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3.1.3 Groundwater Elevations and Contours Prior to purging and sampling, each well was sounded for water depth using a weighted electronic sounder, and the static water level was recorded on a well data sheet (Appendix C). The groundwater elevations were calculated for each well by subtracting the depth-to-water measurement from the top-of-casing reference elevation. The current groundwater elevation data for the Austin Unit are summarized in Table 3-4. The groundwater elevation data obtained during this quarterly monitoring period were used to generate the groundwater elevation contour map shown on Figure 3-1, which indicates that groundwater beneath the Austin Unit generally flows to the northeast with an average hydraulic gradient of 0.002 ft/ft. To calculate the approximate linear groundwater flow velocity for the site, conservative assumptions were used, including a hydraulic conductivity of 875 gallons per day per square foot (0.04 cm/sec) and an estimated effective porosity of 35 percent(CH2M Hill 2000). An estimated groundwater flow velocity was calculated using Darcy's Law: Ki Ln I 0.002 sec- ji = [ — (0.04 )* —)* 2835 — , 0.65 Ole sec 0.35 d(15- fi where: V=Groundwater flow velocity. K=Hydraulic conductivity of the water-bearing unit(0.04 cm/sec). i=Hydraulic gradient: i Pz 0.002 for the site during the current the first quarter 2011. n,=Effective porosity(n,=0.35);an estimated value. The groundwater flow rate is calculated to be 0.65 feet/day(237 feet/year). 3.1.4 Detection Monitoring Program Field and laboratory results for the DMP wells for the first quarter 2011 are summarized in Table 3-1. As shown in Table 3-1, a trace concentration of dichlorodifluoromethane (DCDFM) was measured in the sample collected from well AMW-13; tetrachloroethene (PCE) was measured above the PQL, in the samples collected from wells AMW-13 and AMW-14; and trichloroethene (TCE)was measured above the PQL in the sample collected from well AMW-13. For the DMP wells at the Austin Unit, CLs are calculated for the water quality protection standards (WQPS). The CLs shown in Table 3-1 were calculated by Herst &Associates using Sanitaso, a state-approved statistical software program. Based on a comparison of the data obtained during the first quarter 2011 monitoring period,the following analytes were found to exceed a CL. The results of the statistical analyses performed by Herst& Associates are presented in Appendix D. D:\2011 0050TA IQII.doex 9 GeoLogic Associates