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POSSIBLE SOURCES OF GROUNDWATER IMPACT Because the shallow aquifer is very close to the surface, and because the volume of water in the aquifer is not great, a relatively small amount of contaminated water could have a large impact on the quality of the water sampled at the monitoring wells. One possible source of groundwater impact is water percolating through refuse, which could occur if the refuse is not protected by the closure cap. The County performed a geophysical investigation of the eastern edge of the landfill to define the edge of refuse and to investigate the possibility that refuse may lie outside the closure cap. If unprotected by the closure cap, such refuse could be the source of groundwater impact. The results of this study were inconclusive, due to the indirect nature of the indications (electrical conductivity as an indication of refuse, for example) and possible interference with buried petroleum fuel pipes within the property(Figures 2, 3, and 4); however, the study indicates the possibility of refuse beneath the drainage ditch. Ponding on the landfill itself has not been observed, so oilier sources of leachate are not apparent. a At Well MW-4, it is also possible that the shallow aquifer is being recharged and impacted by j surface water from Corral Hollow Creek. Such water could be highly mineralized and could carry I nitrates from cattle grazing south of the creek.jbe County attempted to investigate the source of impact to MW-4 in July and August 2003 by drilling at two locations excavating by hand digging within the nearby Corral Hollow Creek bed, then sampling and testing the groundwater from these locations and from well MW-4. Samples could be collected from MW-4 and from one of the borings, which was converted to piezometer PZ-1. A third boring (MW-8)was found dry. Samples could not be collected from the creek bed explorations (TB-2 and TB-3)with hand tools. Samples were analyzed for fecal coliform and other parameters to determine if the source of impact was due to seepage of water contaminated by cattle grazing activity. Since data could not be sampled from three of five proposed locations, the investigation provided ! little useful information. The results of the boring(TB-1) showed a discontinuous aquifer, j consistent with irregular conditions at the edge of the Corcoran formation. The analysis of samples from MW-4 did not conclusively indicate that the creek was affecting pH and Nitrate trends. Fecal coliforms were not detected in either TB-1 or MW-4 and the inorganic parameters were very similar in both,which is to be expected since the wells are separated by less than 100 feet. i Pian to Evaluate Impacts to Groundwater Z San Joaquin County Public Works/Solid Waste Corral Hollow Sanitary landfill February 10,2004