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4.3 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 French Camp Landfill are summarized in Table 5. The groundwater elevation data obtained during this quarterly monitoring period were used to generate the groundwater elevation contour map shown on Figure 1, which indicates that groundwater flows to the southeast with 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 300 gallons per day per square foot(0.014 cm/sec), and an estimated effective porosity of 35 percent(CH2M Hill 2000). An estimated range in groundwater flow velocity was calculated using Darcy's Law: Ki cm 0.002 sec–ft V = —= [(0.014 —)* ]*2835 0.227 ft/day ne sec 0.35 cm –day where: V=Groundwater flow velocity. K=Hydraulic conductivity of the water-bearing unit(0.014 cm/sec). i=Hydraulic gradient:i;z�0.002 for the site during the current(third quarter 2007) monitoring period. ne=Effective porosity(ne=0.35);an estimated value. The groundwater flow rate is calculated to be 0.227 feet/day(83 feet/year). 4.4 DETECTION MONITORING PROGRAM Third quarter 2007 field and laboratory results for DMP monitoring wells (MW-6A, MW-7A, MW-8A, MW-9A, MW-913, andMW-lOA)are summarized in Table 2 and time-series plots are presented in Appendix D. As shown in the time-series plots, general chemistry and metals constituent concentrations are generally consistent with historical concentrations. Organic constituent results indicate that trichlorofluoromethane(TCFM) was detected above the practical quantitation limit(PQL) in the sample collected from well MW-9A; and 1,2-dichloroethene(1,2-DCE)was detected at trace concentration in the sample collected from well MW-9B. In a letter dated January 16, 2007, the RWQCB was notified of the non-statistical indication of a release at well MW-9A,which was suspected to be related to the recent placement of a final cover system on the landfill and landfill gas(LFG) generation. In a meeting with the RWQCB on March 9, 2007, the RWQCB agreed to quarterly monitoring of existing groundwater monitoring wells and soil-pore gas probes through May 2008 to evaluate the significance of LFG at the site. D A2007-0M\FC_su m07.doc 3 Geologic Associates