Description
This final report was prepared by Geosyntec Consultants of Columbia, Maryland for the Environmental Research and Education Foundation (EREF) of Raleigh, North Carolina to describe findings from case study application of the Evaluation of Post-Closure Care (EPCC) Methodology at the closed Mohawk Valley Landfill (MVLF) located near Frankfort, New York. EPCC was developed by Geosyntec for EREF with the aim of providing a technically defensible, performance-based approach for evaluating the site-specific elements of post-closure care (PCC) at municipal solid waste (MSW) landfills regulated under Subtitle D of the Resource Conservation and Recovery Act (RCRA). Subpart F of Subtitle D stipulates that an owner/operator of a closed MSW landfill is responsible for its maintenance, monitoring, and condition for 30 years, or for an alternative period as necessary to protect human health and the environment (HHE). PCC activities include monitoring of leachate and landfill gas (LFG) emissions and potential receiving systems (e.g., groundwater, surface water, soil, and air) as well as maintenance of the cover, leachate, and LFG collection systems. Protection of HHE is demonstrated when potential threats to HHE are minimized to acceptable levels at the relevant point of exposure (POE), which is typically identified as the closest property boundary location at which a receptor could be exposed to contaminants and receive a dose via a credible pathway. Demonstrations are made at the point of compliance (POC), which must be located no further from the landfill than the POE.
Consistent with the above, demonstration of no unacceptable threat at the relevant POE in the absence of active care can thus serve as a foundation for demonstrating when regulatory PCC can end. EPCC defines the end of PCC in these terms as ‘functional stability,’ and provides a modular approach for sequentially evaluating the three primary control elements of PCC (i.e., leachate management, LFG management, and cover maintenance) in this context. Once these elements are demonstrated to meet conditions for functional stability, active control and maintenance can be terminated following a confirmation monitoring (CM) program that includes monitoring of groundwater and methane probes to demonstrate that the proposed conditions of passive care or no care are protective. Responsibility for the landfill can then transition from regulated PCC to a custodial care program of land management. Custodial care includes some de minimis level of care to protect against disturbance of buffer zones or passive barriers (mainly the cover), satisfy institutional controls, deed restrictions, or covenants, and facilitate beneficial reuse of the property. By assessing long-term monitoring and management requirements in this way, PCC activities can be demonstrated to be completed, if appropriate, or optimized to focus specifically on providing an environmentally protective level of PCC.
The EPCC approach focuses on site-specific leachate and LFG characteristics, the performance of passive control systems, and an evaluation of potential threats derived from the sensitivity of the surrounding environment and a defined end use for the landfill property. Evaluations involve analyzing statistical trends in leachate quality, LFG generation, and landfill settlement to demonstrate that leachate quality is constant or improving, groundwater is not impacted, LFG production is stable or decreasing, and settlement is essentially complete. The evaluations are based on conservative assumptions and driven by data. Proactive data collection is therefore critical for successful application of EPCC at any landfill. Fundamentally, the EPCC framework of data collection and analysis is designed to demonstrate that active leachate and LFG management can ultimately transition to passive care under a non-regulatory custodial care status. A key benefit of EPCC is that all analyses are site-specific; therefore, measures to proactively stabilize waste (e.g., bioreactor operation) or include more sustainable design components (e.g., gravity drains for leachate, alternative all-soil final covers) will be reflected in the outcome. Two key elements of EPCC are that the POE is defined by the ultimate property end use and the potentially iterative nature of PCC system shutdown and CM is built into the process itself. This avoids the need for subjective definitions or categorization of in-situ waste characteristics inherent in other approaches.
In conjunction with Waste Management of New York (WMNY), Geosyntec identified MVLF as an ideal case study at which to conduct a series of retroactive functional stability analyses for leachate management, LFG control, and cover stability. The site was selected because it had all the control systems required for PCC under Subtitle D (unusual for a pre-Subtitle D landfill) and a comprehensive dataset dating back over 20 years (again, an unusual feature for an older site). Importantly, working with the New York State Department of Environmental Conservation (NYSDEC), WMNY had already negotiated active control system shutdowns for LFG and leachate management, which had been progressively implemented through the years of post-closure. This added an extra dimension to this study, because the outcomes modeled through retroactive application of EPCC could be directly compared to changes that had already been made; in other words, WMNY and NYSDEC had already implemented a prototype CM approach to support incremental transition from active to passive care. This unique set of circumstances were extremely beneficial to this study in terms of understanding expected outcomes from sequential application of EPCC over a two-decade period. However, this report does not intend to suggest that MVLF is typical of closed Subtitle D landfills. It is designed to provide the industry with an illustration of the types of data needed to complete a performance demonstration and a user’s guide to implementing the EPCC process if said data are available.
The facility is a 29-acre MSW landfill on an 80-acre property that operated between the early 1970s and 1991. About 2.2 million cubic yards of MSW were placed in the landfill. Closure construction was completed in 1993, after which the site commenced a 30-year PCC term regulated by NYSDEC in accordance with Section 2.15 of 6CRR-NY Part 360. The final cover is an all-soil design absent a geomembrane barrier. A LFG-to-energy plant operated between 1990 and 2001, after which methane production could no longer sustain plant operations. The site does not have a modern geomembrane liner system; however, the geologic strata underlying the landfill are dense, low permeability glacial till deposits overlying shale bedrock. The lower gray till comprises a de facto liner, with basegrades established to drain leachate to a leachate collection system (LCS) at the downgradient toe of the landfill from where it gravity flows to onsite leachate storage tanks. Until 2012, leachate was hauled off site for treatment and disposal; since then, however, a constructed wetlands treatment system (CWTS) was installed to provide onsite leachate treatment with permitted discharge of treated effluent to the Mohawk River through a control vault and stabilized drainage channel. Groundwater occurs under both confined and unconfined conditions in the area, the latter where bedrock is overlain by glacial till. Groundwater generally flows vertically downward through the till to the top of bedrock and then laterally along the top of bedrock to the river, although groundwater flow can be influenced by the presence of local sand and gravel lenses and alluvial deposits within the upper brown till. The predominant direction of groundwater movement in the unconsolidated units is laterally towards the river.
MVLF has been considered an “accidental bioreactor” due to its siting and design allowing subsurface infiltration of groundwater prior to collection in the downgradient LCS. This was intensively investigated under a solids sampling and analysis program conducted during the 2001-2002 timeframe. Although solids sampling is not recommended under EPCC, these existing data were reviewed and analyzed as part of this study with the objective of ascertaining whether they provided valuable information regarding the status of organic and, by association, functional stability. However, the data were found to be inconsistent and of little value in terms of understanding landfill performance, supporting EPCC’s position that it is extremely difficult if not impossible to obtain representative solids samples to analyze the relative state of waste degradation within MSW landfills.
Routine monitoring data that are useful for this study in term of evaluating functional stability at MVLF have been collected since 1987. The approved environmental monitoring plan (EMP) covers LFG, groundwater, and surface water monitoring as well as analysis of leachate samples. Of unique value, a leachate and LFG database spanning over 20 years was available that allowed a series of functional stability analyses to be performed retroactively approximately five, ten, and 20 years after closure (the precise timing of evaluation events was established following more detailed review of site conditions and operational/PCC history). The focus of each evaluation is to approximate where MVLF is on the functional stability “curve” with respect to the primary components of PCC (leachate management, LFG management, and cap stability). For the purposes of these assessments, the groundwater monitoring component is used to confirm that the functional stability aspects for these primary components have not (and will not) cause an impact above applicable performance standards.
Defining the end use of the landfill property is critical in conducting a functional stability analysis since it assigns expected conditions for custodial care and can also define an appropriate POE, which may be different than the property boundary. For simplicity, at MVLF it was consistently assumed in all evaluations that the landfill property will be maintained as green space set-aside with human contact minimized throughout the post-closure period and beyond into custodial care. Specifically, access to the site will be controlled through maintenance of perimeter fencing, institutional controls will preclude the consumption of groundwater or surface water at the site, leachate will continue to be drained passively from the LCS at the base of the landfill and be managed on site, and the existing cover system and other surface features such as the stormwater management system will remain in place and maintained to function as necessary to retain the character of the landscape. This simple end use strategy was effective in this study, allowing demonstration of functional stability with regard to LFG, leachate, and the cap.
Two retroactive evaluations of LFG management using EPCC were performed in this study, the first in 1997 (Evaluation G1-Y5, Year 5 of PCC) with a follow up in 2001 (Evaluation G2-Y9, in Year 9 of PCC). Evaluation G2-Y9 showed that active LFG collection was no longer the best available control technology (BACT) and suggested that passive venting could serve as the BACT for residual gas control, supplemented with the methane oxidation capacity of the all-soil cover system. This finding is fully consistent with NYSDEC’s regulation of the landfill in that eliminating active LFG management in favor of passive venting was approved at MVLF in 2002. For this study, a hypothetical CM program was developed to demonstrate the validity of this finding. The POC was assumed as an existing methane migration monitoring probe GP01, located approximately 35 feet from the toe of the landfill in the direction of the nearest potential sensitive receptor (an occupied house), and equidistant from the western property boundary (POE). The duration of CM based on the maximum time of travel for gas migration in the vadose zone from the landfill toe to probe GP01 was 21 months, with monthly monitoring required. Assumed that CM would have been initiated in July 2002, at the time that NYSDEC approved transition to passive gas venting, CM would have been scheduled to be completed in April 2004. WMNY’s monitoring data for this period shows that no gas impacts were detected at monitoring probes; therefore, eliminating active LFG controls and converting to a fully passive venting system in 2002 was an acceptable and sustainable strategy. Conditions for functional stability have been demonstrated with respect to LFG management.
Two retroactive evaluations of cap settlement were also performed, the first in 1997 (Evaluation C1-Y5, in Year 5 of PCC) with a follow up in 2001 (Evaluation C2-Y9, in Year 9 of PCC). EPCC assumes that significant post-closure settlement will be limited to secondary settlement resulting from waste degradation, which can be modeled based on historical LFG generation and the remaining LFG potential. Significant secondary settlement is assumed complete (i.e., the cap is functionally stable) when it can be demonstrated that the annual rate of settlement is de minimis, that is less than 5% annually relative to the cumulative total post-closure volume reduction at the landfill. Both evaluations projected that functional stability with respect to cap settlement would be achieved no later than 2004, 12 years after closure. This timeframe is supported by findings from the LFG evaluation described above. To confirm that actual cap settlement rates are consistent with modeled predictions, topographic survey data are needed to calculate reductions in the landfill volume over time (which can be translated to settlement). As-built surveys of the landfill cover were conducted as part of closure construction but subsequent surveys have not been conducted, which means that CM for cap settlement cannot be conducted. However, the site is now 12 years past the date at which functional stability with regard to cap settlement was expected, and routine inspection of the cover since closure has not indicated any significant issues related to differential settlement or subsidence leading to surface irregularities, damage, poor drainage, or ponding of water. WMNY reports there has been little to no cap repair required since closure. Therefore, for this study it is reasonable to assume that CM would not have yielded results that conflict with the assessment that the cap is functionally stable.
Finally, three retroactive evaluations of leachate management were performed in this study, the first in 1997 (Evaluation L1-Y5, in Year 5 of PCC) with follow ups in 2002 (Evaluation L2-Y10, in Year 10 of PCC) and 2011 (Evaluation L3-Y19, in Year 19 of PCC). The long-term leachate management strategy at MVLF was that leachate would continue to gravity drain from the LCS to onsite storage tanks. However, offsite trucking and disposal of leachate would be abandoned in favor of full onsite management using the CWTS from 2012. Evaluation L3-Y19 in 2011 reflects this planned change in strategy. As with LFG management, it is noteworthy that NYSDEC’s regulation of the landfill in terms of approving transition to a mainly passive CWTS for on-site leachate management is empirically consistent with the recommended approach under EPCC.
In summary, by 2011 leachate is on the brink of meeting conditions for functional stability. If minor modifications are made to on-site stormwater ponds to function as GW infiltration basins, and the CWTS is modified for passive (internal) discharge via GW infiltration with the Mohawk River recognized as the POE, the only remaining migration route for leachate to the river would be via superficial GW, with ammonia the single remaining analyte of potential concern. Once these modifications have been made and confirmed to be performing as intended, alternative risk-based criteria could be established for ammonia and variances sought from current discharge permit conditions to reflect these modifications and allow transition to post-regulatory custodial care in which no active leachate controls would be required. Another, potentially simpler, option is to continue monitoring for ammonia under the status quo until leachate source concentrations fall below 60 mg/L, the target value for source leachate consistent with meeting conditions for functional stability in SW. In other words, at this concentration, leakage of leachate from the landfill toe drain or CWTS and subsequent migration via superficial GW to the river would not impact GW or SW quality above their respective limit values.
At MVLF, custodial care would likely involve providing de minimis oversight of the cap, stormwater swales, ponds, and CWTS; general grounds maintenance; compliance with local land-use requirements, deed restrictions, covenants, and local zoning ordinances; and undertaking typical property management responsibilities such as paying property taxes and controlling access. It is reasonable to expect that such residual care requirements could be provided by a caretaker or landscape gardener outside of the requirements of a Part 360 Permit.
The report illustrates that EPCC is relatively easy to use (demonstrating completion of seven evaluations for three different PCC control systems) and that defining functional stability is meaningful: at MVLF, it allows tangible conclusions to be drawn about transitioning active PCC controls to passive management within the 30-year presumptive PCC period. It is recognized that MVLF was fortunate in its construction and siting (e.g., passive drainage can continue for the long term without need for LCS maintenance). This may not be the case at other sites, which would be reflected in an EPCC evaluation performed and could be a limiting factor in defining functional stability. This study marks significant progress towards understanding the impact of site conditions, PCC system design, and availability of data on the EPCC process; however, it is noted that additional experience is still needed on the wider issues that might arise from its broader application. While it is not appropriate to draw firm conclusions about the general efficacy of EPCC, there is reason to be confident that issues will be related to site conditions and/or data availability rather than with the concept of functional stability or the EPCC process itself.
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