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® Vadose Zone Soil Gas <br /> In accordance with the Waste Discharge Requirements and the Monitoring and Reporting <br /> program for the site, soil gas wells GW-1 through GW-7 are monitored for combustible gas <br /> quarterly, and sampled for VOCs (EPA Method TO-14) semiannually. The TO-15 samples are <br /> collected in the 1St and 3rd quarters. <br /> Samples for VOC analysis have been collected from each well, starting in June 2006. At wells <br /> in which combustible gas was detected,the VOC sample was be collected from the probe with <br /> 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 <br /> an obstruction to the probe, in which case a sample is collected from the next higher probe. <br /> Sampling 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 <br /> well provides its own background data) for groundwater analysis, and to use the interwell <br /> tolerance limit method for surface water analysis. Concentration limits are updated quarterly, <br /> and were most recently updated this quarter. When a statistically significant trend exists for the <br /> current data, or when greater than 50 percent of the data consists of non-detects, or when there <br /> are insufficient data,then the latest calculated concentration limit is presented(Table 1). <br /> Analytical results from the current quarter monitoring event were compared to the concentration <br /> limits shown. <br /> The Sanitas®program is used for statistical analysis. Calculation of tolerance limits for <br /> intrawell analysis requires less than 50 background observations. In cases where the total <br /> number of observations is more than 50 and there is no increasing trend, the 50 most current <br /> observations are used for intrawell tolerance limit analysis. <br /> • <br /> Corral Hollow Sanitary Landfill 9 Department of Public Works/Solid Waste <br /> 41'Quarter and Annual 2012 Groundwater Monitoring County of San Joaquin—January 15,2013 <br />