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MONITORING RESULTS Statistical Analysis The FSL has been approved to perform interwell tolerance limits (upgradient wells as the background). Concentration limits are updated quarterly and were most recently updated in the fourth quarter of 2002. Analytical results from the third quarter monitoring event were compared to the concentration limits shown in Table 1. Concentration limits are updated quarterly where possible,using interwell tolerance limit methods. In cases where the historical data contains more than 50%non detects, the concentration limit cannot be updated. In these cases, the established concentration limit as listed in the WDRs is used for comparison. The County also performs a Rank Sum analysis in cases where concentration limits cannot be updated. The Rank Sum analysis determines if there is a significant difference between the median values of historical and recent concentrations within a well. With the adoption of new WDRs in January 2003, concentration limits will only be updated annually. Groundwater Ouarterlv Monitoring and Field Parameters Groundwater levels were measured and samples collected from all wells on October 14,2002. A comparison of the quarterly monitoring parameters to established concentration limits show that there was one exceedance this quarter(Table 1). The concentration of Chloride in MW-3 exceeded the established concentration limit. There have been other indications of impact to the groundwater in the vicinity of MW-3. The County has submitted a plan to evaluate the groundwater conditions in the vicinity of this well, in an attempt to delineate the nature and extent of the suspected release. VOCs Methyl-tert-butyl-ether was reported in MW-1 and MW-3. 1,4-Dichlorobenzene was also reported in groundwater collected from MW-3. County did not resample either well. VOCs have been detected sporadically in MW-3 since 1995. The County has submitted a revised EMP which addresses all requests made by the RWCQB in a letter dated June 21, 2002. While under review by Board staff,the County will continue to monitor the groundwater in the vicinity of MW-3 in accordance with the WDRs. It should be noted that the two compounds(Trichloroethene and 1,1-Dichloroethene)which initially indicated impact to groundwater at MW-3 are consistently"non-detect." TCE has not Foothill Sanitary Landfill Department of Public Works/Solid Waste 41h Quarter and Annual 2002 Groundwater Monitoring Report 6 County of San Joaquin