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® bromoform, chloroform, chloromethane, dibromochloromethane, and methylene chloride) were detected in the QA/QC field and trip blanks during the fourth quarter 2006 monitoring event. Review of the primary sample results indicates that several of these VOCs were detected in the primary samples collected, and were flagged as suspected laboratory contaminants. Several of these constituents were also detected in blank samples during the third quarter 2006 monitoring event where BC concluded, during an internal audit, that the suspected laboratory contaminants were the result of contaminated VOA containers. It appears that BC may have provided ATC with several of these contaminated VOAs. During the fourth quarter 2006 monitoring event, a duplicate sample was collected from well MW -15 and labeled MW-DUP. As shown on Table 2-2, with the exception of potassium, the duplicate sample analyses yielded good correlation (within 10%) with the primary sample results. C7 Review of fourth quarter 2006 sampling dates and laboratory analytical certificates indicates that the laboratory analyses were completed within required holding times. 2.1.3 Groundwater Elevations and Contours Prior to purging and sampling, each well was sounded for water depth using a weighted electronic sounder to an accuracy of 0.01 foot, 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 Forward Unit is summarized in Table 2-3. The groundwater elevation data obtained during the fourth quarter 2006 monitoring period were used to generate the groundwater elevation contour map shown on Figure 2-1, which indicates that groundwater generally flows to the north and northeast towards the Austin Unit, at an average hydraulic gradient of approximately 0.001 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 0.001 sec – ft V = — _ [( 0.04 —) * ] * 2835 ;z; 0.324 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 0.001 for the site during the fourth quarter 2006. ne= Effective porosity (n, = 0.35); an estimated value. The groundwater flow rate is calculated to be 0.324 feet/day (118 feet/year). D:X2003_021\1ForwardAustin_t06.doc\ 5 Geologic Associates