ARCHIVED REPORTS XR0007793 - Laserfiche WebLink

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1.0 EXECUTIVE SUMMARY This document reports the results of the first quarterly groundwater sampling event conducted on the three (3) existing groundwater monitoring wells presently installed at Stockton Plating Incorporated (SPI), 632 South El Dorado Street, Stockton, California (the Site) The work reported here was accomplished under a contract to provide for a single sampling event of the existing wells to establish the present groundwater conditions with respect to gasoline range hydrocarbons and groundwater flow directions prior to initiating further assessment or remedial actions The three existing wells were installed by Condor Earth Technologies (Condor) on 9-10 February 1994 and sampled by Condor on 22 February 1994 Soil samples were collected during well installation, however fuel hydrocarbons were detected in samples from monitoring well I MW-3 only Significant concentrations of benzene were found in samples taken from the 20, 30 and 45 foot BGS levels in MW-3 Groundwater samples collected by Condor on 22 February 1994 indicated that the maximum impact from gasoline fuel hydrocarbons was observable in MW-3 at a concentration of 48 mg/L TPHg and 13,000 ug/L (or 13 0 mg/L) benzene TPHg was not detected in either MW-1 or MW- 2, however benzene was detected at concentrations of 2 0 ug/L and 3 5 ug/L, respectively RFA sampled the three existing wells on 24 October 1994 Benzene was not detected in MW-1 during this round of sampling/analyses Benzene was found in MW-2 at 0 72 ug/L, representing a nearly fourfold decrease in concentration from the initial sampling event Benzene was found ' in MW-3 at a concentration of 8,800 ug/L, roughly two-thirds of its initial value of 13,000 ug/L Water levels as measured on 24 October 1994 in the three wells averaged about 1 17 feet lower than during the initial sampling conducted on 22 February 1994 Groundwater flow gradient was calculated to be the same as found during the initial well sampling The decrease in concentrations as measured in all three wells for fuel hydrocarbon constituents may be related to a slightly lower water table, but could also be related to other factors, such as lower water saturations in the vadose zone during the drier month of October as compared to a much wetter month in February, however the exact reason is not clear Stockton Plating Incorporated (SPI) has indicated an interest in continuing to assess the extents and nature of soil and groundwater impacts at the site The area considered to be downgradient from the presumed source provides for considerable limitations to further groundwater impact extent assessment due to low overheads, tight clearances, buildings and parts racks Immediately beyond SPI's property, in the downgradient direction, lies a known diesel contaminated site and the Mormon Slough Should the groundwater impact be beyond SPI's property boundaries, exploration for the downgradient extent may prove problematical At present, a target location has been selected, permitted and contracted for MW-4, on SPI's property about 60' downgradient from MW-3 Well MW-4 will be installed on 16 November 1994 366-001 _ i _