Oil Spill Shallow

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Environmental Software, V ol . ] ,0 , N o 4 , pp . 241 - 249 , 1995C o p y r i g h t 1 9 9 6 E l se v i e r S c i en c e L t dP r i n t e d i n G r e a t B r i t a i n . A l l r i g h t s r e s e r v ed0 2 6 6 - 9 8 3 8 / 9 5 $ 9 . 5 0 + 0 . 0 0

M o d e l l i n g t h e b e h a v i o r o f c r u d e o i l s p i l l s i ns h a l l o w b o d i e s o f w a t e rA. Maure ,* M. Cerro lazat & R. Berr ios~

* Facultad de lngenieria, Universidad Nat ion al de/Cuyo, Mendoza, Argentina. Fax: +54-6 I-2,I. 80.80+ tnstituto de Materia les y Modelos Estructurales, Facultad de lngenieria, Universidad Central de Venezuela,PO Box 50.361, Caracas 1050A, Venezuela. E-mail: [email protected]. Fax." +582-693.1477+ h~'cuela de Ingenieria Mecdnica, Fa adtad de lngenieria, Universi&ut Central de Venezuela, Caracas( R e c e i v e d 1 9 J u n e 1 9 9 5 ; a c c e p t e d 2 2 N o v e m b e r 1 9 9 5)

A BSTRACTlu order to ewtluate the risk of pollution related to the petroleum indust ry in Llancane lo saltwater lake,;m accura te numerical model o f water contaminat ion using f ile finite element meth~)d and Ihe finitedi/lierencc method is discussed.

The study ~uea is located in tile south of the Mendoza province (Argent ine!. In l!)~q) ~( wasdeclared a faunistic reserve and it constitutes a genetic bank of worldwide importance. This siterepresenls a special environmental area for wild life, and it covers a diversity of habitats which cansu: w,r t many types of resident migrato ry species. Nearby of the occidental coast of lhis shalh)w brady~,I w.alcr, there are many active oil fields. Contaminants from oil spills can enter the lake, liom r~verinlh~ws, from run off and from adjacent lands influenced by water body dynamics. Within lhe lake,c~llam inants are transported by convective currents, disperssive processes and many other c~mq~lexr, h e n o m c ' l l a thal finally kill sensitive individuals or disturb nesling zones.

Due to tile scale and seasonal variability of the lake perimeter, the numerical simulafi~mm~,dcls :~re calibrated and updated by the use of salellital images. Tim final g(ml is Io predictec,~logic:d impacts of petroleum conlamination and to evaluate remediation actions proposed t~{ ' \ 2 1 c ~ ) m e these impacts.

KEY W~)RI)S( ) i spills, nnmerical simulation, finite elements, shallow waters

S( WTWARE AVAI LABILI TYNamc of the sotiware: PPFE2D (Pollnent Propagation with Finite Elements in 2D1Developers and contact address: M. Cerrolaza, Fax: +582-693.1477

E-mail: [email protected]. A. Maure, Fax: +54-61-24.80.g0Yc'~u lirst available: 1993Hardware required: Compatible personal computer 380 or higherSoqw.tre required: MicroSoft Disk Operating System, ver 3.0 or higherProgram Language: PascalPn,gr am size: Approx. 500 Kbytes including the three modules: preprocessor, analysis package amlgraphic postprocessor.Availability and cost: Contact the developers.

I. INTROI)UCTIONThe risk of polluticm related to hydrocarbon substancestram lhe petreleum industry near Llancanelo saltwater lakeneeds an accurate numerical model in order to predictenvironmental impacts. Two coupled numerical models,ihe Finite Element Method (FEM) and the Finite

241

Differences Method (FDM) have been concerned withspace prediction of total hydr9carbon concentration andhow fast the contaminat migrates ,m lhe lake riverentrances.

The associated numerical m o d e , l to conduct tilephenomena o f pollutant propagation i:~ called


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2 42 A. Maure e t a l .b id imen s io n a l sch em e an d a fu l l y z - i n teg ra t ed a lg o ri t h m toso lv e co n v ec t i v e t e rms u s in g f ir s t and seco n d F i ck ' s l awsin d iscrete fo rms. Special boundary condi t ions to s imulatet ime- sp ace d ep e n d en ce o f p o l l u tan t co n cen t r a t i o n s a t sp il lspo in ts on lake coast and r iver en t rances were also used . Inth is sense, the approach by a numerical model l ing usingth e F E an d t h e F D meth o d s r ep resen t s a n o n d i s t u rb in gmeth o d o f su rvey .

As a co n seq u en ce o f t h e r esul t s o b t a in ed ru n n in g t h emod els wi th d i f feren t sp i ll scenar ios , a p rope r sensi t iv i tyanalysis and cont igen cy p lans involv ing preven t ive act ionscan b e t ak en . Ho w ev er , it sh o u ld b e emp h as i zed h e re t h a tt h e v a l i d it y o f th e mo d e l d ep en d s u p o n l ab o ra to ry an df ield scale tes ting of the chemical , physical and b io log icalp ro cesses g o v ern in g t h e co n t amin an t t r ansp o r t.

This work i s o rgan ized as fo l lows: Sect io n 2p ro v id es an o v erv i ew o f t h e f au n i s t i c r ese rv e o f t h eLlancanelo lake, detai l ing some of the l iv ing species there.Sect ion 3 deals wi th the act iv i t ies which con taminate thelake, as wel l as i t p rov id es a b r ief descr ip t ion of theeco to x i co lo g y o f t h e h y d ro carb o n s . Th e way h o w th epol lu tan ts are d ispersed in to the lake i s addressed inS ec t i o n 4 . Th e n u mer i ca l mo d e l u sed t o s imu la t e th econcent rat ion of the po l lu tan ts i s concerned to Sect ion 5 ,which also d iscusses some prel iminary resu l t s ob tained .

2 . T H E F A U N I S T IC R E S E R V E O F L L A N C A N E L OL A K E

The s tudy area (35 30 ' S - 36 00 ' S and 69 00 ' W - 69 15 ' W) involves four ty thousand hectares which arelo ca t ed i n t h e so u th o f M e n d o za p ro v in ce , Arg en t i n e . I n1980 i t was declared a faun is t ic reserve and const i tu tes ag en e t i c b an k o f wo r ld wid e imp o r t an ce .This s i te represen ts a special env i ronmental area fo rwi ld l i fe and i t covers a d ivers i ty o f hab i tats which cansu p p o r t man y r es id en t an d mig ra to ry spec i es o f b i rd s . I no rd er t o o f f e r an ad ecu a t e co n cep tu a l f r ame , t h e l ak e an dthe coastal zones was d iv ided in to s ix land un i t s based onthe analysis o f satel l i ta l images ob tained f ro m theL A N D S A T o r bi te r s:Uni t 1 :Uni t 2 :Uni t 3 :Un i t 4 :Uni t 5 :Uni t 6 :

Volcan ic cones and basal t ic landsCo n es fo rmed f ro m a l l uv i a l r u n o f f d eb r i s an dplain landsInundable f lat landsWate r f l o o d ed co as t sSt rong sal ine coasts wi th occasional waterf loode d areasWate r su r f ace

Abundance and diversityRecen t l y , b io lo g i s t s h av e r ep o r t ed t h a t o v er 9 0 b i rd ssp ec i es l iv e i n L l an can e lo . Ap p ro x im ate ly twen ty -e ig h t o ft h em a re r es id en t an d co n g reg a t ed i n n es t i n g co lo n i esd u r in g th e r ep ro d u c t i o n p e r i o d f ro m Au g u s t t o M a y . Th esi te i s located in the rou te o f eig th migratory species thatn es t in No r th A mer i ca an d v i s i t L l an can e lo in t h e su m mer .In the win ter season , the lake receives ind iv iduals f rom

Patagonian zones wi th most sensi t ive species and nest ingzones being main ly located in the nor th o f the sal t waterl ak e , n ea r t o M o c h o an d M aia rg u e r i v e r en t r an ces . Th eto tal num ber o f ind iv iduals are approx imate ly 132 ,000b i rd s. S o m e o f t h em wi l l b e l i s ted b e lo w in c lu d in g t h e i rlocal name and par t ial nest ing act iv i t ies (uned i ted repor tso f t h e Wi ld L i f e F o u n d a t i o n , 1 9 83 ):P h o en i co p t e ru s Ch i l en s is (F l amin g o ) 8 ,0 0 0Cy g n u s M elan co ry p h u s (C i sn e d e Cu e l l o Neg ro ) 1 ,1 00P o d icep s Occ ip i t a l is (M aca P l a t ead o ) 2 0 0Ny c t i co rax Ay c t i co rax (Garza Bru j a ) 1 00P o d icep s M ajo r (M aca Gran d e) 5 6Eg re t t a Th u l a (Garc i t a B l an ca) 5 1Eg re t t a Alb a (Garza B lan ca) 5 0F u l i ca Arm i l l an ta (Ga l l a r e ta d e L ig as Ro jas ) 4 3Plegadis Chih i (Cuerv i l lo de Caf iada) 22Laru s M acu l i p en n i s (Gav io t a Cap u ch o Cafe ) 2 1Co ro b a Co sc o ro b a (C i sn e Co sco ro b a) 1 5P o d icep s Ro l l an d (M aca Co mfm) 1 2S te rn a T ru d eau i (Gav io t i n Co m u n ) 1 1A r d e a C o c o i ( G a r z a M o r a ) 9F u l i ca Ru f i f ro n s (Ga l l a r e t a Escu d e t e Ro jo ) 4Himan to p u s Himan to p u s (Tero Rea l ) 4S tu rn e l la Lo y c a (P ech o Co lo rad o Gran d e) 4Zo n o t r i ch i a Cap en s i s (Ch in g o lo) 4P h l eo cry t es M elan o p s ( Ju n q u ero ) 3Ox y u ra Vi t t a ta (P a to Zam b u l l i d o r Ch ico ) 2Bu b u lcu s Ib is (Garc i t a Bu ey era ) 2P o ly b o ru s P l an cu s (Caran ch o ) 2M i lv ag o Ch iman g o (Ch iman g o ) 1An as S ib i la t r ix (P a to Ov er t ) 1An as F lav i rost r i s (Pato Barcino) 1Ix o b ry ch u s In v o lu cri s (M i raso l Co m u n ) 1P seu d o se i su ra Gu t tu ra l is (Cach a lo t e P ard o ) 1Ag e l a iu s Th i l iu s (Al f e r ez ) 1Lep t as th en u ra Aeg i t h a lo id es (Co lu d . P ard o ) 1

3. C O N T A M I N A N T A C T I V I T I E SAt presen t there are four ty - two o i l f ie lds spread across theM ala rg u e zo n e , wh ich p ro d u ce o v er 2 ,2 0 0 cu b i c mete r s o fo i l per day . The se o i l f ie lds hav e a to tal recup erab ler ese rv es n ear t o 2 1 ,7 0 0 mi l l i o n o f cu b i c mete r s(Andrada , 1993).Near t h e o cc id en t a l co as t o f t h i s sh a l l o w b o d y o fwate r t o ward s t h e M ala rg u e r i v e r an d Ch acay r i v e r ( twoimportan t t r ibu tar ies that d ischarge in the Llancanelolake) , many o i l f ie lds are in p roduct ion . Th e most l ikelyso u rces o f sp i l l ag e a r e :S p i ll s a n d r u n - o f f i n f l u e n c e s m a i n l y o v e r A o . C h a c a y .* Cer ro M o l l a r an d Cer ro M o l l a r Oes t e (we l l an db a t e ry p l an t sp i l l so u rce )

* Puesto Rojas (w el l and bat tery p lan t sp il l sources)Spi l l s and run - of f i n f l uences m ai n l y ove r M al 'arguer i v e r a n d A t . D e L a s M i n a s .* P u es to Ad o b e (w e l l sp il l so u rces )* P u es to Ho rq u e t a (we l l sp il l so u rces )


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Modelling the behavior o f crude oil spills in shallow bodies o f water 24 3C o a s t a l a n d v e r t i e n t i n f l u e n c e s o v e r L l a n c a n e l o l a k e .* L la n c a n e lo Iw e l l a n d b a t t e ry p l a n t s p il l so u rc e s )Ecotoxicology o f hydrocarbonsP e t ro l e u m i s a v e ry c o m p le x m ix tu re o f d i ff e re n t o rg a n ich y d ro c a rb o n s . M o s t o f t h e 5 , 0 0 0 d i s t in c t iv e h y d ro c a rb o n sc a n b e fo u n d in cru d e o i l. U s u a l ly , t h is c o m p o u n d s a rec las s i f ied in to four broad famil ies such as : sa tu ra ted ( thosew i th o n ly o n e s in g le c a rb o n -c a rb o n b o n d , u s u a l lyc o n s t i tu t in g th e l a rg e s t g ro u p ) , a ro m a t i c s , a s p h a l tn e s s a n dre s ins . T h e p e rc e n ta g e o f e a c h o n e in th e c ru d e o i lim p o s e s d i f f e re n t p h y sic a l a n d c h e m ic a l c h a ra c t e r i s ti c s t othe fluid.

L ig h t o i l s c o n ta in 1 0 to 4 0 p e rc e n t n o rm a l a lk a n e s! t .e .+ m e th a n e C H , 4 , e th a n e C 2 H 6 o r p ro p a n e C 3 H 8 , w h e re( ' s t a n d s fo r c a rb o n a to m s a n d H s t a n ds fo r h y d ro g e n~ to m s ) , b u t w e a th e re d a n d h e a v ie r o i l s m a y h a v e o n ly as m a l l f r a c t io n o f a lk a n e s . H e a v ie r a lk a n e s c o n s t i t u t e 5 to2 0 p e rc e m o f l i g h t oi l s a n d u p to 6 0 p e rc e n t o f h e a v y o i l s .A ro m a t i c h y d ro c a rb o n s a re th o s e c h a ra c t e r i z e d b y th ei , r e s e n c e o t a t l e a s t o n e b e n z e n e (o r s u b s t i t u t e d b e n z e n e )r ing , l igh t t ) i l typ ica l ly con ta in f ro m 2 to 20 pe rcen t ~)fh g h t c o m p o u n d s ( l o w m o l e c u l a r co m p o u n d a s o p p o s i t e tuh ig h h y d ro c a rb o n c h a in s in h e a v y o i l s) , w h e re a s h e a v y o i l s~ o n ta in 2 p e rc e n t or l e ss . H ig h m o le c u la r w e ig h tp o ly a ro m a t i c s c o m p r i s e 2 to 1 0 p e rc e n t o f li g h t o i ls a n dt~p to 35 pe rce n t o f heav y o i ls .F ro m +h e e c o to x ic o lo g ic a l p o in t o f v i e w , th e m o s t

d a n g e ro u s h y d ro c a rb o n s fo r t h e h u m a n a n d a n im a l l i f e a reor ig ina ted when the meth i l rad ica l i s a t tached top o ly a ro m a t i c c o m p o u n d s , s u c h a s fo r in s t a n c e 1 -2b e n z o a n t ra c e n e , 1 -2 -4 -6 b e n z o a n t ra c e n e , 3 -4 b e n z o p i r in e ,e tc . Dire c t exposure o f these subs tances to the sk in o f ah a m s te r p ro d u c e s c a rc in o m a s s i s t e m a t i c a l ly (A t l a s, 1 9 88 ).

T h e m a x im u m p e rm is s ib l e l e v e .l o f e x p o s u re to th e s epo l lu tan ts is ve ry smal l . Typ ica I va lues a re inc o n c e n t ra t io n s o f a f e w m ic ro g ra m s p e r li t er . I t i s e a s y tud e te rm in e th e c o n c e n t ra t io n o f t h e se p o l lu t a n t s i n l a rg eb o d ie s o f w a te r b y u s in g s p e c t ro f lu o ro m e t r i c m e th o d sw h ic h a re u s e d w o r ld w id e to d a y (L e v y , 19 81 ; L e v y , 1 98 l a:E rh a rd t , 1 9 8 3 ) . O b v io u s ly , t h e c a rc in o g e n e t i c r i s k i sim p o r t a n t , b u t i t i s n o t t h e u n iq u e p ro b le m a g a in s t b io t i cfa c to r s. A l s o , m a n y o th e r m a s s iv e a n d lo n g t e rm p h y s ic a lin ju r ies to wi ld l i fe a re ' a lways found when a la rge sp i l l o fc ru d e o c c u r s .

4 . T R A N S P O R T A N D F A T E O F' P O L L U T A N T SPollu tan ts and dangerous subs tances t rom o i l sp i l ls cane n te r t h e l a k e f ro m r iv e r i n f lo w s , f ro m ru n o f f a n d f ro ma d ja c e n t l a n d s in f lu e n c e d b y w a te r b o d y d y n a m ic s(S t re e t e r a n d P h e lp s , 1 9 5 8 ) . T h e s e s u b s t a n c e s a re s u p p o s e dto have an in i t ia l concen tra t ion C ( t=O) a t source pu in tsloca ted in the mouth en trance o f r iw:rs o r in the lake cuas lnear ba t te ry p lan ts o r c rude purg ing we l l .~ . With in the lakc

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Figure 1. Land sat sattelital images of Llancanelo lake." a) January 25, 1 98 5 b) May I6, t985


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2 44 A . M a u r e e t a l .

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Figure 1. bcon t ami nan t s a r e t r anspor t ed by co nvec t i ve cu r r en t s,d i s p er s iv e p r o c e s se s a n d m a n y o t h e r c o m p l e x p h e n o m e n at ha t e i t he r cou l d a f f ec t t he mos t s ens i t i ve i nd i v i dua l s o rcou l d d i s t u r b nes t i ng zones .C r ude o i l and pe t r o l eum d i s t i l l a t e p r oduc t si n t r oduced t o t he l ake env i r onment a r e i mmedi a t e l ysub j ec t ed t o a va r i e t y o f phys i ca l , chemi ca l and b i o l og i ca lchanges . B i o l og i ca l w ea t he r i ng p r oces ses may i nc ludeevapor a t i on o f l i gh t f r ac ti ons , d i s so lu t i on o f w a t e r so l ub l ecompounds , d i spe r s i on , pho t ochemi ca l ox i da t i on , o i lemul s i f i ca t i on ( o f t en ca l l ed mousse), absor p t i on on t osuspended pa r t i cu l a te ma t e r i a l , s i nk ing and s ed i m en t a t ion .

O i l d i spe r sed nea r t he sho r e adhe r e s r ap i d l y t o pa r t i cu l a temat ters and s inks in a process cal led oi l sedimentat ion.T he s ed i men t a t i on ra t e depends on bo t h o i l and s ed i men tp r ope r t i e s . O i l adhe r e s qu i ck l y t o c l ay , s and , s i lt o r w o odyf r agment s . Mo usses o f up t o 2 - 3 i nch shee t s may no t on l yma i n t a i n p l ank t on i c t ox i n concen t r a t i on l eve l s i n t hesu r f ace l aye r s , bu t ma y oc c l ude sun l i gh t, t he r eb y l i m i ti ngphy t op l ank t on p r oduc t i v i t y . B i o l og i ca l p r oces ses i nc l udei nges t i on and so r t i ons by o r gan i sm as w e l l a s mi c r ob i a ldegr ada t i on . T he mi c r ob es r e spons i b l e fo r t he b r eakdow nof o i l a r e r e l a t i ve l y abundan t i n coas t a l and e s t ua r i neenv i r onmen t s and i n t he a r ea o f ch r on i c po l l u t i on .


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M o d el l i n g t h e b eh a v io r o f c ru d e o i l sp i ll s in sh a l l o w b o d i es o f w a t er 245P s e u d o m o n a d s a n d A lc a l ig e n e s , a lo n g w i th Mic ro c o c c i .C o ry n e b a c te r i a , My c o b a c te r i a , N o c a rd ia , C a n d id a a n dP e n ic i l l i u m a re th e m o s t c o m m o n m ic ro b e s (L e a h y a n dC~1we11 ,1990) . While these na tu ra ls o i l remova lm e c h a n i s m s a re e f f e c t iv e , t o o m u c h o i l m a y o v e rw h e lmth e m . I t w o u ld t a k e m a n y y e a r s t o c o n tro l a m a jo r o i l s p i llwithout active reme.diation.L a ke b o u n d a q / va r ia t i o nsDue to sca le and seasona l va r iab i l i t ie s o f the lakep e r im e te r , t h e g e o m e t ry o f t h e l a k e i s u p d a te d b y th e u s eo f sa t te l i ta l im a g e s . T h e f ig u re 1 d i s p la y s a L a n d s a ts a t te l i ta l im a g e o f L la n c a n e lo l a k e , s h o w in g th e v a r i a t io n

o f th e l a k e p e r im e te r i n tw o m o n th s d u r in g 1 9 85 (Ma u reand Ir igo , 1994)I t c a n b e o b s e rv e d th a t t h e l a k e ' s p e r im e te r r e d u c e sc o n s id e ra b ly f ro m J a n u a ry to Ma y . T h i s f a c t m u s t bcc o n s id e re d w h e n a p p ly in g a n u m e r ic a l m o d e l t o th e w a te rs u r fa c e . F ro m D e c e m b e r to Ma rc h , t h e Ma la rg u e r iv e rrun off is le s s than 2 cub ic m ete rs pe r second . Nea r o f 61ipercen t o f the f low is de r ived m Ihe c rops ands imultaneous ly , the ra te o f evapora t ion over 33 . ()01hec ta re s o f wa te r su rface is 1 l ie g rea lc r . For th is s implyreason , the lake pe r imete r is t ime ly t luc tua t ing and i{c o n t r ib u te s to r e s u s p e n d a n d to p ro p a g a te Io x ic s u b s ta n c e sin to the Fake body .

I ) , ~ I A F R O M I , A N D S A 'I " S Al l+ + I+ I, II I~ ( M S N S E N S ( ) R . 6 0 X 8 ( t M T S . P I X E I . i1 9 8 5 1 9 8 5J A N , 2 5 M A ~ , 16

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Mo delling the behavior o f crude oil spills in shallow bodies o f water 2 47Recen t r eev a lu a t i o n s o f t h e mo rp h o lo g i ca l l ak e

dynamics show that a nor th -sou th ax is leng th can f luctuatef ro m 2 9 .2 t o 3% 5 k m an d t o ta l we t t ed co ast a l a reasin c rease an d d ecrease b y ab o u t 4 2 p e rcen t ( see a l so f i g u re2) .5 . T H E N U M E R I C A L M O D E L :S O M E R E S U L T SNu mer i ca l meth o d s a r e u n d o u b ted ly o n e o f t h e mo s tp o wer fu l t o o l s t o s imu la t e n a tu ra l p h en o m en a i n en g in ee-r i n g an a ly s is . Th i s wo rk h as emp lo y ed a n u mer i ca ls imu la t io n m o d e l b ased i n t h e F in i t e E l em en t meth o d an din t h e M e th o d o f We l l M ix ed Ce l ls . As r ep o r t ed incurren t technical l i teratu re, the FEM is wel l es tab l i shed asa t ech n iq u e t o an a l i ze en g in eer in g t r an sp o rt a t io n p ro b l ems(L ick , 1 9 9 2 ; Co n n o r an d Breb b i a , 1 9 78 ; Z i en k i ewicz an dTay lor , 1991). amon g o thers . In th is resea rch , thed e t e rmin a t i o n o f t h e wa t e r v e lo c i t y co mp o n en t s i n t heh ike, p roduced by curren ts and wind f ields over thesu r f ace , were ca r r i ed o u t b y u s in g a b id imen s io n a l m o d e l .F igure 2 i l lus t rates the FEM mesh used , which was basedon 271 noda l po in ts and 255 quadr i lateral e lements . Also ,f igure 2 shows the main d im ensions o f the lake, whichvary depe nding on the e, ason .In th is anNysis , the th ree d imensional ef fects that usual lyap p ear i n t h e wa t e r b o d y were n eg l ec t ed .

Th e r esu l t s p ro v id ed b y t h e F EM mo d e l a r e n o d a lv e lo c i ti e s u f t h e wa t e r su r f ace , wh i l e th e b o u n d aryco n d i t i o n s imp o sed t o t h e mo d e l r ep resen t t h e r i v e ren t r an ces . Th e y were in t ro d u ced b y imp o s in g in i ti a lconcent rat ions o f the po l lu tan t at the th ree key-po in ts A, Band C (s ee f igure 3 ) .Th e sex:on d mo d e l u sed h e re i s t h e Wel l M ix ed Ce l l smodel (Banks, 1974) , which i s based on an exp l ici t f in i te-d i f f e r en ces b id imen s io n a l sch eme. A fu l l y z - i n t eg ra teda lg o r i t h m t~ co m p u te co n v ec t i v e t e rms u s in g F i rs t an d

S eco n d F i ck ' s l aws i n d i sc r e t e fon ~n was a l so u sed h e re(Brebbia and Orszag , 1987; ThomaJan and Muel ler , 1987;Banks, 1974) . The refo re, th is mod el i s set up by using ace l l mesh , wh ich i s su p er imp o sed o v er t h e f i n i t e e l emen tmesh . F ig u re 3 sh o ws an e l emen ta ry we l l -mix ed -ce l l a swel l a s t h e b as i c sch eme o f a b o u n d ary co n d i t i o n ssimulat ing a sp i l l in to the lake.Th e f in i te-d i f feren ces mo del thus al i lows the s imulat ion oft h e p o l l u t an t p ro p ag a t i o n p ro d u ced b y man y co n cen t r a t edsp i l l s in to a b id imensional domain . The basic equat ion canbe wri t ten as fo l lows

, I,J = J +c r ua -u + -T- , t J - . l l t , J - I~ T + D TC ~ J O c,, J _ C ~ J ~ l ,J ( l )

being C ~T = po l lu tan t concent rat ion in cel l I , J a t t ime T~mdo ~ ' J = U I x J * D T / D X( t I - l , J I - 1 J= [ l x ' * D T / D X~ l , J : U I ;J * D T / D Y~ l, J- -1 = l [ I , J - I , DT / DY

(2 )

Both tx and 13 are the ad imen sional t ran spor t factors whichmust be less than 0 .5 in o rder to p rov ide numericalstabil i ty. UlxJ- and " ' l .~ are the veloci t ies ~f the po l lu tan tin X,Y d i rect ions respe ct ively , D T is r~he t ime inc rem entan d DX, DY are t h e ce l l d imen s io n s . Th e p o l l u t an tveloci t ies were ob tained wi th the f in i te elemenl modelp rev io u s ly d esc r i b ed .

The c onserva t ion of mass , which t ruest be cnfo rced at

t k F [. t M I ( [ I ) { I . I J q I E R , t)x/

/

I ~ V * I ) 1I J rX*J~l

/ /

m m

] ) ' ~ "

Figure 4. Well-mixed-cell scheme


8/9

248 A. Maure et al .every t ime s tep T al l over the cel l s system, can be wri t tena s

NI NJ C ~ J] ~ 2 * D T * D Y = C L J o * V o l s pil lI = U = I

(3 )b e in g NI an d NJ t h e n u mb e r o f su b d iv is i o n s i n X ,Ydirect ions res pect iv ely and C~J__o the in i t ia l conc en t rat ionof the sp i l l. F igure 4 d isp lays the basic sch em e of thewel l -mix ed -ce l l sIn the analysis repor ted herein , i t was assumed that thepol lu tan t w ere sp i l led in th ree d i f feren t po in ts at theb o u n d ary l ak e : M ala rg u e r i v e r (p o in t A) , Car i Lau q u ens t r eam (p o in t B) an d Carap ach o s s t r eam (p o in t C) . Th eeffect o f these th ree sp i l l s can be seen in f igure 5 , whichd i sp l ay s t h e r esu l t s o b t a in ed b y t h e n u mer i ca l mo d e l a ts t ep t imes T (1 )=0 .1 h o u r s an d T (2 )=1 0 h o u r s .

I t can b e n o t ed t h e r e l a t i v e ly f as t p ro p ag a t i o n o f t h epol lu tan t as wel l as the way i t i s d is t r ibu ted over the lakesu r f ace . Th i s p ro p ag a t i o n i s i n c remen ted b y t h e win d f ie l dove r the lake surface as wel l as the in ternal curren ts . Alsonote that the in i t ia l concen t rat ion i s s t ronger at po in t A(Malargue r iver) than at the two o ther po in ts (B and C) ,s i n ce t h e M ala rg u e r i v e r i s t h e cu r r en t s t ro n g es t so u rce o fco n t amin a t i o n i n t h e lak e . Th e co n cen t r a t io n s o f t h ep o l l u tan t s were as su med as fo l lo ws : A=2 * C=4 * B .

Th e r esu l t s d i sp l ay ed i n f i g u re 5 were o b t a in ed b yapply ing the f in i te element mesh as the base in terpo lat iong r id . On ce t h e v a lu es o f t h e n o d a l v a lu es o f t h e p o ll u t an tco n cen t r a t i o n a r e k n o wn a t a g iv en t ime s t ep T , a p o wer fu lg rap h i c p o s tp ro cesso r mo d u le i s i n v o k ed t o ca r ry o u t anoveral l in terpo lat ion of the resu l t s a t many o ther po in ts o ft h e mesh . As a co n seq u en ce , r esu l ts can b e d i sp l ay ed in asmooth manner , thus al lowing the analyst to a bet terin terpretat ion of the mo del behaviour . Th e possib i l it ies

A

T ( 1 ) = o . 1 L o w I I 0

A

Relat iveconcentrat ions t

n ig h I l o o

T ( 2 ) = l o H

Figure 5. Contaminant distribution fo r T(1)=O.1 hours and T(2 )= l O hours


9/9

M o d e l l i n g t h e b e h a v i o r o f cr uc le , o i l s p i l ls i n s h a l l o w b o d i e s o f w a t e r 2 4 9a r e ve r y a t t r a c t i ve . , s i nc e t he a na l y s t c a n va r y t he bounda r yc ond i t i ons a s w e l l a s t he e x t e r na l e nv i r onm e n t a l a c t i ons( w i nd f i e l d s a nd w a t e r c u r r e n t s ) i n o r de r t o e va l ua t e t her e s p o n s e o f ~ h e m o d e l t o s e v e ra l k n o w n c o n d i t i o n s .

t he i r s uppo r t t o t h i s re s e a r c h . T he c a r e f u l r e v i s i on o f t hem a n u s c r i p t b y H . B r i c e n o i s a l s o a c k n o w l e d g e d

R E F E R E N C E S

6 C O N C L U I ) I N G R E M A R K SL t a n c a n e l o s a l t w a t e r l a k e i n M e n d o z a ( A r g e n t i n a )p r e s en t s c o n f l i c t i n g s i tu a t io n s b e c a u s e o f th e p r o x i m i t y o fpe t r o l e u m a c t i v i t y t o t he r e s e r ve pe r i m e t e r . I n t he e ve n t o fl a r ge o il s p il l s, t he c on t i n ge n c y p l a ns c ou l d be ou t l i ne d bya num e r i c a l m ode l l i ng o f t he r es u l t s .

T h e f i na l t a r ~ m t i s t o p r e d i c t t he e c o l og i c a l i m pa c t sa ~ d t o t a ke i n t o a c c oun t t he r e s pons e t i m e a nd t her e m e d i a t io n a c t io n s to o v e r c o m e th e o c c u r r e n c e o f l a r g eo i l s p i ll s . I n t h i s s e ns e , t he a pp r o a c h by num e r i c a lm ode l l i ng u s i ng t he f i n i t e e l e m e n t m e t hod a nd f i n i t ed i f f e r e n c e m e t h o d h a s p r o v e n t o b e u s e f u l a n d a c c u r a t e .M or e o ve r , ; f l t hough the f i n i t e e l e m e n t m e s h i s no te x t r e m e l y d e n s e ( o n l y 2 5 5 q u a d r i l a te r a l e l e m e n t s w e r eus e d i n t h i s p r e l i m i na r y s t udy ) , t he r e s u l t s a r c a c c u r a t em m u g h f r o m a n e n g i n e e r in g v i e w p o i n t , t h u s p r o v i d i n gr e le v a n t i n f o r m a t i o n a b o u t t h e p r o p a g a t i o n ~ I t h ec ( m t a m i na n t . T h e m ode l a l low s t he a na l y s t t o s i nm l a t e aw i de va r i e t y o f r e a l- l i fe s i t ua t ions b y s i m p l y a d d i ng o rs ub t r a c t i ng o i l s p i ll s t o t he l a ke . T h e pos s i b i l i t y o f t o va r yt he in t e ns i t y o f the s p il l s due t o p r o pos e d r e m e dy a c t i onsi s a l s ~ an a t t ra c t i ve c a pa b i l i t y f o r e nv i r o nm e n t a le ng i ne e r s .

A C K N O W L E D G E M E N T S' l 'h e a u t ho r s w i s h t( ', e xp r e s s t he i r a c kno w l e dge m e n t t o t heF a c u l t a d d e I n g e n i e r i a o f t h e U n i v e r s i d a d N a c i o n a l d e lC u y o ( A r g e n - t in a ) a n d t o th e C O N [ C I T ( V e n e z u e l a ) f o r

Andrada, L. (1993) Llancan elo oil f ield, Pro< Of the XII ( 'ong.of Geology in Argentina Basins. V,~fl Ifl. (V Ra mo s Ett),October , Mendoza, Argent ina

Atlas, R.M. (1988) Biodegradat ion of hydrocarb ons m the envir-onment , in Environmental Biotechnolo~v ((;.S . O wen Ed),Plenum Press , New Y orkBanks R.B . (1974) Well mix ed cells mod el to calculate dissol-

ved oxygen and BOD in r ivers and lakes , ,1 ofb;ngineerm,~,Vol 12, UNAM, MexicoBrebbia C .A and Orszag S.A. ( 1987~ Modellm.e the plum e risedispersion, Spring er Verlag, Ber l inCo nn or J. and Brebbia C.A. ( I 978) Finite element techniques/or

f l u i d f l o w , Buttherworths , LondonErhardt M. (1983) Method s m sea ware," m~alys;s . SpringerVerlag, Berlinl ,evy A. (1981 ) Polyn uclear hydrocarb(ms in sediments , MarinePollution, No. 550, pp. 50-50I,ev y A. (1981a) Spectrofluorometric ana!ysis. Ah~rine P olltl-

tion, No. 549, pp. 39-53Leah y .I .G. and Colwel l R.R. (1990 ) Microbial degradat ion of

hydrocarbons in the environmem, .~ticrahiologtcal Review.Vol 4l,ick W. (1992) M od elling the transporl arid late c~mtam inantsin shal low bodies o f water , Proe . q/dze /nt . Cong. on Num.

Meth. in Eng . an d Appl. Sciences. Vol I I Concepc ion, Chi leMaure A. and Ir igo C. (1994) Environmental s tudies of theLlancanelo area, edit ed Internal Repor~ ?~o. 26. Faculty ~fl"

Engineer ing, Univ. Nac. del Cuy o, Mendoza, A rgentinaStreetcr H.W. and Ph elps E.B. (1958J A s tudy of the pol lut ionand natural purification Hf the ()hi~ river. Pubhc kfealthBulletin, No. 1 46, U.S. Pu bl ic Heal th Service , W ashington

Tho man n R .V. and M uel ler .I.A. (1987!~ Principles ~?]'stoJa(~ewater quali ty modell ing and control. Harper & Row Publ is-hers , New YorkZienk iewicz O.C. and Taylor R,L. (1991) 7he Finite Elemem

Method, M cGraw Hil l, 4th E di t ion, Swm~sea. UK

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