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Constituents of Concern Both groundwater and surface water samples are analyzed for constituents of concern(COCs) every 5 years(Appendix E). The last 5 year COC groundwater monitoring event occurred in the fourth quarter of 1999. The next monitoring event will occur during the fourth quarter of 2004. Surface water samples were analyzed for COCs during the monitoring event of February 14, 2000. The next monitoring event for surface water will occur during the first quarter of 2005. Statistical Analysis The County has been approved to use the intrawell tolerance limit method(each well provides its own background data) for groundwater analysis and the interwell tolerance limit method for surface water analysis at the CHSL. Concentration limits are updated quarterly and were most recently updated this quarter. Analytical results from the current quarter monitoring event were compared to the concentration limits shown in Table 1. The County uses the Sanitas®program for statistical analysis. Calculation of tolerance limits for intrawell analysis requires less than 50 background observations. In cases where the total number of observations is more than 50 and there is no increasing trend, the 50 most current observations are used for intrawell tolerance limit analysis. Concentration limits are not updated if the data contains more than 50%non-detects or if there is a statistically significant increasing trend in the data. In these cases,a Rank Sum analysis is performed. The Rank Sum analysis determines if there is a significant difference between the median concentrations of the compliance and background data. VOC History VOCs (Trichlorofluoromethane (CFC 11) and tetrachloroethene(PCE))were first detected in groundwater samples collected from MW-5 during the fourth quarter of 1991. Confirmation samples were collected in February 1992, and the presence of VOCs at monitoring well MW-5 was confirmed. The County believes that.the presence of VOCs in MW-5 is due to landfill gas influence. A corrective action plan(CAP),which includes the closure and capping of the landfill,has been implemented. A LFG control and collection system has also been operated onsite since December ' 2001 as corrective action. The effectiveness of the CAP is indicated by the trend of reduced VOC detections(see monitoring results). Corral Hollow Landfill 3 Department of Public Works/Solid Waste e Qtr/Annual 2003 Groundwater Monitoring County of San Joaquin-January 28,2004