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PLAN TO EVALUATE IMPACTS TO GROUNDWATER As Part of Corrective Action At the Corral Hollow Landfill PURPOSE The purpose of this Plan is to evaluate possible sources of apparent impact to groundwater indicated at shallow aquifer monitoring wells MW-4 and-5. The proposed work includes investigating the potential for percolation of rainwater through refuse outside of the closure cap and for impact by percolation of feces-contaminated water running into the nearby creek bed from grazing land south of the site. BACKGROUND The Corral Hollow Sanitary Landfill(CHSL) is an unlined, municipal solid waste landfill, located in southern San Joaquin County at the intersection of Interstate 580 and Corral Hollow Road (Figure 1). Annual rainfall is approximately eight inches. The landfill was closed(received closure cap per Title 27) in the fall of 1995. The CHSL is currently monitored through a Corrective Action Program under Waste Discharge Requirements (WDRs) Order No. 5-01-176 due to VOC impacts at one well(see below). Two aquifers are present below the landfill: one shallow and one deep. The shallow aquifer is discontinuous, being perched on the Corcoran formation which ends under the landfill footprint. The depth of this aquifer in the vicinity of the landfill varies between 15 feet and 67 feet. Shallow aquifer groundwater monitoring system is provided by one up-gradient well(SB-1) and two down-gradient wells (MW-4 and MW-5). Well MW-5 had previously shown impact by VOCs, prompting the installation of a landfill gas (LFG) system. Data from both of these wells have shown a statistical decreasing trend for pH_ In well MW-4 an increasing trend in nitrate is present. In well MW-5 increasing trends in chloride, TDS, Sulfate, and electrical conductivity are present. These trends are characteristic of impact by leachate. The landfill access road extends along the eastern perimeter of the refuse footprint. A drainage ditch immediately west of this road, carries much of the landfill rain runoff to the retention pond in the eastern corner of the site(Figure 2). It was intended that both the road and drainage ditch be off the landfill footprint. The drainage ditch is not provided with a liner. The original closure construction included a half- round corrugated metal pipe to carry water collected at the northern corner of the landfill to the retention pond. Significant erosion occurred under this pipe because water flowing from the northeast encountered the lip of this half-round pipe, causing water to flow along and under the pipe rather than within the pipe. This condition was corrected by removing the half-round pipe, grading the ditch surface to drain, and installing anti-erosion mesh and grass seed in December 2003. Plan to Evaluate Impacts to Groundwater San Joaquin County Public Works/Solid Waste Corral Hollow Sanitary Landfill February 10,2004