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Results Groundwater: The groundwater samples from the temporary borings were found free of contaminants (Appendix A). These results are consistent with data collected from MW-3 since January 2002. Soil Gas: Among the many VOC compounds detected in the LFG, four VOC compounds were also detected in the soil gas collected near MW-3. However, these compounds are not detected in the water samples taken at MW-3 or at the temporary borings of this Evaluation Monitoring. Additionally,the two VOCs (1, 1-Dichloroethene, and Trichloroethene)which initially indicated impact to the groundwater were not detected in either the soil gas or the landfill gas sample. CONCLUSION The results of the EMP reveal that groundwater is not impacted by the presence of the Foothill Landfill at this time. Corrective action is,therefore,not warranted. No further investigation is proposed at this time. Monitoring of the groundwater will continue in accordance with Detection Monitoring. Thep resence of VOCs in soil gas apparently does not cause impact to groundwater, at least at the concentrations measured in the soil in these tests. DISCUSSION The detection of VOCs at MW-3 from March 2000 to October 2001, and the lack of verified detections since that time indicate a transient effect. In an attempt to identify the mechanism of this transient effect, Solid Waste staff interviewed the site operator and past Solid Waste staff. This investigation indicates that an area(approximately 40'x 60') over refuse fill, immediately adjacent to MW-3, experienced ponding in the winters of 1995-1998. The slope of this area was corrected, and future ponding prevented,by regrading this area in the spring of 1998. It is probable that the cause of the detections at MW-3 was the result an increase in LFG production in those portions of the refuse to which moisture intruded. This increase in LFG production would increase the movement of LFG through the soil, and with it the potential transport of VOCs. It is speculated that this increased transport was sufficient to cause impact to ' be detected at MW-1 while it was occurring. Once the ponding was corrected in 1998,the LFG production in the wetted refuse subsided and the impact gradually dissipated to the"non-detect" status indicated today. The dissipation of the Engineering Feasibility Report 3 Foothill Sanitary Landfill October 1,2003 San Joaquin County Solid Waste