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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 Q. 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 3-5. 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 north at an average hydraulic gradient of approximately 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 cm 0.002 sec– ft V = —= [(0.04—)* ]*2835 0.65 ft/day ne sec 0.35 cm – day ® where: V=Groundwater flow velocity. K=Hydraulic conductivity of the water-bearing unit(0.04 cm/sec). i=Hydraulic gradient: i z 0.002 for the site during the current the first quarter 2008. 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 2008 are summarized in Table 3-1 and time-series plots are presented in Appendix D. As shown in Table 3-1, estimated trace VOC constituent concentrations of ethanol (a suspected field/laboratory contaminant) were measured in samples collected from wells AMW-7 and AMW-12, and PCE in the sample collected from well AMW-6. In addition, 1,1-dichloroethane (1,1- DCA), cis-1,2-dichloroethene(cis-1,2-DCE), dichlorodifluoromethane (DCDFM), PCE, TCE, and trichlorofluoromethane (TCFM)were measured above the PQL in the sample collected from well AMW-13; and DCDF , PCE, and TCE were measured above the PQL in the sample collected from well AMW-14. 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 Sanitas®, a state-approved statistical software program. Based on a comparison of the data obtained during the first quarter 2008 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. DA2007_0043\FA 1Q08.doc 9 Geologic Associates