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Harding Lawson Associates <br /> May 28, 1993 <br /> 10433 005 <br /> Mr. Chuck Kolesar <br /> Shell Oil Company <br /> 511 North Brookhurst Street <br /> Anaheim, California 92803 <br /> Response to RWQCB Letter <br /> Preliminary Results of Plume Characterization <br /> Shell Oil Distribution Plant <br /> Stockton, California <br /> Dear Chuck: <br /> Per your request, we have prepared this response to Comments 2 and 3 of the Regional <br /> Water Quality Control Board, Central Valley Region (RWQCB) letter dated May 10, <br /> 1993 regarding the Preliminary Results of Plume Characterization, Shell Oil <br /> Distribution Plant, Stockton, California. <br /> Comment 2 <br /> The RWQCB indicated that the extent of the plume in the vicinity of hydropunch <br /> samples 26, 28, and 31 had not been defined and that a monitoring well should be <br /> installed "to establish the zero line of contamination and ascertain that BP Oil Company <br /> is not a source." <br /> On the basis of data collected to date, it appears that the extent of groundwater with <br /> dissolved hydrocarbons in the area of hydropunch samples 26, 28, and 31 has been <br /> adequately characterized and that no further investigation in this area appears <br /> warranted due to the following reasons: <br /> The regional groundwater gradient at the Shell Stockton Plant is to the south. Based on <br /> this southern groundwater flow direction, the concentrations detected in hydropunch <br /> sample 26 would be expected to migrate in the direction of hydropunch samples 28, 30, <br /> and 31. Hydropunch samples from these locations showed low to nondetect chemical <br /> concentrations, and suggest that dissolved hydrocarbons in the northeast corner of the <br /> site is of limited areal extent and has not migrated significantly. The presence of the <br /> fine-grained aquifer materials beneath the site (predominantly silts and clays) suggest <br /> the potential for chemical migration from the localized area would be limited. Also, <br /> moderate chemical dispersivity rates characteristic of these types of fine-grained <br /> materials would tend to enhance dilution of chemical concentrations over relatively <br /> short distances. This rationale is supported by the significant decrease in chemical <br /> Engineering and 7655 Redwood Boulevard. P.O. Box 578, Novato,CA 94948 4151892-0821 <br /> Environmental Services A Subsidiary of Narding A saciures•OQins Nationwide <br />