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0 0 TECHNICAL MEMORANDUM Evaluation of Bed Ash Disposal Forward Landfill San Joaquin, California November 1, 2016 Page 5 Because both selective and mass excavations are not feasible, the evaluation described below was completed to assess potential water quality impacts associated with bed ash disposal at the landfill. POTENTIAL IMPACTS TO WATER QUALITY Qualitative Assessment Available site-specific and bed ash disposal information suggests that the potential for copper to leach from the bed ash and affect the underlying groundwater is remote because: • The acidic leaching conditions modeled by the STLC WET procedure are not likely to occur in the more pH-neutral landfill environment and thus the potential for generating copper concentrations in leachate at or above the STLC criterion in leachate is questionable (the most recent [2015]Annual Monitoring Report for the landfill indicates the pH of leachate from the FU cells is about 7.4); • Copper in solution is reactive and if leached from the bed ash, would be expected to be removed from solution by natural attenuation as it migrates downward through the landfill (particularly in intermediate soil layers), and if leakage through the liner were to occur,the remaining copper would likely further attenuate in the subsurface;3 • The bed ash disposal areas are underlain by a Subtitle D composite liner system the includes a composite compacted clay/geomembrane containment layer that is overlain by a LCRS. In accordance with Subtitle D requirements,the LCRS is designed to allow no more than 1 foot of head at any location in the disposal area. Although it is possible that the leachate head acting on the liner could exceed 1 foot, this would be a short term, transient occurrence and the average head acting on the liner should be 1 foot or less 3 The United States EPA defines natural attenuation as"a variety of physical,chemical,or biological processes that,under favorable conditions,act without human intervention to reduce the mass,toxicity,mobility,volume,or concentration of contaminants in soil or groundwater.These in situ processes include biodegradation;dispersion; dilution;sorption;volatilization;radioactive decay;and chemical or biological stabilization,transformation,or destruction of contaminants." The most important of these with respect to copper include dispersion,dilution, sorption,and chemical stabilization.