Leachate modeling for a municipal solid waste landfill for upper ...

KSCE Journal of Civil Engineering (2010) 14(4):473-480 DOI 10.1007/s12205-010-0473-1

Environmental Engineering

www.springer.com/12205

Leachate Modeling for a Municipal Solid Waste Landfill for Upper Expansion Jee-Eun Min*, Meejeong Kim**, Jae Young Kim***, In-Sun Park****, and Jae-Woo Park***** Received February 4, 2009/Accepted December 10, 2009

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Abstract This study sought to predict leachate levels within a municipal solid waste landfill and evaluate design alternatives for landfill expansion using the U.S. Geological Survey modular flow model with new flow and contaminant transport packages (MODFLOWSURFACT). This was used to overcome the drawbacks of the more widely used Hydrologic Evaluation of Landfill Performance (HELP) model in predicting leachate levels and movement within a landfill. The valley landfill in this study had two imminent issues regarding leachate management. One was its current high leachate levels, and the other was the selection of a design option for the bottom of the expanded landfill above it. According to the model calibration, relatively high leachate levels in the current landfill were attributed to unsatisfactory leachate removal efficiency by the leachate collection system. Additional leachate pumping was necessary to prevent possible leachate level increase after the landfill expansion over the current landfill. Based on numerical investigations of the design options of the expanded landfill bottom, the separation of leachate generated in the upper (expanded) landfill from the lower (existing) one is recommended, indicating that a low permeable layer such as a liner is an essential component. MODFLOW-SURFACT can be successfully used to calibrate landfill properties and predict hydraulic performance with given landfill conditions, particularly for a valley landfill. Keywords: numerical simulation, leachate management, landfill expansion, liner system, design option ···································································································································································································································

1. Introduction Leachate management has been an issue of great concern for Municipal Solid Waste (MSW) landfills all over the world. To estimate leachate generation properly, dynamic simulation models, such as the Hydrologic Evaluation of Landfill Performance model (HELP), have been applied in numerous cases (Cortazar and Monzon, 2007; Schroeder et al., 1994; Berger, 2002; Chanthikul et al., 2004; Demirekler, 2004). The HELP is a model used to design waste landfills and evaluate the hydrological performance of the landfills to determine details of engineering components, such as liners, leachate and runoff collection systems, surface slope, and subsurface drainage. A typical landfill may be represented as a set of profiles in the HELP. Water balance in a landfill is calculated separately for each sub-region represented by a single profile, and total water balance is calculated. This framework is, however, not suitable for simulating leachate movement and distribution within a landfill. Leachate flow inside landfill can be treated as subsurface flow and thus the leachate movement and levels within a landfill can

be simulated in groundwater flow models. However, groundwater flow models are not designed to represent details of engineering components of a landfill readily. Despite these shortcomings, their capability of simulating subsurface flow is very useful in leachate flow modeling within and near a landfill during and after landfill operations. This enables a comparison of alternative design options for landfill design and helps to select the economical and better-engineered option. The sanitary MSW landfill in this study is approximately 45 hectares in size and is located at a valley near a metropolitan city in South Korea. The landfill has been in operation since 1990. The local government plans to expand the existing landfill by a height of 10~50 m and by an area of 40 hectares after the capacity of the current landfill is filled in the near future. This, of course, was not the original plan when the local government planned and built the current MSW landfill. Due to several problems including “Not in My Backyard (NIMBY)” attitudes of residents near newly-proposed landfill areas, expansion of the current landfill became the only viable option. Recently, two critical issues were raised regarding leachate management of this landfill. These were due to the imminent

*Member, Researcher, Dept. of Civil and Environmental Engineering, Hanyang University, Seoul 133-791, Korea (E-mail: [email protected]) **Researcher, Dept. of Civil and Environmental Engineering, Hanyang University, Seoul 133-791, Korea (E-mail: [email protected]) ***Member, Professor, Dept. of Civil, Urban, and Geo-systems Engineering, Seoul National University, Seoul 151-742, Korea (E-mail: jaeykim @snu.ac.kr) ****Member, Researcher, Dept. of Civil and Environmental Engineering, Hanyang University, Seoul 133-791, Korea (E-mail: [email protected]) *****Member, Professor, Dept. of Civil and Environmental Engineering, Hanyang University, Seoul 133-791, Korea (Corresponding Author, E-mail: [email protected]) − 473 −

Jee-Eun Min, Meejeong Kim, Jae Young Kim, In-Sun Park, and Jae-Woo Park

compliance required for new leachate level criteria in MSW landfills, which was to be effective in Korea soon. One issue was to lower the leachate levels of the current landfill to comply with the newly-set government regulation, namely 5 m during landfill operations and 2 m after landfill operations. The other issue was to select the design option of the border layer between the existing and the newly-added upper landfills to help effectively comply with the new regulation. The objectives of this study were, therefore, to: 1) evaluate the hydrological performance of the current landfill in this study, 2) to predict leachate levels of the existing lower and new upper landfills in the event of the construction of a new landfill above the existing one, and 3) help to design the border layer between the existing and the expanded landfills using a numerical model. For the numerical simulation and investigation of temporal and spatial distribution of leachate levels, a groundwater flow model MODFLOW-SURFACT was used. This is a three-dimensional flow model based on MODFLOW and can be used for modeling both saturated and unsaturated water flow. Other concerns of landfill expansion, such as structural stability and so on, were beyond the scope of this study.

2. Site Description The existing landfill in this study is currently in operation, and it is expected to be filled by the year 2009. Average annual waste volume and intermediate cover soil in the landfill amount to 6.8×105 and 1.5×105 m3, respectively, according to the available record over the past 10 years, from 1991 to 2000. The existing landfill consists of two sectors, each being approximately 27 and 20 hectares, respectively. Polyethylene (PE) and High Density Polyethylene (HDPE) liners were installed in sectors 1 and 2, respectively. A major difference between sectors 1 and 2 is the location of the leachate collection pipes. The pipes were placed in the waste layer in sector 1 and in the drainage layer in sector 2, respectively. Municipal solid waste, amounting to 1.0×107 m3, has been put in the landfill with 2.2×106 m3 of intermediate cover soil as of the year 2000. The height ranges between 20~30 m as of the year 2005 and is expected to reach 20~45 m at the landfill completion. Average annual leachate volumes collected through the collection system amounted to 3.5×105 m3. Leachate levels in the landfill were measured once in 2005 at 105 wells covering the landfill area. Observed leachate levels showed a wide range of 4~20 m, even reaching approximately 70% of the landfill height in a certain area of sector 1. These high leachate levels can be attributed to the characteristics of valley landfills. Valley landfills usually have high landfill heights, and effective drainage of precipitation inflowing through the sides of landfill may not occur due to the difficulty in the drainage layer installation. Moreover, Korean food waste contains high water content due to Korean food culture. Since a complete ban of direct filling of food waste in the landfill began only in 2005, the high water content of food waste could have contributed to the high leachate levels in this

landfill. The landfill height is to be increased above the existing landfill (sectors 1 and 2), and a new landfill area (sector 3) is to be constructed as illustrated in Fig. 1.

3. Numerical Analysis 3.1 Conceptualization of a Landfill in MODFLOW-SURFACT In MODFLOW-SURFACT, fractions of precipitation contributing to runoff and evapotranspiration are not considered, while those can be calculated from precipitation, temperature, and Surface Curve Number (SCN) in the HELP. Thus, to determine the inflow from precipitation into a landfill, the recharge rate option should be used in MODFLOW-SURFACT. Empirically 20~40% and 40~70% of total precipitation infiltrate a landfill before and after landfill closure, respectively. In this study the recharge rate was estimated along with other hydraulic properties during model calibration. The recharge rate estimated in this manner is consequently not limited to precipitation, and includes any form of water input such as the water content of food waste or runoff from a mountain side due to an ineffective drainage system. The leachate collection and removal system cannot be simulated in MODFLOW as in the HELP, and thus the drain option was used. With this drain option, there is no general formulation to calculate conductance because drainage could be formulated in various forms depending on the specific drainage system within MODFLOW. The conductance value was then adjusted during the model calibration to match measured leachate volumes to the values from the model. Since 50% of the bottom of the existing landfill was lined with HDPE and the other 50% was lined with PE, 1×10-13 cm/sec and 1×10-9 cm/sec were used as hydraulic conductivity values, respectively, for the bottom boundary condition. Hydraulic conductivity of 6×10-6 cm/sec was used for the slopes of the existing landfill as boundary conditions, as the

Fig. 1. (a) Plan View of the Expanded Landfill (1: lined with PE, 2: lined with HDPE), (b) Side Views of the Height of the Existing and Expanded Landfills

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KSCE Journal of Civil Engineering

Leachate Modeling for a Municipal Solid Waste Landfill for Upper Expansion

slopes were not well-lined as the bottom. 3.2 Model Implementation A model domain of the existing landfill site (1473 m×993 m) consists of 80×60 meshes and corresponding vertical layers as seen in Fig. 1(b). The time step was 30 days and a constant inflow rate into the landfill was considered for the entire simulation period. This no time-varying inflow condition can be different from real situations where precipitation shows seasonal and even daily variation. The intermediate cover soil layer was not distinguished from the waste layer, and thus their lumped properties were determined. This approach is acceptable as a conservative estimate, since intermediate cover soil layers might contribute to the mitigation of excessive leachate accumulation on the landfill bottom. The final cover soil layer was treated as an individual layer to evaluate its hydraulic properties. 3.3 Model Calibration MODFLOW requires a recharge rate and a conductance value of the drainage system as well as hydraulic properties such as the hydraulic conductivities of the layers in the landfill, as listed in Table 1. Due to lack of data, all these parameters needed to be estimated in this study. The values of recharge rate, conductance, and hydraulic conductivity were determined through calibration,

and those of other hydraulic parameters were estimated with reported values in literatures (Benson and Wang, 1998; Jang et al., 2002; Dho et al., 2002; Bou-Zeid and EL-Fadel, 2004) to make the calibration process simpler and efficient. Simulation results appeared insensitive to the parameters for which literature values were given in Table 2. Model calibration was done with the leachate levels and accumulated volume from the leachate collection system. Since a single set of monitored leachate level distribution was available, the generation of landfill leachate had to be simulated from the initial stage of the landfilling process to the monitoring time for the model calibration. Fortunately, a limited number of combinations of parameter values could be identified to make the unique spatial distribution of the leachate levels. The inflow rate of 1050 mm/yr best simulates the total collected leachate volume and monitored leachate levels. With that inflow rate, the resulting average annual leachate volume removed by the leachate collection system was 3.4×105 tons, which was close to the average value of available records, 3.3×105 tons. This estimated inflow rate was comparable to the average annual precipitation at this site over the past 15 years (1100 mm/yr) and implied the possibility of subsurface flow addition and/or high water content of waste, in particular food waste, although each contribution cannot be individually assessed. The significant contribution of

Table 1. Landfill Properties Given or Estimated in the Model Calibration Parameters

Values used in this study

Remarks

Waste layer Hydraulic conductivity**

2×10−4 cm/sec

1×10−4~1×10−3 cm/sec [Kling and Korkealaakso (2006)]

Porosity

0.45

Effective porosity Specific yield

0.45 0.25

0.5 [Benson and Wang (1998)]; 0.3~0.5 [Jang, Kim, and Lee (2002)]; 0.6 [Bou-Zeid and EL-Fadel (2004)]; 0.48~0.58 [Kling and Korkealaakso (2006)] given as the same as total porosity given as the difference of porosity and residual saturation

Van Genuchten parameters

0.17/cm, 2.3

0.26/cm 2.2 [Benson and Wang (1998)] 0.08/cm, 2.4 [Kling and Korkealaakso (2006)]

Residual saturation

0.2

0.1 [Benson and Wang (1998), Bou-Zeid and EL-Fadel (2004)]; 0.25 ~ 0.3 [Kling and Korkealaakso (2006)]; 0.3 Field capacity [Bou-Zeid and EL-Fadel (2004)]; 0.3 Field capacity of a landfill in Korea [Jang, Kim, and Lee (2002)] (Field capacity is approximately equal to residual saturation)

Hydraulic conductivity

5×10−3 cm/sec

5×10−3 cm/sec [Bou-Zeid and EL-Fadel (2004)]; 1×10−3 cm/sec [Jang, Kim, and Lee (2002)]

Total porosity

0.4

0.4 [Jang, Kim, and Lee (2002)]

1×10−5 cm/sec

8×10−6~3×10−5 cm/sec [Jang, Kim, and Lee (2002)] 0.35 ~ 0.4 [Jang, Kim, and Lee (2002)]

α, β

Drainage layer

Cover soil Hydraulic conductivity Total porosity

0.4

Average inflow rate*

1050 mm/yr

Average leachate volume collected*

3.3×105 m3

3.3×105 m3 average value based on records over 5 years

*Calibrated values. **Conductance values used in drainage layer of block 1 where the leachate collection pipes are buried in the waste layer. Vol. 14, No. 4 / July 2010

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Table 2. Sensitivity of the Model Results with Respect to Model Parameters Case

Residual mean (m)

RMS (m)

Normalized RMS (%)

Average annual leachate volume drained

Calibrated one

0.139

1.085

2.268

3.2×105 m3/yr

α =0.05 cm−1 α =0.51 cm−1

-0.047 0.192

1.089 1.090

2.278 2.279

3.2×105 m3/yr 3.2×105 m3/yr

β =1.6 β =3.0

0.143 0.143

1.085 1.085

2.269 2.268

2.9×105 m3/yr 3.3×105 m3/yr

θ r =0.03 θ r =0.3

0.139 0.140

1.085 1.085

2.268 2.268

3.0×105 m3/yr 3.5×105 m3/yr

Sy = ne =0.3; nt = 0.4

0.193

1.091

2.280

3.5×105 m3/yr

Note: 1. α and β are van Genuchten parameters of the soil water retention curve. θ r, Sy, ne, nt and are residual saturation, specific yield, effective porosity, and total porosity, respectively. For simplicity, specific yield and effective porosity were given the same values that were smaller than the total porosity. 2. The smaller the α values, the larger the capillary fringe above the water table. Coarse medium has a large value. 3. β, greater than unity, typically 1.3 to 6 depending on the soil. The larger the β values, the more uniform the grain size distribution

food waste to the high water content of the leachate is not a completely groundless presumption, considering that Korean food usually contains high water content and there has been no in-sink disposal system for food waste. The contribution of the initial water contained in waste to total water inflow in a metropolitan landfill in Korea was measured to be comparable to or even exceeds that of infiltrated precipitation (Dho et al., 2002). The hydraulic conductivity of the waste layer, 2×10−4 cm/sec, gives the best match between the computed and monitored leachate levels, which falls within the range reported for a metropolitan landfill in Korea (Jang et al., 2002; Dho et al., 2002). Lower conductance was estimated for sector 1 where leachate collection pipes are placed in the waste layer than for sector 2 where leachate collection pipes are placed in the drainage layer. This lower conductance value of sector 1, along with higher leachate levels in sector 1, might be attributed to damage and clogging of the leachate collection pipes by surrounding waste. Leachate distributions computed with calibrated input parameters are displayed along with the observed ones at limited number of cross-section and vertical section in Fig. 2 and 3. Computed leachate levels matched the observed levels reasonably well, although high deviations were observed in the landfill border area with a high base elevation. 3.4 Management of the Leachate Levels in the Current Landfill The first objective of this study was to find optimal leachate management schemes, which allows the current landfill to satisfy the compliance criteria. According to simulation results, leachate levels will continue to rise unless additional leachate removal action, such as pumping through vertical pipes, is used. Since less than a year remained until the effective date of the newly-set government regulation when this study was initiated, there was no choice but to begin leachate pumping immediately and at a high rate. More challenging was to maintain the leachate level to meet the regulatory criterion 3 years after closing the current landfill because the leachate level would increase if the leachate

Fig. 2. Vertical Cross-sectional Views of the Leachate Level Distributions in 2005. Numerically computed leachate distributions are presented in dark gray, and the monitored leachate levels are shown with observation wells. Small figures are top views showing the positions of the vertical crosssectional plane.

left behind in the landfill migrated downward at a substantial rate exceeding the efficiency of the leachate collection system.

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Fig. 3. Model Calibration Results with Leachate Levels

Fig. 4 presents total leachate volumes removed by pumping out through vertical pipes and draining through the collection pipe system with four different combinations of water inflow rates during and after the waste filling period. Fig. 5 shows average leachate levels after termination of pumping for the four combinations in Fig. 4. An inflow rate of 1050 mm yr−1 during the waste filling period represented the same conditions as estimated in the calibration. An inflow rate of 750 mm yr−1, for example, represented the case where water inflow is reduced as a result of reduced initial water in the waste or reduced subsurface inflow. It is possible that the ban of food waste filling since 2005 reduced the inflow rate, though the degree of reduction is uncertain. The inflow rates after completion of the waste filling were assumed to be 750 mm yr−1 and 450 mm yr−1. According to the HELP model output, about 40% of total precipitation infiltrated through the final cover soil with a hydraulic conductivity of 5×10−5 cm sec−1, and it did not change substantially at various slope conditions of 0 to 50%. From empirical data in literatures, the fraction of infiltrated precipitation to total precipitation ranges from 20 to 40% (Bou-Zeid and El-Fadel, 2004). The assumption of an inflow rate of 450 mm yr−1 after the final cover soil is reasonable with an average precipitation of 1100 mm/yr in Korea. For a conservative estimation, 750 mm yr−1 was also assumed, which was the same inflow rate as the one estimated through the model calibration. In Fig. 4, the additional pumping process helped to enhance the leachate removal more in sector 1 than in sector 2. In the case where the water inflow during the waste filling period is higher, i.e., 1050 mm yr−1-750 mm yr−1 and 1050 mm yr−1-450 mm yr−1, no additional pumping seems necessary after initial intensive pumping is done to meet the regulatory criterion. Comparing two cases with the same total accumulated inflow volume, 1050 mm yr−1-450 mm yr−1 and 750 mm yr−1-750 mm yr−1, the former appeared to render greater leachate removal in total and the latter has a higher peak in the leachate level, as shown in Fig. 4 and 5. Fig. 5 represents that only the 750 mm yr−1-450 mm yr−1 case barely satisfied the leachate level criterion of 2 m even after pumping termination among the four combinations. Based on the numerical investigation, quite a number of Vol. 14, No. 4 / July 2010

Fig. 4. Total Leachate Volumes Removed by Pumping Out through Vertical Pipes and Draining through the Collection Pipe System with Four Different Combinations of Water Inflow Rates during and after the Waste Filling Period. The earlier and later three years correspond to the additional waste filling period and post-filling period, respectively.

Fig. 5. Average Leachate Levels Reached after the Pumping Termination at Various Combinations of Inflow Rate Conditions. The first and second numbers indicate the inflow rate during the waste filling period since the monitoring time and the post-filling period, respectively.

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pumping pipes should be placed more or less uniformly over the landfill site for efficient reduction of leachate levels and leachate should be pumped for a potentially long period and at a potentially high rate to prevent a leachate mound after pumping termination. The model prediction shown in Fig. 4 and 5 indicate that it is crucial to maintain a low rate of inflowing water, particularly during the waste filling period 3.5 Design Option for the Border Layer between Two Landfills The second objective of this study was to predict leachate levels of the existing lower and new upper landfills and to find the optimal option for the in-between border layer in the event of the construction of a new landfill above the existing one. The leachate level of the overlying landfill was determined by a combined condition of water inflow rate and its drainage efficiency. Depending on the design option of the border layer between the two landfills, leachate can build up in either lower or upper landfill. Three border layer options were considered as illustrated in Fig. 6: CD (final Cover soil+Drainage layer), CL (final Cover soil+Liner), and CLD (final Cover soil+Liner+ Drainage layer). The basic leachate collection system was assumed in all options. Although CD with no-liner option was

Fig. 6. Design Options of the Expanded Landfill Bottom Bordering the Top of the Current Landfill (Not drawn to scale)

expected to result in undesirable leachate buildup in the underlying landfill, it was included to investigate the economical feasibility of applying cost-saving soil compaction technology instead of liner option. Based on the monitored high leachate levels and model calibration results, a newly expanded landfill cannot avoid high leachate levels with the currently estimated inflow rate and leachate collection system. Since the estimated high inflow rate is undesirable and should be reduced for the new landfill, a condition with lower infiltration needed to be assumed. From this, the inflow rate into the underlying landfill for the rest of the filling period and new landfill construction period was given as 750 mm yr−1-450 mm yr−1 in Fig. 4. With CD of no-liner option, as expected, leachate was built up in the lower landfill when the same leachate removal systems as in sectors 1 and 2 in the lower landfill were assumed. A large portion of the leachate was not collected by the leachate collection system of the upper landfill and flowed into the lower landfill, thereby generating high leachate levels in sector 1, which exceeded the regulatory criterion. Leachate infiltration from the upper landfill into the lower landfill was investigated at various leachate removal efficiencies of the collection system, i.e., the conductance value defined per unit area per unit time in the MODFLOW and hydraulic conductivity of the cover soil. The final cover soil appeared to prevent leachate infiltration to the underlying landfill at acceptable levels when its hydraulic conductivity was reduced down to 1×10−7 cm sec−1 Fig. 7. Although the CD system saves cost for the liner installation, this advantage might be counteracted due to the requirement for high degree of compaction of the cover soil. Unlike no liner option, leachate buildup in the overlying landfill was of concern in CL and CLD options. The CL option has a higher possibility of leachate buildup in the lower landfill,

Fig. 7. Three Cross-sectional Views of the Leachate Levels in the Existing and Expanded Landfills with the CD Option at t = 3, 15, and 27 yrs. Black areas represent saturated leachate, and gray areas represent the filled waste layer (It should be noted that although gray levels represent the degree of water saturation, the gray version appears inconsistent with what is intended in color scale). Small figures are top views showing the positions of the vertical cross-sectional plane. − 478 −



Fig. 8. Three Cross-sectional Views of the Leachate Levels in the Existing and Expanded Landfills with the CL Option at t = 3, 15, and 27 yrs. Black areas represent saturated leachate, and gray areas represent the filled waste layer (It should be noted that although gray levels represent the degree of water saturation, the gray version appears inconsistent with what is intended in color scale). Small figures are top views showing the positions of the vertical cross-sectional plane.

Fig. 9. Three Cross-sectional Views of the Leachate Levels in the Existing and Expanded Landfills with the CLD Option at t = 3, 15, and 27 yrs. Black areas represent saturated leachate, and gray areas represent the filled waste layer (It should be noted that although gray levels represent the degree of water saturation, the gray version appears inconsistent with what is intended in color scale). Small figures are top views showing the positions of the vertical cross-sectional plane.

which exceeds the leachate level criterion. Numerical simulations gave results consistent with this expectation as shown in Fig. 8 and 9. In the CL system, leachate rose so high as to overflow the landfill surface. In the CLD option, leachate buildup was observed in the low-elevation area Fig. 9. However, the leachate buildup in the upper expanded landfill was predicted from mass balance analysis with inflow and drainage rates during the early few years.

4. Conclusions The MODFLOW-SURFACT was successfully used to calibrate Vol. 14, No. 4 / July 2010

and predict the hydraulic performance of landfill properties and engineering components, especially for a valley landfill. If specific engineering components are calibrated, the performance of the landfill design consisting of the calibrated components can also be evaluated. However, it has limitations, since it is not suitable for presenting specific details of design parameters such as drainage length or slope when it is required to suggest an enhanced system. The calibrated inflow rate was comparable to the total precipitation, indicating the possibility of high water content in the waste or the addition of subsurface inflow. Relatively high leachate levels in sector 1 could be attributed to unsatisfactory

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leachate removal efficiency of the leachate collection system. Reduction of the inflow rate during and, in particular, after the waste filling period was a necessary condition for maintaining leachate levels that would satisfy the regulatory criterion. Although leachate was to be pumped out from the landfill completely during the pumping period, leachate buildup could occur in the future after pumping was terminated. It was suggested that a sufficiently long period of time should be given prior to the landfill expansion to allow the leachate to be sufficiently drained to the landfill bottom liner and thus prevent future leachate buildup. Various design options of the border layer between the existing lower landfill and new upper landfill to be constructed was evaluated with respect to leachate levels reached during the waste filling period. Given an inflow rate of 75% of the calibrated value for the current landfill and the same leachate collection system as the existing underlying landfill, only the CLD (final Cover soil+Liner+collection system in a Drainage layer) option complied with the leachate level criteria for both the underlying and overlying landfills. The CL (final Cover soil+Liner+collection system in a waste layer) option caused significant leachate rises similar to observations in the current landfill, despite a reduced inflow rate. In the no-liner option, based on the numerical simulation, the hydraulic conductivity of the final cover soil should be reduced to at least 1×10−7 cm sec−1 to prevent substantial infiltration through the cover soil. However, this high level of compaction might counteract the cost-saving advantages. Moreover, these options might have a high risk of increasing leachate levels in the lower landfill due to the low efficiency of leachate removal of sector 1. Numerical investigations confirmed that continuing leachate pumping for a possibly long period, i.e., even during construction of the landfill expansion after closure of the current landfill site, is helpful. Low water inflow into the landfill during the pumping period appeared necessary using appropriate leachate design options.

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