r e s s u r e s t o i n d u c e t h e r e q u i r e d f lo w r e g im e . A c t i v e m e t h o d s

r e q u i r e t h e a r t i f i c ia l g e n e r a t i o n o f d i f f e r e n t i a l g a s p r e s s u r e s t o a c c o m p l i s h t h e

s a m e fl ow p a t t e r n . P r a c t i c a l e x p e r i en c e h a s d e m o n s t r a t e d t h a t a c t i v e

g e n e r a t i o n o f n e g a t i v e d i f f e r e n ti a l g a s p r e s s u r e s t y p i c a ll y p r o v i d e s t h e m o s t

f a v o r a b l e f ie l d r e s u l t s .

T h e a i r f lo w g e n e r a t e s a d v e c t i v e v a p o r f l u x es t h a t c h a n g e t h e v a p o r - l i q u i d

e q u i li b r iu m , i n d u c i n g v o l a t i l i z a t io n o f c o n t a m i n a n t s . T h e a d v a n t a g e o f t h is

m e t h o d i s t h a t i t i s i m p l e m e n t e d i n p la c e , c a u s i n g m i n i m u m d i s r u p t i o n . T h i s is

e s p e c i a l l y i m p o r t a n t a t a c t i v e f a c i l i t ie s o r s i te s h i n d e r e d b y p h y s i c a l o b s t a c le s .

V a c u u m e x t r a c t i o n l a b o r a t o r y s t u d i e s a re d e s c r ib e d i n T h o r n t o n a n d W o o t a n

(1982) , Mar ley and Hoag (1984) , and T .R . I . (1984) . A f ie ld- sca le exper iment i s

d i s c us s e d in C r ow e t a l. (1985, 1987) . S im u la t ion t e c h n iq ue s u se d to s im u la t e

v a c u u m e x t r a c t i o n a r e p r e s e n t e d i n D . E . W i l s o n e t a l . ( 1 9 8 7 ) , J o h n s o n e t a l .

(1988) , K r i sh n ay y a e t a l . (1988), S te ph an a to s (1988) , B aeh r e t a l . (1989) , and

M a s s m a n n (1 98 9). A p p l i c a t i o n s t o h a z a r d o u s w a s t e s i t e s i n c lu d e A g r e l o t e t a l.

(1985) , C on ne r (1988) , Re ga lb uto e t a l . (1988), an d H ut z le r e t a l . (1989) . Kno w ing

s u b s u r f a c e s p a t i a l v a r i a b i l i t y a n d p r o p e r d e s i g n c o n t r o l t h e e f f e c t iv e n e s s o f a

v a c u u m e x t r a c t i o n a p p l i c a t i o n .

V a c u u m e x t r a c t i o n c a n b e e f fe c ti v e t o re m o v e N A P L a t r e s id u a l s a t u r a t i o n

f r o m t h e v a d o s e z o n e . A c c o r d i n g t o H u t z l e r e t a l . (1 9 89 ), m o s t c h e m i c a l s t h a t

h a v e b e e n s u c c e s s f u l l y e x t r a c t e d h a v e a l o w m o l e c u l a r w e i g h t a n d h i g h

v o l a t il i ty . M o s t o f t h e c o m p o u n d s h a v e v a l u e s o f H e n r y s l a w c o n s t a n t s

o f > 0 .0 1. V a c u u m e x t r a c t i o n t e n d s t o b e m o s t e f fe c t iv e i n h o m o g e n e o u s ,

p e r m e a b l e m e d i a . I f t h e w a t e r t a b l e i s l o w e r e d , i t c a n a l s o b e u s e d t o r e m o v e

r e s i d u a l N A P L f r o m b e l o w t h e o r i g i n a l w a t e r - t a b l e e le v a t io n . G r o u n d w a t e r

p u m p i n g a n d v a c u u m e x t r a c t i o n a r e b e i n g u s e d t o g e t h e r t o r e c o v e r D N A P L

c o n t a m i n a n t s a t t h e T y s o n s S u p e r f u n d s it e, P e n n s y l v a n i a , U . S. A . ( W a s s e rs u g ,

19 90 ). V a c u u m e x t r a c t i o n a l s o c a n i n c r e a s e n a t u r a l b i o d e g r a d a t i o n p r o c es s e s

b y i n t r o d u c i n g a d d i t io n a l o x y g e n i n t o t h e s u b s u r fa c e . F i n a l l y , v a c u u m

e x t r a c t i o n i s g e n e r a l l y u s e d i n c o n j u n c t i o n w i t h a t h e r r e m e d i a l m e t h o d s .

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44

i o r e c l ama t i on

J W M E R C E R A N D R M C O H E N

Many water-table aquifers support aerobic microorganisms that degrade a

variety of organic contaminants. According to J.R. Wilson et al. (1986),

examples include benzene, toluene, xylenes, and other alkylbenzenes from

gasoline or solvent spills (Lee and Ward, 1984); naphthalene , the methyl-

naphthalenes, fluorene, acenaphthene, dibenzofuran and a variety of other

polynuclear aromatic hydrocarbons released from spilled diesel oil or heat ing

oil (B.H. Wilson and Rees, 1985); acetone, isopropanol, methanol, e thanol, and

t-butanol from solvent and gasoline spills (Jhaveri and Mazzacca, 1983; Lokke,

1984; Novak et al., 1984); and many methylated phenols and heterocycl ic

organic compounds found in certain industrial waste waters. Many synthetic

organic compounds can also be degraded, including dichlorobenzenes (Kuhn et

al., 1985), the mono-, di- and tri-chlorophenols (Suflita and Miller, 1985), the

detergent builder nitrilotriacetic acid {Ward, 1985), and some of the simpler

chlorinated compounds such as methylene chloride (dichloromethane)

(Jhaver i and Mazzacca, 1983). Many DNAPL chemicals are halogenated

compounds that degrade under anaerobic conditions or are resistant to deg-

grada tion (Feens tra and Cherry, 1988). Also, when they do degrade, equal ly

undesirable chemicals may result (Vogel et al., 1987).

J.R. Wilson et al. (1986) present an overview of bioreclamation as a ground-

water remediation technique. In the sa turated zone, oxygen needed for aerobic

biodegradation is available in a dissolved state at relatively low concentra-

tions. Many of the bioreclamat ion systems are derived from a paten t process at

Suntech, Marcus Hook, Pennsylvania, U.S.A. (Raymond, 1974). The first step

in this process is to recover mobile NAPL. Next, laboratory studies are

conducted to determine if the native microbial population can degrade the

contaminants. Finally, a combination of oxygen and nut rien ts is injected into

the contar~inated zone to enhance natural biodegradation. Controlling

groundwater flow in the sa turated zone is critical for transporting oxygen and

nutrients to the contaminated zone and enhancing the degradation process.

Therefore, the limiting process in bioreclamation is convective transport of

oxygen and nutrients, not rate limitations due to degradation. Hence, site

characterization providing a detailed definition of hydraulic conductivity is

required for a successful bioreclamation design. Bioreclamation has been

applied mainly to LNAPL s (e.g., gasol ine) with varying success (e.g., Yaniga

and Mulry, 1984). According to Downey (1990), while microbiologists have

proven the principles of biodegradation in the laboratory, engineers are having

less success achieving a uniform reac tion in heterogeneous aquifers.

There is also related work on bio-emulsification of oils for microbial en-

hancement of oil recovery from petroleum reservoirs (Cooper and Zajic, 1980)

that should be directly applicable to petroleum product spills. Ehrlich et al.

(1985) demonstra ted tha t bacteria from a well contaminated by JP-5 ~ jet fuel

could emulsify the fuel if the well water was supplemented with phosphate and

nitrate. In favorable geologic situations, these emulsions should be mobile and

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A R E V I E W O F I M M I S C I B L E F L U I D S I N T H E S U B S U R F A C E 4 5

could be removed by pumping for treatm ent on the surface (J.R. Wilson et al.,

1986). Microorganisms can also mobilize hydrocarbons by transforming them

to polar compou nds such as alcohols, ketones, phenols, or org anic acids (Perry,

1979). Here again, the success of the mobilization will depend on the site

characterization.

DISCUSSION AND FUTURE RESEARCH

NAPL chemicals re sult from a variety of indus trial processes and are found

at nu merous contaminated sites. Where present, NAPL s are a long-term

source of miscible groun dwater contamination. Site charac teriz ation and re-

mediation depend on underst andin g the NAPL behavior in the subsurface. This

understanding depends on knowledge of NAPL properties, which is the main

topic of this paper.

Density determines if the immiscible fluid is lighter than water (LNAPL) or

heavier th an water (DNAPL). LNAPL s are often associated with petroleum

hydrocarbons. Because of work performed in the petroleum industry, consider-

able information has been developed on properties associated with petroleum

products. In addition, because LNAPL is at or near the water table, charac-

terization and reme diation are gene rally more successful than at DNAPL sites.

Remediation of NAPL contamination often involves a combination of

techniques to form a tre atme nt train. Mobile NAPL should be recovered using

a pump-and-treat technology. This will leave behind a residual sa tura tion that

might be remediated by another technique such as vacuum extraction or EOR

methods. Residual NAPL recovery, especially for DNAPL, is difficult with no

routine proven technology available. At some sites, hydraulic containment

may be the only practical con taminan t management option.

Although much work has been accomplished to better understand NAPL

behavior, additional effort should be focused on the following areas:

Properties

(1) Development and demonstr ation of improved methods to mea sure in situ

saturations.

(2) Measurement of relative permeability functions, especially to improve

understanding of the field behavior of solvents.

(3) Because of the importan ce of residual sat urat ion as a continuin g source

for vapor transport and/or dissolved chemical transport, an improved un-

derstanding of in situ volatilization and dissolution is needed.

odels

(4) Although much of the data required for NAPL simulation are unavail-

able, model improvements can be made by adding gas and dissolved chemical

7/24/2019 Mercer 1990


46


transport to existing NAPL codes. This would also require an improved un-

derstanding of mass-transfer mechanisms.

(5) Few DNAPL simulation studies have been published. Conceptual

simulation analyses of the effects of variable source and subsurface conditions

on NAPL transport, containment and recovery should be conducted to provide

insight to site characterization, chemical migration and remediation.

haracterization

(6) As with any contaminat ion problem, the abili ty to define spat ial variabil-

ity, including fracturing, is important to understanding NAPL migration and

recovery.

(7) Improved manuals that provide documentation and guidance on

procedures and priorities for investigating NAPL contamination sites are

needed by NAPL site investigators.

Remediation

(8) Many NAPL remediation technologies are largely untested; field-scale

demonstration studies are required to determine which technologies work

effectively under what conditions. These studies require careful control to

evaluate actual mass recovered and left in situ.

Ongoing research and experience from current field work at sites such as

S-Area and Hyde Park in New York (Cohen et al., 1987) and Tyson s site in

Pennsylvania (Wassersug, 1990) should significantly advance our understand-

ing of the feasibility of using different methods to investigate and remediate

subsurface NAPL contamination during the 1990 s.

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R E V I E W O F I MM I S C I BL E F L U ID S I N T H E S U B S U R F C E

47

4~

o

g:U

. . . . . . . . . . . . .

e4 ~ ~ e4 ~ e4 e~ ~ ~

T

I I I I I I I I

7 ~

~ ~ ~ ~ ~ ~ ~ ~~ ~

~ ~ ~ ~ ~ ~ i ~ ~ ~ ~

o

o

o

• .~ ~ .,~ ,.~ ~ ~ ~ -~

o ~ ~ ~ ~o ~

o o o

T~

T

o

o ~

7/24/2019 Mercer 1990


48

J W M E R C E R A N D R M C O H E N

. .

° ~

Z

~ =

i

~ ~

. . . . . . . . . . . . . . °

~ ~ ~ ~ ~ ~

I = I =

o=

i

~ - ~

~ g ~

° ° °

I : I

. . . . . 0

~ ~ v

7

. . i

° ~ 0 ~

7/24/2019 Mercer 1990


R E V I E W O F I M M IS C I B L E F L UI D S I N T H E S U B S U R F C E 4 9

o o °

.....

....

~ T ~ T T

I ~ I

~ = 6 v M N

T ,

il

o ~ . q

, , ~ Z ~

~ - ~ , ~

• ~ c ~ ~ ~ ,

c n 8 ~ . ~ , ~ . . .

~ _ ~

~ ~ ~

~ ~ ~o

•~ - . ~

~ { ~ ~

7/24/2019 Mercer 1990


1 5 0 J W M E R C E R A N D R M C O H E N

A

m l

° i i

i = 1

0

° 1 - i

° 1 - i

° . ~ A

~0 ~ ~ 0 ~

. . . ~ ~ .

~ A A ~

~ ~

~ o o o o o

° ~

~ ~

0 0 0

o ~

° ~ ~ o o

~ ~ ~

~ ~°~°~°~°

7/24/2019 Mercer 1990


A R E V IE W O F I M M I S C I B L E F L U I D S I N T H E S U B S U R F A C E 1 5 1

m ~ m

°

~ ' .

~ r =-.- m~

• ,

~ m m m m m m

- . o m

m M m m

<

m m m

2 ,

® m m m ~ m

. . . . . . . . . . . . o O . .

~ i C ' + 4 t r + ~ ' ~ + , , ~ , . . + t r, +

i ~ t t ~

C~

, , - + + + , .~ ~ ~ ~ ~

,:: , ,

~ . , . , ?

<3,I

~

~ t . ~ - + - - . r

°

~ ~

, , . . . . , . . ~ ,~

~ . -,

t = ~

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+ + . - , + . , , ~

m ,

7/24/2019 Mercer 1990



X

<

r~

• ~

~ ~ ~ ~ •

°.

,=.

~

b ~

E-,

i

L

o

,=,

~=,

7/24/2019 Mercer 1990


A REVIEWOF IMMISCIBLEFLUIDS N THE SUBSURFACE 153

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