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SEP 2 8 W2 <br /> Harding Lawson Associates <br /> ENVIRONMENTAL HEALTH - - - - - - <br /> P0MITfPF-gyIQFS <br /> September 2, 1992 <br /> 10433 382 <br /> Shell Oil Company <br /> 511 North Brookhurst Street <br /> Anaheim, California 92803 <br /> Attention: Mr. Chuck Kolesar <br /> Dear Chuck: <br /> Concerns Regarding RWQCB Requests <br /> Deep Plume Delineation Work Plan <br /> Shell Oil Distribution Plant <br /> Stockton, California <br /> Harding Lawson Associates (HLA) has reviewed the California Regional Water Quality Control Board <br /> (RWQCB)letter dated August 20, 1992 requesting a Work Plan for investigating the vertical extent of <br /> soil and groundwater contamination at the Shell Distribution Plant, Stockton, California. We would <br /> like to take this opportunity to present you with our review of the RWQCB comments, specifically, <br /> their request to perform chemical analyses of saturated soil samples. <br /> The RWQCB requested that Shell delineate the vertical extent of the petroleum hydrocarbon plume in <br /> both soil and groundwater. Specifically, the RWQCB believes that the definition of"soil contamination <br /> below the water table" is required to effectively remediate the site. The RWQCB cites a paper(Haley, <br /> et al, 1991)which they claim advocates collection and chemical analysis of saturated soil samples. We <br /> believe that chemical analysis of saturated soil (below the water table) is unnecessary, and merely <br /> provides information on groundwater chemistry. Our position is based on evaluation of fundamental <br /> concepts of contaminant hydrogeology. <br /> The petroleum hydrocarbons present in unsaturated soil and groundwater at the Shell Facility <br /> originated from releases from the above-ground storage tanks and from the above- <br /> ground loading rack (HLA, 1991). Under the influence of gravity and capillary forces, the <br /> hydrocarbons infiltrated through the unsaturated zone to the capillary fringe and water table. Because <br /> the petroleum hydrocarbons are less dense than water, they tend to accumulate at the groundwater <br /> table; light free-phase petroleum hydrocarbons do not sink to the bottom of the aquifer. Vadose zone <br /> water or groundwater in contact with the petroleum can then dissolve the hydrocarbons. The dissolved <br /> hydrocarbon plume in groundwater can be then transported horizontally, and potentially vertically <br /> under ambient hydraulic gradients and the effects of molecular diffusion and hydrodynamic dispersion. <br /> Engineering and 7655 Redwood Boulevard, PO. Box 578, Novato,CA 94948 4151892-0821 <br /> Environmental Services A Su udiary„/Ha Ni,M111 iurcs•Officer Naiinnwide <br />