Water Resources Management (2006) 20: 109–132 DOI: 10.1007/s11269-006-4634-4 C Springer 2006 Modeling Leachate Contamination and Remediation of Groundwater at a Landfill Site I. K. TSANIS ∗ Department of Civil Engineering, McMaster University, Hamilton, Canada (Received: 17 June 2003; in final form: 21 March 2005) Abstract. Leachate contamination of an aquifer from a landfill site was simulated using the groundwa- ter flow and transport model SUTRA developed by the U.S. Geological Survey. The model calibration was performed by spatially adjusting the hydraulic conductivity in order to capture the measured hy- draulic head spatial variation and then by adjusting the dispersivity and porosity match the measured chloride plume. Based on the simulations it was found that without remedial action the contaminants in the existing leachate plume would remain above acceptable regulatory concentration levels for longer than 2010. The chloride loading of an adjacent stream exceeds acceptable levels under Ontario’s Reasonable Use Guidelines. Simulations indicate that a pump-and-treat system using additional two purge wells could remediate the leachate contamination within approximately 10 years from now. Key words: chloride loading, groundwater modelling, landfill, leachate contamination, remediation Introduction Solid municipal waste is one of the most pressing forms of pollution that society has created. The most prevalent form of disposal of solid waste is by interring the waste into a sanitary landfill that typically consists of a containment system, leachate collection and drainage system, numerous waste cells with daily cover, methane collection system, and a final cover. Unfortunately, sanitary landfills are not an ideal solution to the disposal problem of solid waste. Water, which infiltrates into the landfill through the top cover, saturates the waste and becomes laden with high levels of soluble contaminants which are released during the waste decomposition process. The leachate can contaminate groundwater resources in the immediate vicinity of the landfill by diffusive transport or direct infiltration through the landfill liner, and/or by failure of the engineered leachate collection system. Typical contaminants and concentrations found in landfill’s highly contaminated waters (leachates) are reported by Rowe (1994). This paper describes the application of the groundwater flow and transport model SUTRA with respect to simulating a leachate contamination problem at a landfill site. Following the introduction and the theoretical background of the simulation ∗ On leave, Department of Environmental Engineering, Technical University of Crete, Chania, Crete, Greece.
Solid municipal waste is one of the most pressing forms of pollution that societyhas created. The most prevalent form of disposal of solid waste is by interring thewaste into a sanitary landfill that typically consists of a containment system, leachatecollection and drainage system, numerous waste cells with daily cover, methanecollection system, and a final cover. Unfortunately, sanitary landfills are not anideal solution to the disposal problem of solid waste. Water, which infiltrates into thelandfill through the top cover, saturates the waste and becomes laden with high levelsof soluble contaminants which are released during the waste decomposition process.The leachate can contaminate groundwater resources in the immediate vicinity ofthe landfill by diffusive transport or direct infiltration through the landfill liner,and/or by failure of the engineered leachate collection system. Typical contaminantsand concentrations found in landfill’s highly contaminated waters (leachates) arereported by Rowe (1994).
This paper describes the application of the groundwater flow and transport modelSUTRA with respect to simulating a leachate contamination problem at a landfillsite. Following the introduction and the theoretical background of the simulation
∗On leave, Department of Environmental Engineering, Technical University of Crete, Chania, Crete,
Greece.
110 I. K. TSANIS
model SUTRA, the case study of the Brock West Landfill Site in Ontario, Canada isdescribed. The results of simulations for calibration and verification are presentedas well as those for a pump-and-treat groundwater remediation.
THEORETICAL BACKGROUND
The computer program SUTRA (Saturated-Unsaturated TRAnsport) developed bythe U.S. Geological Survey is capable of modelling non-homogeneous, anisotropicaquifers, steady-state and transient conditions, and fully saturated or saturated-unsaturated flows. The model assumes a stationary aquifer, and employs the flowequations developed by Bear (1972, 1979).
SUTRA employs a two-dimensional hybrid finite-element and integrated-finite-difference method to approximate the following equation for groundwater fluidmass balance:(
SwρSop + nρ∂Sw
∂p
)∂p
∂t+
(nSw
∂ρ
∂U
)∂U
∂t− ⇀∇ ·
[(kkrρ
μ
)· (
⇀∇ p − ρ⇀
g)
]= G̃
(1)
where Sw = the degree of saturation, ρ = fluid density [M/L3], Sop = specificpressure storativity [(L · t2)/M], p = pressure [M/(L · t2)], t = time [t], n = porosity,
U = general tem for concentration or temperature [M/M or ◦C], k = permeabilitytensor [L2], kr = relative permeability to fluid flow, μ = fluid viscosity [M/(L · t)],⇀
g= gravity vector [L/t2], and G̃ = mass flux per unit volume [M/(L3 · t)]. For solutetransport, the term U in Equation 1 represents the contaminant concentration as amass fraction of the total solution (kg(solute)/kg(solution)), and solute mass balanceis simulated using the expression
∂(nSwρc)
∂t= − f − ⇀∇ · (nSwρ
⇀vc) + ⇀∇ · [nSwρ(D∗
d I + D) · ⇀∇c]
+nSwρ�w + G̃c∗ (2)
where c = solute concentration [M/M], f = the volumetric adsorbate source –which is the gain of adsorbed species by transfer from fluid per unit total volume –[M/(L3 · t)],
⇀v= average seepage velocity [L/t], D∗
d = the bulk diffusion coefficient[L2/t], I = the identity tensor, D = the dispersion tensor [L3/t], �w = the solutemass source in the fluid due to production reactions [M/(M · t)], and c∗ = soluteconcentration of fluid sources. If the solute production, reaction, and adsorptioncomponents are ignored, Equation 2 simplifies to
∂(nSwρc)
∂t= −⇀∇ · (nSwρ
⇀vc) + ⇀∇ · [nSwρ(D∗
d I + D) · ⇀∇c] + G̃c∗ (3)
The dispersion tensor is symmetric, and is expressed as
D =[
Dxx Dxy
Dyx Dyy
](4)
MODELING LEACHATE CONTAMINATION AND REMEDIATION OF GROUNDWATER 111
where the elements are
Dxx =(
1
v
)(αLv2
x + αT v2y
)(5)
Dyy =(
1
v
)(αT v2
x + αLv2y
)(6)
Dxy = Dyx =(
1
v
)(αL − αT )(vxvy) (7)
where vx and vy represent the x- and y-components of the average velocity respec-tively, αL = longitudinal dispersivity [L], and αT = transverse dispersivity [L]. Voss(1984) indicates that SUTRA assumes a stationary solid phase; therefore, the av-erage velocity term used in SUTRA’s groundwater flow and contaminant transportequations is actually the average velocity of the fluid phase. All elements within thefinite-element mesh are quadrilateral. Pressures and concentrations are solved forthe nodes within the finite-element mesh, and velocities are solved for the centres ofthe elements based upon the results at the nodes. A full discussion of SUTRA’s nu-merical methods is provided in Voss (1984). SUTRA has been applied to simulatingseawater intrusion in coastal aquifers by various members of the U.S. GeologicalSurvey and is recommended by the ASCE (Quality of Ground Water, 1996).
Case Study: Brock West Landfill
SITE CHARACTERIZATION
The Brock West Landfill Site encompasses an area of 123 hectares, of which 64.4hectares are used for waste disposal. Operated by the Municipality of MetropolitanToronto since 1975, it was closed and decommissioned in 1995, after more than 11million tons of municipal solid waste was interred (Beach, 1994).
The site is located to the north of West Duffins Creek and immediately to the westof Ganatsekiagon Creek, a left-bank tributary of West Duffins Creek (Figure 1).These streams are at elevations of between 105 and 120 metres above sea level in thevicinity of the site. The landfill rises to an elevation of approximately 180 metresabove sea level with side slopes of between 2:1 and 3:1 (Dixon Hydrogeology,1997).
The landfill was constructed in the remnants of an aggregate quarry. During theconstruction, gravel and sand backfill was utilized to partially fill the quarry. Asubdrain consisting of perforated pipe, was installed below the liner along a north-south axis in order to allow the groundwater table to be lowered in the immediatevicinity of the landfill. The liner consists of 100 mm of compacted bentonite clay.Above the liner, a leachate collection system consisting of a header-lateral arrange-ment of perforated pipe was installed. The purpose of the leachate collection systemis to remove leachate produced from the landfill for treatment off-site and to pre-vent hydraulic mounding in the landfill. The waste cells are located directly above
112 I. K. TSANIS
Figure 1. Location of the Brock West Landfill.
the fill protecting the leachate collection system. A methane collection system islocated above the waste cells, immediately below the final cover of the landfill. Themethane collected in the gas collection system is provided to the Eastern PowerLimited Generating Plant, which converts methane into electricity for distributionto the power grid (Dames and Moore Canada, 1997).
The maintenance records of the site indicate that in 1991, increased hydraulicmounding was observed within the landfill (Dames and Moore Canada, 1994).Trouble-shooting procedures concluded that the leachate collection system hadfailed, preventing the removal of water from within the landfill. Modificationswere made to the methane collection system to limit the total height of the
MODELING LEACHATE CONTAMINATION AND REMEDIATION OF GROUNDWATER 113
hydraulic mound by pumping leachate out of the landfill through the main gasheader. Prior to completion of the modifications, records show that the hydraulicmound increased in height to above the level of the liner lip, resulting in aleachate spring into the aquifer along the southeast edge of the landfill. Levelsof contamination from the Brock West Landfill site measured during monitoringcontravened the levels allowable under the Reasonable Use Guideline (MOEE,1994).
HYDROGEOLOGY
The nearest climate station to the site is at Oshawa, where the average annualrecorded precipitation between 1980 and 1992 is 891 mm, of which 116 mm falls assnow (Dixon Hydrogeology, 1997). In the past three years, periods of low (667 mm)and high (1062 mm) precipitation have been recorded. According to Dixon Hydro-geology’s (1997) calculations, the average annual water surplus in a sandy loamregion typical of this area is 342 mm/year. Infiltration to the waste is expected tobe variable, depending on factors including cover placement and type, slope, andvegetation. Infiltration values in uncovered or daily covered waste would likelyexceed 200 mm/year and could exceed the average estimated water surplus (Sibulet al., 1977).
The landfill is situated in a partially buried valley located east of the glacialLake Iroquois shoreline bluff. The landfill occupies an area between the bluff anda till-cored island further east. The valley, which is cut into the Halton till, is par-tially infilled by approximately 30 m of unconsolidated Quaternary aged lacustrinedeposits. The buried valley is theorized to have been formed by subglacial ero-sion as found elsewhere in the area (Ostry, 1979). Deposition of coarse-grainedaquifer materials within the till, as identified under the central and eastern partof the landfill, is typical of formations associated with erosion due to such anevent.
Four main hydrogeological units are identified under the site. These units, fromthe surface down, are: (i) a sand and gravel aquifer containing the water table, (ii)a clay/silt-till confining unit, (iii) a gravel unit within the till unit in the southern,central, and eastern parts of the site, and (v) the shale bedrock of the Ordovician agedWhitby formation, primarily monitored at its contact with the overburden (bedrockcontact zone) (Ostry, 1979). Figure 2 shows a cross-section of the succession underthe site (Dames and Moore Canada, 1997).
The upper aquifer underlies all but a small portion of the landfill to the southwest.It is comprised of fine to medium sands with some silty areas. The unit is thickest(∼30 m) near the southern limit of landfill and thins to the west where the landfilldirectly overlies a clay and sand till. Narrow and shallow stringers of water-bearingsand have been identified in north-south trending valleys in the southwestern por-tion of the landfill site (Dames and Moore Canada, 1997) shown as in Figure 3. Theupper aquifer is under confined water table conditions under the lined portion of the
114 I. K. TSANIS
Fig
ure
2.C
ross
-sec
tio
no
fB
rock
Wes
tL
and
fill
Sit
e.
MODELING LEACHATE CONTAMINATION AND REMEDIATION OF GROUNDWATER 115
Figure 3. Site configuration of Brock West Landfill.
landfill and under confined artesian conditions to the extreme southeast. Both WestDuffins Creek to the south and Ganatsekiagon Creek to the east are incised with thisaquifer, which contributes base flow to these streams. The hydraulic conductivityof the upper aquifer is considered to be typical of similar sandy formations, rangingfrom 10−4 to 10−3 cm/s (Freeze and Cherry, 1979), based on data from various moni-toring and purge well construction reports (Dames and Moore Canada, 1991, 1994).The average hydraulic conductivity is estimated to be approximately 3.8×10−3 cm/s(3.3 m/day); however, values as low as 1.15 × 10−3 cm/s (1 m/day) have beencalculated using well pumping tests in the aquifer (Dames and Moore Canada,1994).
MODEL CONFIGURATION
The region of study consists of a region approximately 1300 m by 1000 m, en-compassing the entire landfill, with the southerly boundary corresponding with theportion of West Duffins Creek that is impacted by the pollutant plume. Figure 3 il-lustrates the region selected for modeling. In order to model the Brock West Landfill
116 I. K. TSANIS
Figure 4. Finite element SUTRA mesh.
site using SUTRA, the region must be represented by a finite-element mesh. Theirregular meshes of approximately 40 m × 40 m were selected, as shown in Fig-ure 4. The depth of each element was extrapolated from additional cross sectionsprovided by Dixon Hydrogeology (1999).
Constant head and constant concentration boundary conditions were selectedfor the modeling effort. The western (slopes to the north), northern (slopes to theeast), and eastern (slopes to the south) regions were modeled as having specifiedheads as determined from site measurements by Dames and Moore Canada (1997).Concentrations along these boundaries were set at the assumed background con-centration of 25 mg/l. Finer mesh was used to the remaining boundaries such as theaxis of the subdrain and at the purge wells where the nodes were forced to locate.General assumptions for the simulations included:
• Short-term transient effects were neglected, as the observed hydraulic headsrepresent long-term steady state conditions;
• The influence of domestic water supply wells within the aquifer was assumed tobe insignificant because the net withdrawal rate is low;
MODELING LEACHATE CONTAMINATION AND REMEDIATION OF GROUNDWATER 117
• The fluid density is assumed to be constant, without variation due to dissolvedsolids or temperature;
• SUTRA, a 2-dimensional confined aquifer model, could adequately model un-confined flow in an areal extent to serve as a decision-making tool at a planninglevel by assuming that the thickness of the unconfined aquifer could be repre-sented by the equivalent thickness of a confined aquifer.
• Infiltration of leachate through the landfill liner is modeled as a source at thenode. The SUTRA model assumes that each element is completely mixed andaverages the mass of solute injected at the node across the depth of the element.
• The change of leachate concentration with respect to time can be represented asa first-order function.
INFILTRATION
Recharge of the aquifer occurs due to both prevalent groundwater flow and infil-tration during storm events, because the groundwater aquifer that is being contam-inated by the Brock West Landfill site is an unconfined aquifer. SUTRA is unableto model an unconfined aquifer in an areal extent because it can not predict aquiferdraw-down (Voss, 1984). As a result, it has been assumed that a confined aquifermodel will provide adequate approximation to allow preliminary decision-makingsupport. In the confined aquifer model, infiltration is simulated by locating a fluidsource at each of the nodes composing the areal extent of the site, other than thatactually composed of the landfill. The area of the site is approximately 1318370 m2,of which 707636 m2 contributes to groundwater recharge through infiltration.The remaining region is the landfill, which contributes to groundwater pollution.With an estimated infiltration of 342 mm/year (Dixon Hydrogeology, 1997), thegroundwater recharge due to infiltration is approximately 1.03×10−8 m/sec per m2
area. These values represent a time-averaged estimate, as the actual precipitationand infiltration were observed to be highly variable (Dames and Moore Canada,1994).
LEACHATE PRODUCTION AND INFILTRATION
Dames and Moore Canada (1994) performed a complete water balance on thelandfill to determine the volume of leachate that infiltrates through the landfill liner.The reported results indicate that the total leachate penetration through the liner andthrough the failures of the pipes is approximately 44 m3/day (Dames and MooreCanada, 1994). After the leachate calibration, it is found that the 40% of source ofleachate was evenly distributed over the nodes that represent the landfill the rest ofthe 60% was concentrated over the lower west side of the landfill.
Studies by Rowe (1994) suggest that the strength of landfill contaminant de-creases with respect to time, due to mechanisms such as biological degradation,precipitation, or dilution, and may be approximated as a first-order exponential func-tion. In the case of a conservative solute, where no reaction occurs, the decrease in
118 I. K. TSANIS
concentration is a result of dilution in the waste due to infiltration through the finalcover. A landfill may therefore be modeled as a Continuously Stirred Tank Reactor(CSTR), with an instantaneous source of pollutant at t = 0.
C = Coe(−kt) (8)
where k = qo/Hr ; Hr = pMo/(ACo) and p = proportion of total mass of waste thatis contaminant of interest; A = area of landfill [L2]; Mo = total mass of waste [M];Co = representative source concentration [M/L3]; qo = rate of infiltration [L3/t];Hr = representative height of leachate [L].
Site observations indicated that the actual leachate concentration ranges from3500 mg/l to over 7000 mg/l (Dames and Moore Canada, 1997). The decay rate,k, was calculated by Kaasalainen (1995) to be 0.03. On the other hand, usingthe same methodology, Rowe (1994) estimated the decay rate for chloride to beapproximately 0.065. As the decay rate is strongly influenced by the infiltrationflow rate, differences in the estimation of the permeability of the clay liner canaccount for the discrepancies between Rowe (1994) and Kaasalainen (1995).
LEACHATE SUBDRAIN
In order to model the leachate subdrain, nodes were forced to locate at the axis ofthe subdrain, as shown on Figure 4. Since subdrain flow is not well-documentedat the site and has been shown to possess highly variable flow rates, a rough esti-mate of flow was obtained using a solute mass balance. Assuming a closed systemwith no solute losses due to mixing or dispersion, 44 m3/d of leachate (Dames andMoore Canada, 1994) at a concentration of assumed to be approximately 3500 mg/l(Rowe, 1994) penetrates the liner and enters the aquifer. Typical subdrain concen-trations of 280 mg/l have been observed at the site (Dames and Moore Canada,1994). Assuming that no pollutant is lost, the subdrain flow rate was estimatedusing a mass balance approach (Ci Qi = Co Qo). Solving for Qo ((44 m3/d ×3500 g/m3)/(280 g/m3)) results in a total estimated subdrain flow of 550 m3/d or6.3 × 10−3 m3/s.
PURGE WELLS
During routine monitoring of the landfill site, a plume was observed possessingunacceptably high levels of sodium chloride. This plume resulted from two factors:infiltration of leachate through the landfill liner; and failure of the leachate collectionsystem, resulting in hydraulic mounding in the landfill and a temporary leachatespill over the southeastern liner lip. The MOEE recognized that the temporaryleachate spill would result in elevated chloride concentrations that could potentiallycontaminate the West Duffin’s Creek. As a result, they issued an order pursuant toits authority under the OWRA requiring that the leachate plume be mitigated tolevels consistent with the Reasonable Use Guideline (137.5 mg/l).
MODELING LEACHATE CONTAMINATION AND REMEDIATION OF GROUNDWATER 119
Table I. Observed performance of extraction wells (Dixon
Hydrogeology Limited, 1999)
Well # Depth (m) Flow (l/min) [Cl−] (mg/l)
PW-1 38.7 76 699
PW-2 30.2 61 1254
PW-3 20.8 13 444
PW-4 29.0 13 273
PW-5 26.2 11 148
The Ontario Ministry of Environment and Energy (MOEE), based upon themonitoring reports issued as required, ordered the construction of purge wells in1991. Mass balances performed during hydrogeological surveys indicated that thecross-sectional aquifer flow in the direction of Lake Ontario was approximately600 l/min (Dames and Moore Canada, 1995). In order to reduce the potential con-tamination of West Duffins Creek, a total of six wells were specified, each of acapacity of 100 l/min. The well system was finally installed and placed into servicein 1994. All wells performed below the predicted yield, with the sixth well havingno appreciable yield. This well is maintained as a monitoring well only. Duringoperation from 1994 through 1998, occlusion of the well screens occurred due tomigration of particulate matter in the groundwater that further reduced the yieldsto unacceptably low values (Dixon Hydrogeology, 1999). The remaining five wellspossess flow rates as well as chloride concentrations for 1998 as tabulated in Table I.
Results and Discussion
Three sets of simulations were undertaken: one to calibrate the model; one to predictlong term natural attenuation of the leachate; and one to perform a sensitivityanalysis on the major parameters of SUTRA.
CALIBRATION
In order to calibrate the modeled data to the observed site data, a trial and errorprotocol was used to determine the best parameter values. The calibration wasundertaken in two parts. In part one, the cell thicknesses and hydraulic conductivitieswere adjusted spatially to calibrate the simulated head to the spatial field data. In parttwo of the calibration, adjustment of the longitudinal and transverse dispersivitieswas undertaken to calibrate the chloride plume concentrations to the observedconcentrations at the monitoring wells. Calibration was deemed to be satisfactorywhen no other combination of parameters provided were judged to provide a betterrepresentation of the field data.
Calibration of the model for contaminant transport was hampered by the pre-viously discussed leachate springs, which resulted in the release of a large volume
of chloride in a short period of time. Site records indicate that detailed monitoringdata is not available prior to this instantaneous source. In addition, no estimate ofthe volume of leachate spilled, or its concentration, exists. As a result, values forlongitudinal and transverse dispersivity have been selected, based on values thatare within the appropriate range on the scale of measurement as indicated byGelhar (1986).
SENSITIVITY ANALYSIS – HYDRAULIC CONDUCTIVITY
In order to determine the model’s sensitivity to hydraulic conductivity, a sensitivityanalysis was conducted by using the conditions pre-1994. Table II lists the parameterexamined, and the range that was examined.
Figure 5. Model sensitivity to hydraulic conductivity: a) 10−6 m/s, b) 10−5 m/s.
(Continued on next page)
MODELING LEACHATE CONTAMINATION AND REMEDIATION OF GROUNDWATER 121
Figure 5. (Continued)
Figure 5a illustrates the effect of decreasing the hydraulic conductivity to1.0×10−5 m/s. The most significant difference when compared to Figure 5b occurswith 110 m contour curve. The much lower hydraulic conductivity creates a localminimum hydraulic head in the lower portion of the subdrain. Both of 115 m and120 m contours rise further north and produce a sharper pointed shape of contours.This means more groundwater flows toward to subdrain horizontally. With the re-duction in the hydraulic conductivity, the effect of the subdrain on head is muchmore noticeable. Figure 5b illustrates the effects of increasing the hydraulic conduc-tivity to 1.0×10−4 m/s on the simulated head. This lowers the hydraulic heads in thesubdrain section. The subdrain becomes less effective with attracting groundwater.
STEADY-STATE HYDRAULICS WITH TRANSIENT SOLUTE TRANSPORT
Table III summarized the values used for the major model parameters. The simulatedand measured head profiles of the site under steady-state conditions are in goodagreement.
122 I. K. TSANIS
Table III. Partial list of model input parameter
Average Mesh Size 31 m × 31 m
Time step 2.62980 × 106 s (1 month)
Total Simulation Time 9.467280 × 109 s (300 years)
Number of Nodes 1389
Number of Elements 1313
Hydraulic Mode Steady-state
Solute Mode Transient
Longitudinal Dispersivity 10
Transverse Dispersivity 3
Hydraulic Conductivity 2.2 × 10−5
Density 1000 kg/m3
Porosity 0.30
Element Depth – Region 15
Element Depth – Landfill 10
Figure 6. (a) Simulated potential heads and (b) Simulated velocity field for Brock West Landfill
before the Purge Wells Installed (pre-1994).
(Continued on next page)
MODELING LEACHATE CONTAMINATION AND REMEDIATION OF GROUNDWATER 123
Figure 6. (Continued)
Figures 6a and b illustrate the simulated head and velocity profiles of the ground-water at the site corresponding to the optimized set of parameters where simulatedand measured head profiles are in good agreement prior to the installations ofpurge wells in 1994. The sub-drain intercepts the water traveling from the west,resulting in much lower velocities throughout much of the landfill than would oth-erwise occur. The greatest velocities are observed in the southwest, correspond-ing to regions of high hydraulic gradient. The average velocity is 71.8 m/yearwhich is slightly above the observed range of 1.1 m/year to 66.1 m/year (IWA,1994).
Figures 7a and b represent the simulated heads and velocity profiles after theinstallations of the purge wells by using the optimized parameters. The simulatedpotential heads as in Figure 7a shows the strong agreement with the observedvalues obtained from Dixon Hydrogeology (1999). Also the effect of the purgewells is clearly shown with the displacement to the north of the potential headcurves.
124 I. K. TSANIS
SIMULATION CONFIGURATION
In order to simulate the effect of the extraction wells, SUTRA was executed in threeseparate steps. In the first step, SUTRA was executed without the extraction wellsfor approximately 19 years, from the beginning of leachate contamination until theinstallation of the purge wells. The second step utilized the output of the first step asinput, with the addition of purge wells from 1994 to 2000. The third step introducedthe decay rate, k, to the leachate concentrations from the year of 2000.
SIMULATION
The reduction in concentration of a conservative substance occurs due to dilutionof the contamination in the aquifer. A simulation was undertaken using SUTRAto determine the plume concentrations for 1994, 1998, 2000, and 2010 after com-missioning of the landfill in 1975. The changes in the concentration contours with
Figure 7. Comparison of (a) Simulated and observed potential heads and (b) Simulated velocity
field for Brock West Landfill after the Purge Wells Installed (post-1994).
(Continued on next page)
MODELING LEACHATE CONTAMINATION AND REMEDIATION OF GROUNDWATER 125
Figure 7. (Continued)
respect to time provide insight into the behaviour expected at the site, and into theoverall pollution potential.
The year of 1994 is the end of the first step of simulation. Figure 8 indicates thatafter nineteen years of contamination in 1994, the plume possesses concentrationsin excess of the Reasonable Use Objective of 137.5 mg/l. More than 500 mg/l ofchloride concentration is simulated to attain the landfill boundary. Interception ofthe plume with chloride concentrations in excess of 137.5 mg/l by West DuffinsCreek will potentially result in the pollution of surface water.
PURGE WELLS
From 1994, a set of five purge wells is operated in the southern boundary of landfill toreduce the plume. Since the remediation system has not been operating as specified,simulations were undertaken to predict the performance of the system under actualconditions listed in Table I. Figure 9 displays the concentration profile of the siteafter four years of operation of the wells in 1998. Chloride in excess of 300 mg/l is
126 I. K. TSANIS
Figure 8. Simulated chloride plume in 1994.
simulated to be outside of landfill property limit and more than 1500 mg/l is foundwithin the purge wells area. Figure 10 shows the both simulated and observedchloride concentrations near the southern boundary. Chloride concentrations in thevicinity of 137.5 mg/l are predicted to enter the West Duffins Creek, causing apotential danger in the water quality. The chloride concentrations are in agreementat the purge well 2.
Figure 11 displays the simulated concentration contours in the year 2000. Thecontaminant concentrations are well above levels acceptable to the MOEE reason-able and the chloride concentrations in the purge well area has been increased. Thepurge wells prevent the plume concentration to increase significantly near the WestDuffins Creek.
Figures 12 and 13 show the observed and simulated chloride concentrationsrespectively in the five purge wells. Both the simulated and observed results showthat the maximum chloride concentration is at well PW-2 reaching a level of1200 mg/l with second the well PW-1 with concentrations varying between 600 mg/lto 800 mg/l. The simulated values for PW-3, PW-4, and PW-5 are lower than theobserved values.
From the year of 2000, the decay rate of the source concentration is introducedto the model as suggested by Kaasalainen (1995). Assuming the performance of the
MODELING LEACHATE CONTAMINATION AND REMEDIATION OF GROUNDWATER 127
Figure 9. Simulated chloride plume in 1998.
Figure 10. Comparison of simulated and observed chloride concentration in 1998.
128 I. K. TSANIS
Figure 11. Simulated chloride plume in 2000.
Figure 12. Observed chloride concentration in the purge wells.
five purge wells will not change, the plume reaching West Duffins Creek isgreatly improved in 2010 as shown in Figure 14. There is not chloride concentra-tion in excess of the Reasonable Use Objective of 137.5 mg/l appears reaching thecreek. Chloride concentrations over 300 mg/l is still predicted near the propertysouth boundary limit in 2010.
MODELING LEACHATE CONTAMINATION AND REMEDIATION OF GROUNDWATER 129
Figure 13. Simulated chloride concentration in the purge wells.
Figure 14. Simulated chloride concentration for Brock West Landfill in 2010 under current
conditions.
130 I. K. TSANIS
Assuming that the modelling assumptions are reasonable and prevail over theanalysis period, the Brock West Landfill Site is expected to remain a contaminantsource contributing to degradation of groundwater quality at least into the next 10years. Consequently, remediation of the site will be necessary to reduce the periodof time required for the site to reach contamination levels acceptable under MOEEguidelines. Further efforts are required to comply with the MOEE Reasonable UseGuidelines in an effective and timely manner.
ADDITIONAL EXTRACTION WELLS
Increasing the number of extraction wells helps the reduction of the contaminantconcentrations. The locations of the additional wells were chosen close to the areasthat showed the higher chloride concentrations. Both extraction wells were selectedto operate at a rate of 60 l/min. Figure 15 shows the locations of the two additionalextraction wells among the five original purge wells and the simulated chlorideconcentration contours in 2010. The Reasonable Use objective of 137.5 mg/l isattained at the property boundary. The maximum concentration of 600 mg/l is sim-ulated at the site which is significantly reduced from the maximum concentrationof 800 mg/l shown in Figure 14.
Figure 15. Simulated chloride concentration for Brock West Landfill in 2010 with additional
extraction wells.
MODELING LEACHATE CONTAMINATION AND REMEDIATION OF GROUNDWATER 131
Most site conditions are homogeneous owing to the complex variations in thesoil properties that occur. Under many conditions, particularly where soils withhigh hydraulic conductivity are involved, an assumption of mean parameters maynot be appropriate enough to predict the plume shape since hydraulic conductivityhas the most sensitive effect on the fluid flow. Since SUTRA is a 2D aquifer modelit simulates the average mass of solute source concentration over the entire depth ofthe element (fully mixed conditions in vertical). SUTRA can adequately model flowand pollutant transport in an areal extent and the results can serve as a preliminarydecision-making tool at a planning level. The modelling results are limited sincethe actual pollutant plume is three-dimensional in nature. A 3D pollutant transportmodel can be used for detailed 3D pollutant transport but will require for validationadditional data.
Conclusions
The groundwater flow and the leachate transport under the Brock West Landfill wasmodelled using the USGS simulation model SUTRA. It was found that groundwaterflow is most sensitive to the changes in the hydraulic conductivity and to a lesserextent to changes in infiltration and leachate infiltration flow. The model calibrationwas performed with field data of the measured chloride plume.
It was found that without remedial action the contaminants in the existingleachate plume would remain above acceptable regulatory concentration levels forlonger than 2010. The current design for the remediation system of five purge wellswill reduce the extent of the leachate plume but still will remain above the regu-latory levels. Remediation of the plume is predicted to have taken approximately35 years. Addition of two extraction wells installed between PW-2 and PW-1 andbetween PW-1 and PW3 will intercept the remaining section of the south leachateplume. Based on the simulations the remediation design will have a substantialimpact upon the plume within ten years from installation, with most of the plumeto be intercepted by the wells.
At this point, the information obtained from the Brock Landfill site is not suf-ficient enough to perform a complete simulation to match the recent field datapast 1999. Continuous monitoring and further chloride inflow information willprovide additional valuable information for the verification of the pollutant plumetransport simulations and the effectiveness of the remediation measures. This datashould be used for the 3D transport model that will take into the account the phreaticpart of the aquifer and the 3D nature of the flow.
Acknowledgments
The present work was financially supported by the National Science and Engineer-ing Research Council (NSERC) Grant RGP157914-02 and is part of the Mr. Li-FanSong’s M.A.Sc thesis.
132 I. K. TSANIS
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