ARCHIVED REPORTS XR0006544 - Laserfiche WebLink

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CLEAWV7ER L R 0 U r Environmerrta! servrces Groundwater Monitoring, Results On April 2, 2001, static groundwater in the wells was present beneath the site at depths ranging from approximately 17 5 to 19 feet bgs Gauging data, combined with well elevation data, were used to calculate groundwater elevations in these wells, and to generate a groundwater elevation contour map The groundwater flow direction was calculated to be toward the west-southwest at a gradient of 0 016 ft/ft (Figure 3) Dissolved-phase contaminant con(entrations were detectable only at extremely low levels again this monitoring event Neither TPHg nor benzene was detected above the method detection limit in any of the wells sampled this event MTBE was detected in extremely low levels in four wells this event MW-1 (6 3 pg/L), MW-2 (1 3 lig/L), MW-4 (0 63 pg/L), and MW-6 (3 4 pg/L) (Figure 4) These extremely low to non-detectable residual dissolved-phase contaminant concentrations are consistent with a long term trend of contaminant level decrease following remediation at the site The samples collected from both sparge wells, screened deeper than the other monitoring wells, were also free of all detectable contaminant concentrations 1 he lack of contamination in these wells is noteworthy in that the results effectively delineate the vertical extent of residual dissolved-phase contamination on-site Samples from the on-site Domestic Well continue to be free of any petroleum contaminants Considering the well was not impacted by elevated contaminants levels prior to remediation, it appears that the well is not threatened by the extremely minimal post-remediation contaminant levels now present in the soil and shallow groundwater beneath the site It is noteworthy that the remnants of the dissolved-phase contaminant following active remediation are now barely detectable by analytical methods, indicating that groundwater beneath the site has effectively been restored to background quality Additionally, the very low levels of dissolved-phase contaminants that do remain are all below primary MCLS for California drinking water TPHg, benzene, and MTBE concentrations for MW-1 and MW-2 were plotted against time (since the start of remediation) (Figures 5a and 5b) The resulting graphs show a distinct downward trend for all contaminants in both wells, and lines fitted to each data set indicate that contaminants will continue to decrease at predictable rates with a high level of confidence Recent and cumulative groundwater sample analytical data are summarized on Table 2, and are shown on graphically on Figure 4 Copies of the laboratory report and chain-of-custody form are attached ZB107C GWMR 2Q01 4 April 9, 2001