REASONABLY FORESEEABLE WATER QUALITY RISKS FROM LINED LANDFILLS LEACHATE AND LANDFILL GAS RELEASES

摘要

The State of California requires MSW landfills to install a funding mechanism to provide groundwater corrective action for a contaminant release from new modules. Development of realistic cost estimates for such a "reasonably foreseeable release" require extensive analysis of potential release mechanics associated with both leachate and landfill gas, the biological processes at work within a decomposing waste mass, and failure potential of environmental controls required in modern landfills. At the Kiefer Landfill, in Sacramento, California, the database from the existing lined module has been used to make such an assessment. Analysis of leachate chemical data collected indicates MSW landfills can pose significant water quality risk within the first few years of operation, but the risk declines rapidly as the cell is sealed from infiltrating rainfall and the waste mass begins anaerobic decomposition. Installation of landfill gas collection systems also reduces water quality risks as volatile organics are removed with the LFG. The first consideration in assessing water quality risk is leachate quantity. The fraction of exposed liner is the predominant mechanism for leachate quantity generation. The primary solution to this problem is phasing the landfill's fill sequence such that installed liner is covered with waste prior to winter rains, and constructing lined cells of sufficient size to allow complete gravity flow of rainfall run-off rather than reliance on storm-water pump stations on the liner floor. Secondary solutions would be multiple leachate collection sumps in individual modules to enable separation of "contact" versus "non-contact" stormwater. Even with initial large volumes of leachate, a properly operated LCRS can reduce the quantity risk factor to the area of the leachate collection sump. Collected leachate quality data from the lined module at Kiefer displays the classic phased decomposition process at work within the waste mass. The methanogenic, or Phase IV, decomposition process is actually serving as a contaminant source removal "filter" reducing metals and VOC concentrations in collected leachate. The data exhibits some scatter as expected with an active module, but the initial 4 phases of waste decomposition are readily apparent in the data collected thus far. Current trends indicate water quality risks can be expected to reduce further as the mass enters complete anaerobic decomposition. Landfill gas risks tc water quality have been known for some time now. This contaminant mechanism has been predominant in the Kiefer situation for its unlined module. However, lined modules with composite lining systems (60-mil HDPE over a GCL for the Kiefer Landfill) will substantially contain the LFG even without active gas controls systems. The saturated nature of LFG can hydrate the GCL thus sealing any flaws in the overlying HDPE in "dry" areas of the LCRS. However, with "Dry Tomb" management, the water quality risk of a LFG release could increase due to declining moisture levels until the decline begins to throttle gas generation. These factors were used in developing reasonably foreseeable water quality risks and subsequent corrective action cost estimates for the Kiefer Landfill. While specific to Kiefer, these same methods could be applied at any MSW landfill.