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® No VOCs were reported at this well this quarter. VOCs were last detected in this well on April 23, 2004;these detections are believed to have been introduced during well construction, and do not indicate groundwater impact from the landfill. Surface water Surface water samples were last collected on January 27, 2011, but were not available during staff site visits this quarter due to lack of rain. All concentrations of field parameters and constituents of concern were below established concentration limits where the limits could be determined. Unsaturated Zone Soil Pore Gas The Monitoring and Reporting program for the landfill requires semi-annual VOC testing by TO- 15 analysis. Soil gas samples were last collected on January 13, 2013, and are included in this first quarter report. Before sampling, the concentration of major gasses (Methane, Carbon Dioxide, and Oxygen) and pressure were measured using a CES-LANDTEC GEMTm2000 portable gas detector. The GEMTm2000 was operated until a steady-state gas composition indicated that the well was purged and ready for sampling. Soil gas samples were collected in 1-liter Summa canisters and sent for VOC analysis by EPA method TO-15. The concentrations of major gasses were recorded during the first quarter 2013 TO-15 sampling event, and a sample was collected from the probe most likely to have the highest VOC concentration: the sample was taken from the probe with the highest methane content, but if methane was not detected, the sample was collected from the deepest probe. The probe selected in the first TO-15 sampling event conducted in 2006 has been used thereafter for this analysis. Methane concentrations were zero in all perimeter migration monitoring wells during the first quarter. VOCs were detected in all seven soil gas wells during the first quarter monitoring event. The highest VOC concentration was found in well GW-6D (Appendix C), however,there is no shallow aquifer at that location. No "upwind" air sample was taken. Concentrations of major gasses (Methane, Carbon Dioxide, and Oxygen) and pressure were measured using a CES-LANDTEC GEMTm2000 portable gas detector on January 30, 2013 (Table 4). Groundwater Direction and Flow Rate Shallow Aquifer Groundwater elevations at wells SB-1, MW-4, MW-5, MW-8, MW-9A, MW-10A, and MW-11 A indicate that the off-site groundwater flow gradient in the shallow perched aquifer is to the north (Figure 2 and Appendix G). The average calculated groundwater velocity is 8.4 ft/yr based on a gradient of 0.03 feet per foot. Corral Hollow Sanitary Landfill 16 Department of Public Works/Solid Waste 1 5'Quarter 2013 Groundwater Monitoring County of San Joaquin—April 15,2013