ARCHIVED REPORTS_2009 - Laserfiche WebLink

Home Browse Search

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 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 and historical groundwater elevation data for the French Camp Landfill is summarized in Table 5. The groundwater elevation data obtained during the monitoring period were used to generate the groundwater elevation contour maps shown on Figures 1 and 2, which indicates that groundwater generally flows in a southerly direction 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 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– fi V = —_ [(0.014 —)* ]*2835 - 0.227 fi/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;t�0.002 for the site during the 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 Field and laboratory results for DMP monitoring wells (MW-6A, MW-7A, MW-8A, MW-9A, MW-9B, and MW-lOA) from the current monitoring period 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. During the monitoring period, chloroform was measured above the PQL in the samples collected from background well MW-6A. In response to the detection of chloroform, retest samples were collected from this well on June 30, 2009. The results of the retest samples indicate that chloroform was confirmed in both of the retest samples at similar concentrations above the PQL. Review of the historical database indicates that since 1995 chloroform has only been detected three times at three different sample points in on-site wells,while low levels of chloroform have been intermittently detected in background well MW-6A. Since chloroform is also a constituent used to treat drinking water and has been rarely detected in on-site wells, and given the fact that background well MW-6A is located approximately 1200 feet from the landfill and was completed in a traffic box in a residential street, the detection of D:UO09-00ISWC_1SA09.doc 3 Geologic Associates