Upload
others
View
1
Download
0
Embed Size (px)
Citation preview
82 COLORADO SCHOOL OF MINES QUARTERLY
Exploitation of Shale Hydration for
Leachate Control in Disposed Oil Shale
George H. Watson
Amoco Corporation
Naperville, Illinois
David B. McWhorter
Colorado State UniversityFort Collins, Colorado
Juergen Braun
Colorado StateUniversityFort Collins, Colorado
Earl D. York
Rio Blanco Oil Shale Co.
Denver, Colorado
INTRODUCTION
Combusted oil-shale wastes undergo cement-like
reactions that enhance the retention of water and reduce
thehydraulic conductivity ofembankments formed by the
disposal of such materials. This paper describes the
measurement of thechanginghydraulicproperties and the
manner in which those changes function to minimize
water movement and consequent generation of leachate
from embankments of disposed shale.
Production of liquid petroleum from oil shale at a
commercial scale will result in large volumes of retorted
shale. At least a portion of the waste will be disposed in
embankments that will cover large areas and may reach
thicknesses of several hundred meters. Important
environmental considerations are the rates, volumes, and
chemical characteristics of any leachate that may occur
from such embankments.
The potential for leachate generation derives from the
redistribution (drainage) ofwateremplacedwith the solids
and from infiltration that is in excess ofwhat canbe return
ed to theatmospherebyevapotranspiration. It isanticipated
thatwater will be added to the solidwaste for cooling and
dust control. In addition, process waste water may be
codisposedwith the solids.Anyquantityof thesewaters in
excess of what can be retained by physical and chemical
processeseventuallywill drain from theembankment.Net
infiltration (recharge) will slowly penetrate the em
bankment and eventually become leachate, also.
The studies reported herein show that the volume of
emplaced watermat can be retained in an embankment of
combusted shale is greatly increased by cementatious
reactions that occur. In addition, these reactions can be ex
ploited to create one or more layers with small hydraulic
conductivity that act to minimize net infiltration. These
features are quantitatively evaluated for the LURGI-
combusted shale.However, theexperimental and analytical
methodologies are applicable to other combusted shales
that undergo cementatious reactions.
MATERIAL PROPERTIES
Allexperimentalmeasurementswereperformedon fine-
textured,granular solid from theLURGIprocess followed
by combustion at about 700C to remove residual carbon.
The feed oil shale was from the Green River Formation in
thePiceanceBasin inwesternColorado. Figure 1 shows the
grain-size distribution as determined byMcWhorter and
Brown (1985).Thedominantconstituentsof thecombusted
shale are Si02 (34%),CaO (24%),MgO (8%), AI2O3 (7.5%),
Fe203 (3.3%), K2O (3.0%), andNa20 (2.3%), as reported byMarcus and others (1984). McWhorter and Brown (1985)
determined the specific gravity of particles less than 0.045
mm diameter to be 2.743. Particles larger than 2 mm have
a specific gravity of 2.728.
Upon formationofa shale-waterpaste, hydrationbegins
almost immediately as evidenced by the generation of
heat. During curing under saturated conditions, the paste
solidifies and takes on the appearance of a cement block.
McWhorterandBrown (1985) andMcWhorter (1987) found
that the solidified material retained only 3.6% water byweightafterdrying in a vacuumoven at 105C for 24hours.
In addition, the quantity of retained water was found to
depend on the water content at which curing took place.
Water retained at 105C decreased as curingwater content
decreased. The maximum of 3.6% was achieved only for
samples cured at saturation or very near saturation
(McWhorter, 1987).
SATURATED HYDRAULIC
CONDUCTIVITY
The observation that variable amounts of water were
retained, depending on the water content at which the
LEACHATE CONTROL IN DISPOSED OIL SHALE 83
PERCENT FINER BYWEIGHT
0.1 1.0
GRAIN SIZE (mm)
10.0
Figure 1.Grain-size distribution forLURGI-combusted shale
(McWhorter and Brown, 1985).
shale was cured, suggested that relevant hydraulic prop
erties might similarly be dependent on thewater content
during curing. Seven sets of duplicate samples were pre
pared and curedwith the aimofdetermining the influence
ofwater content during curing on subsequentlymeasured
hydraulic conductivity at saturation. Table 1 indicates the
manner in which the various groups of samples were
prepared and cured.Water contents of 10%, 15%, and 20%
byweightwere targeted for thegroup II samples.However,vapor adsorption proved to be much slower than
anticipated, and a final water content of about 1.5% was
achieved for all group II samples. The permeameter cells
containing group I and group II samples were stored in
containers in which relative humidity was controlled to
values corresponding to 0, 10%, 15%, and 21% water con
tents. Sri Ranjan (1989) explained how the appropriate
relativehumiditieswere estimated.Allgroup I and II sam
ples were weighed weekly to assure that water contents
remained unchanged during the 90-day curing period.
Following the 90-day curing period, all the samples
listed in Table 1 were vacuum saturated. Thewell known
falling-head procedure was used to measure hydraulic
conductivities at various times over a 216-day period.
Between measurements, the permeameter cells were im
mersed in water to maintain the materials in a saturated
condition. These cells were constructed with a rubber
sleeve in contact with the test material. External pressure
Table 1. Distinguishing features of samples cured for subsequent hydraulic conductivitymeasurements.1
Sample
Group Designation Sample Preparation and Curing Process
0A
0B
No water added. Samples exposed to dry atmosphere for 90 days.
II 1040A Water added by vapor adsorption from vapor-saturated nitrogen
1040B continuously flowing through sample. Target water content = 10%.
1540A Water added by vapor adsorption from vapor-saturated nitrogen
1540B continuously flowing through sample. Target water content = 15%.
2040A Water added by vapor adsorption from vapor-saturated nitrogen
2040B continuously flowing through sample. Target water content = 20%.
Ill 10A Water added"instantaneously"
by spraying and mixing to achieve 10%
10B water content.
15A Water added"instantaneously"
by spraying and mixing to achieve 15%
15B water content.
20A Water added"instantaneously"
by spraying and mixing to achieve 20%
20B water content.
1. All samples 1,400kg/m3
dry bulk density and cured in rubber-sleeved permeameter cells for 90 days.
84 COLORADO SCHOOL OF MINES QUARTERLY
was exerted on the sleeve to assure that no annular space
formed between the permeameter wall and the sample.
Figure2 shows the time trend forhydraulicconductivities
measured for the three groups of samples. Because data
from group II sampleswere indistinguishable from group
I data, the measurements for both groupswere plotted as
filled circles.The salient feature of thedata forgroups I and
II is the rapid decline of hydraulic conductivity, which at
216 days had declined to about 3 xIO-8
cm/s. Most of the
reduction occurred in the first 50 days of the test period. In
marked contrast to the group I and II samples, group III
samples showed only a slight decline in hydraulic con
ductivity. Immobilization of water resulting from
cementatious reactions are believed responsible for the
dramatic decline of hydraulic conductivity in the group I
and II samples.An armoring effect promoted by thecuringconditions is offered as an explanation for the markedly
di fferentbehaviorof thegroup III samples.The concepts of
immobilization and armoring effect are further discussed
below.
SATURATED HYDRAULIC CONDUCTIVITY (cm/s)10"
10_
10_
10'
10"
10"
""*K
20%
Group III
15%
^b-H-n 10%
MD
X
l\f\
Groups I & II
1
I>,,,,
50 100 150
TIME (days)
200 250
Figure 2. Saturated hydraulic conductivity of LURGI-
combusted shale.
IMMOBILIZATION OF WATER BY
CEMENTATIOUS REACTIONS
The observed three-order-of-magnitude reduction in
hydraulic conductivity suggests thatan increasingly largerfractionof theporespacebecomes incapableofcontributingto water flow as curing proceeds. However, practically all
thewater present during curing can be driven off at 105C(Brown andMcWhorter, 1985;McWhorter, 1987). Further
more,no significant changes inbulkvolumewereobserved
to occur during curing. These observations indicate that
the porosity of thematerial remained essentially constant,
and the large reduction of hydraulic conductivitymust be
attributable to immobilization of water.
A great deal of attention has been given to the various
states inwhichwatermay exist in cement pastes.McCarter
(1987)consideredwater incementpastes toexistascapillarywater and asgelwater, the latterbeingpartof thegel phase
responsible for the development of strength of the paste.
McCarter argued thatwater in the capillary stateprimarilyis responsible for the conductance of electrical current at
low frequencies. At high frequency, gel water contributes
to electrical conductance, andMcCarter exploited this fre
quency dependence toestimate the relative volume of gel
in curing cement pastes.
McCarter's (1987) work motivated the use of low-
frequency electrical conductance as a means ofmeasuringthemobilewater in thepresent study.Triplicate samples of
LURGI-combusted shale were prepared in electrical
conductance cells at a drybulk densityof 1,400kg/m3
and
a saturation of unity (Sa = 1 ). The cells consisted ofLucite
rings5.1 cm in diameter and 2.15 cm long. Aluminumdisks
were placed in contact with the shale on both ends of the
cell and held in place by a spring-loaded device designed
to maintain constant force on the disks. Annular spaces
formed by the walls of the rings, and the edge of the disks
were taped to suppress evaporation.Gases generated dur
ing curing could escape because the tape did not provide
a seal. A Hewlett Packard Model 4192A impedance ana
lyzer with a frequency range of 5 Hz to 13 MHz was used
tomeasure electrical conductance.Conductancemeasure
ments were made at frequencies of 100, 1,000, 10,000,
1 x IO5, and 3 x IO5 Hz (Sri Ranjan, 1989). Only the results
measured at 100 Hz are reported herein.
Themeasured electrical conductance (averageof triplicate
samples) is shown in Figure 3 as a function of time. Note
that electrical conductance declineswith time inmuch the
samemanner as for hydraulic conductivity. The decline of
electrical conductance is attributed to a decrease in the
volume of capillary (mobile) water as hydration proceeds.
It is expected that water contributing to electrical con
ductance is the same water that contributes to hydraulic
conductivity. The apparent similarity between the electric
and hydraulic results is not surprising, therefore.
Figure 4 depicts the volumes of air, water, and solid
before and after hydration. The cement-like reactions im
mobilize a portion of the total volume ofwater,but neither
the solid or air volumes significantly change. Immobilized
waterproperly cannotbeconsidered as solid because it can
bedrivenoffby the simpleprocessofraising the temperatureto 105C. Porosities measured prior and subsequent to
hydration are virtually identical.
LEACHATE CONTROL IN DISPOSED OIL SHALE 85
0.08
0.06
ELECTRICAL CONDUCTANCE (S/m)
0.04 -
0.02 -
20 30 40
TIME (days)
Figure 3. Electrical conductance of saturated shale.
50
Before Hydration After Hydration
Air (Va
;>> Mobile Water (Vm)
1'
',
',
'
Immobile Water|
I (Mm) '
I i I I
Figure4.Definition sketch forcharacterization ofwatercontents.
We define the apparent saturation, Sa, as the fraction of
theporevolumeoccupiedbywater.Theapparent saturation
is changed only by the transfer ofmobilewater across the
boundaries of the bulk element shown. The fraction of the
pore volume occupied bymobile water is denoted by Sm.
Based on Archie's (1942) formula relating electrical
conductance to porosity, Sri Ranjan (1989) wrote,
/ct,(U)\0-5
a)
where Sm(l, t) is themobilewater saturation atany time for
Sfl = 1. The corresponding electrical conductance is oc/.(l, 0,
and at(l, 0) is the electrical conductance for Sa = 1 and t =
0. Equation 1 permits themeasured electrical conductance
data to be interpreted directly in terms of mobile water
saturation. Figure 5 shows mobile water saturation as a
function of time as calculated from equation 1 using the
electrical conductance data shown in Figure 3.
MOBILE WATER SATURATION
Estimated data
Fitted curve
-i i i i i_
10 40 5020 30
TIME (days)
Figure 5.Mobile water saturation as a function of time.
The relationship shown in Figure 5 permitsestimationof
the mobile water saturation as a function of time under
conditionsofcomplete saturation (Sfl = 1 ). SriRanjan (1989)
wrote,
Sm(Sa,t) = SaSm(l,t) (2)
from which the mobilewater saturation can be calculated
foranyvalueofapparent saturation.Equation 2 isbased on
a generalization of the work ofWyckoff and Botset (1936)
and extensivemeasurements of electrical conductanceas a
functionof timeatvariousconstantvaluesof Sa (SriRanjan,1989). This result, together with the function Sm(l, t), as
depicted in Figure 5, has direct practical relevance, as will
be demonstrated subsequently.
THE CONCEPT OF ARMORING FOR
Sa LESS THAN UNITY
Group I and II samples, forwhich a dramatic decrease of
hydraulic conductivity was observed, were maintained
nearly dry prior to initiation of the measurements (see
Table 1). Therefore, little or no opportunity was availablefor hydration prior to the time the samples were saturated
86 COLORADO SCHOOL OFMINES QUARTERLY
for themeasurementofhydraulic conductivity. Hydration
occurredduring thecourseof themeasurementsand causedthe immobilization ofwater and theobserved reduction of
hydraulic conductivity.
In contrast, group III samples were exposed to constant
water contents of 10%, 15%, and 20% byweight for 90daysprior to initiation of the hydraulic conductivity meas
urements. Recall that Figure 2 shows only a slight decline
of hydraulic conductivity throughout a 216-day periodsubsequent to being saturated. It is hypothesized that the
90-day curing process at less than full saturation created ashell or armor around the solids that prevented further
hydration upon complete wetting.
Thearmoringhypothesiscanbeunderstood by referencetoFigure6,which shows theextentofwater immobilization
in fully and partially saturated elements of the combustedshale. Cementatious reactions and the products thereof
necessarily are limited to that portion of the pore space
occupied by water during curing. Because group III ma
terialswere less than fully saturated, the largerpore spacesoccupied by air during curingwere not greatly affected byhydration. In effect, the large pores and channels occupied
by air were"frozen"
into place during the curing process.The bounding surfaces of the space occupied by air werearmored by the reactions, and further hydration upon
subsequent saturation of the materials was effectivelyprevented. Thus, the hydraulic conductivity of group III
samples was not greatly affected during curing or sub
sequent thereto.
Cured at Sa = 1 .0 Cured at Sa = 0.5
Figure 6. Fraction of pore space occupied by gel phase after
curing at two different saturations. Fraction of void volume
available for flowupon subsequent saturation is (a) 0.1 in. and
(b) 0.55 in.
The tendency for combusted shale cured at less than full
saturation to become armored and to freeze into place the
pore space contributing to large hydraulic conductivityhas important practical implications. Combusted shale
placed in an embankment at water contents less than full
saturation cannot be expected to undergo the large re
duction ofhydraulic conductivity observed in group I and
II samples.Apparently, a criticaldegreeofsaturation exists
thatmustbeexceeded toachievefull reductionofhydraulic
conductivity. It is to be noted that about 65% of the pore
space of samples 20A and 20B was occupied by water
during curing. Evidently, this degree of saturation is not
adequate toachievefull reductionofhydraulicconductivity.
Subsequent studies showed that the critical saturation is
about85%.At saturationsgreater than85%, little likelihood
exists thatcontinuous, interconnected air-fil led poresexist.
Ways in which the critical saturation and associated re
duction ofhydraulic conductivity can be achieved to form
low-permeability layers in shale embankments are dis
cussed in a subsequent part of this paper.
DRAINAGE OF EMPLACED WATERS
As mentioned previously, one mechanism by which
leachatepotentiallycanbegenerated isby internaldrainageofwatersplaced in theembankment at the time ofdisposal.
Thewatercontentsatwhichdisposal embankmentswillbe
constructed under commercial operations remain to be
established. However, it is unlikely that emplaced water
contents will exceed 20% byweight and, probably, will bemuch less. Any emplaced water in excess of what can beretained by immobilization and by capillarity eventuallywill appear as leachate.
Waters placed in the embankment with the shale will
tend to slowly drain toward the bottom of the pile. Figure
5 and equation 2 indicate that 90% ofemplacedwaterswill
become immobilizedwithin 20days.Twentydays isa brieftime in relation to the time required for significantdrainage
to occur at water contents of 20% or less. Therefore, esti
mation of the quantity of drainable water can bemade byassuming that waters remaining mobile subsequent to
hydration are theonlywaters available fordrainage.All or
a portion of themobile waters will be retained within the
embankment by capillarity, as is demonstrated in the
following example.Suppose LURGI-combusted shale is emplaced in 5-m
lifts at an average drybulkdensityof 1 ,400kg/m3
and 15%
water content (dry weight basis). These figures convert toan emplaced saturation of Sa = 0.5. The total volume of
water emplaced in each lift is 1 .05m3/m2. After 20 days of
curing, the saturation of mobile water will be 0.05 (from
Figure 5 and equation 2). Hence, the volume of potentiallydrainable water in each lift 20 days after emplacement is
0.105 m3/m2. The potentially drainable water is capillarywater, water that is free to drain until mechanical equi
librium is achieved.
Because the shale will be cured at Sa = 0.5, the armoringphenomenon previously described will occur. Capillarywater will tend to drain over a very long time period.
LEACHATE CONTROL IN DISPOSED OIL SHALE 87
During this period of redistribution, relevant hydraulic
properties will be time invariant because of the armoringthatoccurred during curing. In particular, the relationshipbetween capillary pressure and saturation of capillarywaterwill be approximately that of the unhydrated shale.
A popular equation relating the saturation of capillarywater to capillary pressure is the Brooks-Corey (1966)
relation:
c V(3)
where Sc is the saturation of capillarywater, hc is capillarypressure, hd is displacement pressure, and X is the pore-
sizedistribution index. For theLURGI-combusted shale,
hd = 276 cm of water, and X = 0.44.
Redistribution of capillary water will continue until
mechanical equilibrium prevails. There exists amaximum
volume of capillary water that can be retained atmechan
ical equilibrium. If the total volume of capillary water
(mobilewater after hydration is complete) is less than the
maximum volume retained at mechanical equilibrium, no
leachate will result from internal drainage.
At mechanical equilibrium, the capillary pressure head
is distributed linearly with elevation according to,
hc = Z + hc(0) (4)
where Z is measured upward from the base of the em
bankment.Thevalue ofhc(0) is taken tobe zero, a condition
corresponding to a coarse-grained foundation material
upon which the embankment rests. Then the maximum
volume ofcapillarywater that canbe retained perunit area
at mechanical equilibrium is,L L
Vc= (?jScdZ = ^Scdhc + (?hd (5)
0 hf
in which L is total height of the embankment, and (j> is
porosity. Evaluation of the integral in equation 5 using
equation 3 gives,
V =^Jt[(t-) -*]. k*1 (6)c 1-a.L
h/J
To determine the volume of capillary water that
eventually will drain from the pile (if any), the volume of
mobilewater remaining after hydration iscompared with
the volume computed from equation 6. If the volume of
mobile (capillary) water is greater than Vc, the difference
eventually will appear as leachate. Otherwise, no leachate
resulting from internal drainage will be generated.
Suppose the embankment in the example consists of 40
lifts of 5 m, for a total height of 200 m. Then equation 6
indicates that amaximum 22m3/m2 ofcapillarywater can
be retained at mechanical equilibrium. The volume of
mobilewater remaining in each lift after 20 days of curing
is 0.105m3/m2, for a total of 4.2m3/m2 in 40 lifts. Because
the volume of mobile water is less than what can be re
tained at mechanical equilibrium, it is concluded that no
leachatewillbegenerated from thisexample embankment
by themechanismof internal drainage. Furthermore, there
isa "factorofsafety"
of5, avalue sufficient toaccommoda te
a great deal of uncertainty in the parameters used in the
computations.
The importanceof immobilizationofwaterbyhydration
warrants emphasis. If immobilization did not occur in the
example embankment, the total volume of capillary water
would be 42m3/m2. Only 22m3/m2
would be retained at
mechanical equilibrium, and 20m3/m2
eventually would
drain from the embankment as leachate.Thus,hydration is
instrumental in preventing the generation of leachate in
this example. It is expected to be similarly important in
actual field disposal operations.
In closing this section, an important difference is noted
between those waters retained in the shale by immobil
ization and those retained by a balance between pressure
gradient and gravity (i.e., mechanical equilibrium). The
formerare incapableofbeingdisplaced, forexample, as the
result of net infiltration into the embankment. In contrast,
the latterwaters are free tomove shouldmechanical equi
libriumbe disturbed or fail to develop as a result of inflowof net infiltration. Therefore, it is important that net in
filtration beminimized so that any capillarywater will be
retained within the embankment.
CREATION OF
LOW-PERMEABILITY LAYERS
It isclear from the experimental results that the armoringeffectwill prevent full reduction ofhydraulic conductivityif curing takes place at a saturation below some critical
value. Sri Ranjan (1989) speculated that the critical value
should be about Sa = 0.85. It is likely that the bulk of thedisposal pile will be emplaced with water contents in the
range of 5% to 20% by dry weight. Thus, the desirable
effects of hydration on hydraulic conductivity are not
likely to be realized in the bulk of the pile.
On theotherhand, low-permeability layerscanbecreated
by intentionally effecting saturated or near-saturated
conditions at strategic locations (for example, layers near
the bottom and top of the embankment). To prevent
armoring, the saturated or near-saturated condition must
be created before the shale is exposed to saturations less
than the critical value for any significant time. This was
easily accomplished in the experiments described above
simplybymixingdry shalewithwater toachieveconditionsfavorable for maximum hydration. However, premixingshaleandwater toessentially saturatedconditions followed
88 COLORADO SCHOOLOFMINES QUARTERLY
by placement on the embankment likely seems infeasiblefrom the standpoint of materials handling. A possible al
ternative is to placeoneormore liftsofpracticallydry shaleand then create saturated or near-saturated conditions byapplication of water.
Application of water, for example by irrigation, to thesurface of a lift of dry shale will result in saturation valuesthat range from zero below the advancingwetting front to
near unity at the surface. Shale at any arbitrary point in the
irrigated lift gradually will wet up as the wetting front
passes the point in question. Thus, the shale at this point
will beexposed to saturations less than thecritical value for
some period of time. We define the exposure time as the
time interval between first arrival of water at a point and
the time at which critical saturation is achieved. Exposure
time is zero for shale on the surface of the irrigated lift and
increaseswith depth. Shale at depthswith exposure times
greater than about two or three days are expected to
experience significant armoring and less than complete
reduction of hydraulic conductivity.
Sri Ranjan (1989) solved the differential equation for
infiltration using hismeasured time-dependent hydraulic
conductivity function. On this basis he predicted that all
pointswithin 70 cm of the shale surface would reach a sat
uration of 0.85 ormore within two days after initiation of
irrigation. Thus, a 70-cm layer of fully reduced hydraulic
conductivity (3 x 10-8cm/s) was predicted. Due to the
armoring effect, shale at depths greater than 70 cm would
not experience full reduction in hydraulic conductivity.
Figure 7 shows Sri Ranjan's predicted results.
140DEPTH OF PENETRATION (cm)
S - 0i
120
100
X q 0 P5
80
f* S*?'
txposure time = z daysm
/1 cm
60
/ y^ X. = 0.30
40 /y h^
= 331 cm
20
, , i i i i . i _i 1 1
"0 1 2 3 4 5 6
TIME (days)
Figure 7. Depths to Sa = 0 and Sa = 0.85 as function of time.
Braun (1990) conducted experiments to measure the
thickness of the layer of fully reduced hydraulic con
ductivity.Vertical columnsofair-dry shalewere infiltrated
by water until the infiltration rate declined to a small,
approximately constant value. Water was introduced at
the top of the column at practically zero pressure. Figure 8
shows the rapid decline of the infiltration rate into three
0.001
1E-07
600 800
timt. hours
1000 1200 MOO
-*-
column 1 ? column 2 HB-column 3
Figure 8. Infiltration into columns at room temperature.
LEACHATE CONTROL IN DISPOSED OIL SHALE 89
replicate columns. The decline is attributable both to re
duction inhydraulicgradientand to reduction inhydraulic
conductivitydue to hydration.
Braun's columns were constructed of stacked, 5-cm
sections.Following the infiltrationexperiments, the sectionswere separated, and saturated hydraulic conductivity of
thematerial in each sectionwasmeasured. Themeasured
distributionofhydraulic conductivity along thecolumns is
shown inFigure 9. Indeed, thehydraulic conductivity pro
file shows an interval of low hydraulic conductivity,
followed by a sharp increase. These data indicate that a
layerabout 90 cm thickwas formed inwhich the hydraulic
conductivitywas fully reduced.
water and, therefore, the thickness of the low-permeabilitylayer. To investigate this possibility, Braun (1990) again
conducted infiltration experiments, this time into columns
maintained at about 80C. Figures 11 and 12 show the dis
tributions ofhydraulic conductivity and volumetricwater
content, respectively, observed in those experiments.
Indeed, the thickness of the interval of low hydraulic con
ductivity was reduced to about 20 to 30 cm.In summary,Braun (1990)observed in room-temperature
experiments the creation of a 100-cm interval of fullyreduced hydraulic conductivity,belowwhich existed a 40-
cm transition zone. The average hydraulic conductivities
in these two intervalswas found to be 3.3 x 10-8 cm/s and
3* column 1 t column 2 ? column!
Figure9. Saturated hydraulic conductivity as obtainedby infiltration at room temperature.
The volumetric water content in each 5-cm segment of
thecolumnwasmeasuredprior to conducting thehydraulic
conductivitymeasurements.Figure 10 showswater-content
distribution measured at the cessation of the infiltration
experiments.Note that thewater content from 0 to 90 cm is
high (above 0.45). Awater content of 0.45 corresponds to a
saturation of 0.85. Thus, the interval of fully reduced saturation corresponds to the interval in which the water
saturation exceeds 0.85, as predicted by Sri Ranjan (1989).The thickness of the layeroffully reducedhydraulic
conduc-
tivity observed by Braun (1990) is somewhat greater thanthe 70 cm predicted by Sri Ranjan.Retorted oil shale likely will be emplaced in a disposal
pile at a temperature well above the room temperature at
which the above experiments were conducted. Should a
higher temperature cause hydration to proceed more
quickly, one would anticipate that the rapid reduction of
hydraulic conductivity might reduce the penetration of
56 x IO-8cm/s, respectively. Similar experiments at 80C
(approximately) resulted in a 24-cm interval with average
hydraulic conductivity of 5.0 x IO-8cm/s. In the 17-cm
transition zonebelow, averagehydraulic conductivitywas
found to be 22 x IO"8cm/s.
ASSESSMENT OF PILE HYDROLOGY
The pressure of one or more low-permeability layers
within a pile of retorted shale is expected to influence the
rate of water movement within the pile. In particular, a
low-permeability layer located immediately below a layer
of topsoil is expected to significantly decrease the quantityof infiltrating waters passing into the interior of the pile.
This expectation is predicted on the fact that potential
evapotranspiration exceeds average annual precipitation
in the oil shale regions of Colorado, Wyoming, and Utah.
A low-permeability layer would function to impede
90 COLORADO SCHOOL OF MINES QUARTERLY
140
column 1 4 column 2 -S- cokjmni
Figure 10.Water content after infiltration at room temperature.
j-**-
column 1 * eolimn 2
Figure 11. Saturated hydraulic conductivity as obtained by infiltration at elevated
temperature.
downward percolation from the root zone during wet
periods, thus making more water available for evapo-
transpiration during subsequent periods of water
deficiency.
Braun (1990) used theHydrologic Evaluation ofLandfill
Performance (HELP II) model to evaluate the degree to
which a low-permeability layer functions to minimize
deep percolation into the pile interior. It is beyond the
scope of this paper to describe themodel in detail. Briefly,
themodel iscapableofcomputingwatercontent andwater
flux in a waste pile composed of several layers, each with
different physical properties. Themodel receives as input
precipitation on a daily basis. Surface runoff andevapo-
transpiration are computed on a daily basis. Basicallytrw
LEACHATE CONTROL IN DISPOSED OIL SHALE 91
X< column 1 column 2
Figure 12.Water content after infiltration at elevated temperature.
model is a water-budget routine with a mechanism for
calculating flow from layer to layer. Such flow iscalculated
fromknowledgeof thehydraulicconductivityof the layers
as a function ofwater content.
Braun (1990) simulated disposal-pile hydrology on a
dailybasis for a simulation period of20years. Theprimaryoutput of interestwas the long-term average rate ofwater
flux into the pile interior. Precipitation data from Grand
Junction and LittleHills,Colorado,were used to establish
the statistical parametersofprecipitation in thearea. These
statistics then were used by the synthetic precipitation
generator to calculate daily precipitation for the Tract C-asite. Long-term mean precipitation values of 360 mm/yr
and450mm/yrwereused.Evapotranspiration iscalculated
in the model based on water availability in the root zone.
The degree to which a low-permeability layer affects
deep percolation into the pile is shown in Figure 13. The
uppermostcurve representsdeeppercolationasa function
of topsoil depth without a low-permeability layer. The
intermediate curve was calculated for a low-permeabilitylayer only 25 cm thick with a saturated hydraulic
conductivityof6.0 x10-8
cm/s. The lowermost curve is for
a 100-cm thick layer with a hydraulic conductivity of
3.3 xlO"8 cm/s.
It is observed that the presence of the low-permeabilitylayer has the greatest effect when topsoil depth is small.
With deeper topsoil, all three systems perform practically
the same. This is because the greater depth of topsoil pro
vides sufficient storage to accommodate essentially all
incoming water. Even during wet periods, deep topsoil
stores excess moisture for subsequent evapotranspiration
during a dry period.With thinner topsoil, the underlying
low-permeability layer is responsible for minimizingdrainageofroot-zonewaters inexcessof thenatural storage
capacity. Thus, the low-permeability layer functions to
hold up excesswater inwet periods so that it canbe subse
quentlyconsumedbyevapotranspiration.Atgreater topsoil
depths, very littlewaterexistsinexcessofroot-zone storage
capacity, and the low-permeability layer plays a more
minor role.
Of course, the greater the amount of precipitation, the
greater is the importance of the low-permeability layer.
The data in Figure 13 were calculated for average annual
precipitation of 360mm/yr. Figure 14 shows the compu
tations for average annual precipitation of 450mm/yr, all
other parameters being equal to the previous case. Notethat the low-permeability layer significantly reduces deeppercolation over a greater range of topsoil depths.
CONCLUSIONS
Cementatious reactions between water and combusted
oil shale immobilize as much as 90% of available water
within a period of 50 days. This phenomenon, coupled
with capillary storage, virtually precludes leachateproduction as a result of internal drainage of emplaced
waters (for example, dust-control water, water used for
cooling, process-waste water).
92 COLORADO SCHOOL OFMINES QUARTERLY
20 25
DEPTH OT TOPSOL (H)
cose 1 cose 4 cose 5
Figure 13. Percolation into the pile from the bottom of different barrier-layer types
at average precipitation.
20 25
DEPTH OP TOPSOL (N)
cose 2 case 3 case 6
Figure 14. Percolation into the pile from the bottom of different barrier-layer types
at increased precipitation.
The cementatious reactions also result in about a three-
order-of-magnitude reduction in hydraulic conductivity.
However, this reduction is achieved only if the combusted
shale is cured atwater saturations equal to or greater than
about 85%.
Conditions favorable for full reduction of hydraulic
conductivity can be achieved in a25- to 100-cm interval by
irrigating the surface of a lift of dry shale. The 25-cm
thicknesswill resultwhen the shale isabout80C.At room
temperature, thehydration process is slower, and a 100-cm
layer of reduced permeability is achieved.
A low-permeability layer located immediately below a
topsoil layer functions to hold upwater thatwouldother
wise drain from the root zone into the interior of the pile.
Waters thus prevented from rapid drainage are subse
quentlyconsumed byevapotranspiration.Theimportance
of the low-permeability layer in minimizing deep
percolation diminishes as topsoil depth increases.
LEACHATE CONTROL IN DISPOSED OIL SHALE 93
REFERENCES
Archie, G.E., 1942, The electrical resistivity log as an aid in
determining somereservoircharacteristics:AIMETransactions,Petroleum Division, v. 146, p. 54-61.
Braun, J.M., 1990, Hydrology of LURGI-combusted oil shale
disposal piles: Ft. Collins, Colorado State University, M.S.
Thesis.
Brooks, R.H., and Corey, AT., 1966, Properties of porousmedia
affecting fluid flow:AmericanSocietyofCivilEngineers,Journalof Irrigation and Drainage Division, v. 92, no. IR2, p. 61-88.
Marcus, D., Sangrey, D.A., and Miller, S.A., 1984, Effects ofcementation process on spent shale stabilization: Paper pre
sented atSME-AIMEAnnualMeeting,LosAngeles,California.
McCarter, W.J., 1987, Gel formation during early hydration:
Cement and Concrete Research, v. 17, p. 55-64.
McWhorter, D.B., 1987, Retardation of flow in oil shale residues
affectedby in situ hydration, in Proceedingsofthe 1987EasternOil Shale Symposium: University of Kentucky, Institute for
Mining and Minerals Research.
McWhorter,D.B., and Brown,CO., 1985,Adsorption and flowof
water in nearly dry LURGI retorted oil shale: Report to Rio
Blanco Oil Shale Co., 94 p.
Sri Ranjan, R., 1989, Water movement in LURGI-combusted oil
shale affected by time-dependent hydraulic properties: Ft.
Collins, Colorado State University, Ph.D. Dissertation 127 p.
Wyckoff, R.D., and Botset, N.G., 1936, The flow of gas-liquid
mixtures through unconsolidated sands: Physics, v. 7
(September), p. 325-345.