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SECTION 6.0 CONCLUSIONS AND RECOMMENDATIONS i 6.1 CONCLUSIONS Based on a review of historical data and a limited groundwater investigation at the Stockton Yard site, the following conclusions can be made © The well survey found evidence of local private wells approximately 500 feet north 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 corner 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], at the fairgrounds [>500 feet], and the south of the Stockton Yard on 4th Street [>500 feet]) Although the mobile home park wells were located within 500 feet of the Stockton Yard site, these wells have been closed Based on historical groundwater monitoring data (chemical and hydraulic gradient) for the Stockton Yard site, it is unlikely that detectable concentrations of petroleum hydrocarbons have migrated off the Stockton Yard 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 Q Groundwater monitoring of the 12 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 July 1996 TPH-g and BTEX concentrations in groundwater have decreased with an increase to the groundwater elevation over time since July 1996 • Based on the results of the vertical groundwater sampling in wells MW-6 and VW-2, an overall decrease in TPH-g and BTEX concentrations with depth was observed to the total depth of the ' respective wells However, the vertical extent of TPH-g, BTEX, and MTBE has not been estimated by the discrete vertical sampling using the Kabism sampler • The slug tests estimated the hydraulic conductivity to range from 13 x 10'2 to 7 6 x 10"; cm/s, the transmissivity to range from 4 1 to 9 5 em-/s, and groundwater velocities to range from 5 4 x 10'5 to 9 2 x 10'5 cm/s, these data are consistent with a sand/silty sand saturated zone ' The slug tests may have been influenced by the sandpack within the borehole annulus, which is assumed to be more permeable (well-sorted sand) than the sand/silty sand saturated zone • Evidence that natural attenuation is occurring may be assumed by the decrease in TPH-g and BTEX concentrations in groundwater over time however, the results of inorganic analyses and rwater quality parameters were inconclusive in suggesting whether electron acceptors such as • iutrate and sulfate are available and being utilized within the petroleum-hydrocarbon plume in groundwater S IPRo,1141040100041SYYARMMAINTYARDSA.DOC 6-1 URS Greiner Woodward Clyde