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® 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-I 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 /> I st and 3"quarters. <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 /> 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 /> 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 /> used for intrawell tolerance limit analysis. <br /> Corral Hollow Sanitary Landfill Department of Public Works/Solid Waste <br /> V'Quarter 2013 Groundwater Monitoring County of San Joaquin—April 15,2013 <br />