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SECTION 6.0 CONCLUSIONS AND RECOMMENDATIONS 6.1 CONCLUSIONS Based on a review of historical data and a limited groundwater investigation at the Shop 10 site, the following conclusions can be made o The well survey found evidence of local private wells approximately 550 feet northeast of the northern property boundary of the site Specifically, two wells screened from 50 to 180 feet bgs and 250 to 300 feet bgs are owned and operated by Clark Well, located at the comer of South B Street and Charter Way One of the wells is used occasionally, and the other has been shut off Other private wells were located in the vicinity of Stockton Yard (at the adjacent mobile home park [ 500 feet], possibly at the fairgrounds [ 500 feet], and approximately 900 feet southeast of the Shop 10 site on 4th Street [ 500 feet]) Based on historical groundwater monitoring data (chemical and hydraulic gradient) for the Shop 10 site, it is unlikely that detectable concentrations of petroleum hydrocarbons have migrated off the Shop 10 property in the direction of these wells Analytical data for the Clark Well wells and the California Water Service well on 4th Street indicate that TPH-g, BTEX, and MTBE were not detected above laboratory detection limits 0 Groundwater monitoring of the 10 monitoring wells and two vapor monitoring wells has been conducted on a quarterly basis since 1995 The highest TPH-g and BTEX concentrations in groundwater were detected in September and May 1997, respectively TPH-g and BTEX concentrations in groundwater have generally decreased with an increase in the groundwater elevation over time since July 1996 o Based on the results of the vertical groundwater sampling in wells MW-13, and VM-2, concentrations of TPH-g and BTEX did not vary significantly with depth However, the vertical extent of TPH-g, BTEX, and MTBE has not been estimated by the discrete vertical sampling using the Kabis'rm sampler o The slug tests estimated the hydraulic conductivity of the soil to range from 1 2 x 10"Z to 19 x 10"3 centimeters/second, the transmissivity to range from 1 7 to 6 9 square centimeters/second, and groundwater velocities to range from 1 3 x 10-5 to 84 x 10"5 centimeters/second These data are consistent with a sand/silty sand saturated zone The slug tests may have been influenced by the sandpack within the borehole, which is assumed to be more permeable (well-sorted sand) than the sand/silty sand saturated zone ID Evidence that natural attenuation is occurring may be assumed by the steady decrease in TPH-g and BTEX concentrations in groundwater over time, however, the results of inorganic analyses . and water quality parameters were inconclusive in that electron acceptors (nitrate and sulfate) are being utilized within the petroleum hydrocarbon plume in groundwater S U ROJ411040100041SHOP104SHOP10SkDOC 6-1 URS Greiner Woodward Clyde