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u <br />Vadose Zone Soil Gas <br />In accordance with the Waste Discharge Requirements and the Monitoring and Reporting program <br />for the site, soil gas wells GW -1 through GW -7 are monitored for combustible gas quarterly, and <br />sampled for VOCs (EPA Method TO -14) semiannually. The TO -15 samples are collected in the <br />lst and 3rd quarters. In August 2013 two additional soil gas wells (GW -8 and GW -9) were <br />installed. The logs for these wells are included in Appendix 1. <br />Samples for VOC analysis have been collected from one probe at each well, starting in June 2006. <br />At wells in which combustible gas was detected, the VOC sample was be collected from the probe <br />with the highest methane content. Where no combustible gas is detected, the VOC sample is <br />collected from the deepest probe unless the sampling technician determined that there might be an <br />obstruction to the probe, in which case a sample is collected from the next higher probe. Sampling <br />has continued from those probes that were originally selected in 2006. <br />Statistical Analysis of Groundwater Samples <br />1 At the CHSL, the County has been approved to use the intrawell tolerance limit method (each well <br />provides its own background data) for groundwater analysis, and to use the interwell tolerance <br />limit method for surface water analysis. Concentration limits are updated quarterly, and were <br />most recently updated this quarter. When a statistically significant trend exists for the current <br />data, or when greater than 50 percent of the data consists of non -detects, or when there are <br />insufficient data, then the latest calculated concentration limit is presented (Table 1). Analytical <br />results from the current quarter monitoring event were compared to the concentration limits <br />shown. <br />1 The Sanitas® program is used for statistical analysis. Calculation of tolerance limits for intrawell <br />analysis requires less than 50 background observations. In cases where the total number of <br />observations is more than 50 and there is no increasing trend, the 50 most current observations are <br />tused for intrawell tolerance limit analysis. <br />L <br />l <br />1 <br />Corral Hollow Sanitary Landfill Department of Public Works/Solid Waste <br />3d Quarter 2013 Groundwater Monitoring County of San Joaquin — October 15, 2013 <br />