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Former Chevron Service Station 9-0557 Page 2 <br /> 139 South Center Street May 6, 2010 <br /> Stockton, California <br /> feet bsg, and MW-15D, screened between forty-six and fifty-one feet bsg. Although <br /> one can argue that both wells are screened within the `smear zone', still there appears <br /> to be a notable increase in contamination in the deeper-screened well. <br /> 2. MW-18D2, screened between one-hundred and one-hundred five feet bsg, is the only <br /> deep well at this site. It appears that the concentrations of contaminants of concern <br /> have stabilized based on the analytical data for groundwater collected during the two <br /> years this well has been sampled; however, the EHD is not convinced that <br /> concentrations of approximately 1,000 micrograms per liter (Ng/L) of total petroleum <br /> hydrocarbons (TPH) quantified as gasoline (TPH-g) indicate that the plume is defined <br /> vertically. <br /> 3. Grab groundwater samples collected at one-hundred sixteen feet bsg from cone <br /> penetration test (CPT) boring CPT5-W-116, and at one-hundred ten feet bsg from <br /> CPT6-W-110 indicate significant petroleum hydrocarbon contamination in these <br /> deeper zones. Furthermore, the characterization of the subsurface lithology and the <br /> identified potentially permeable units using CPT technology indicate sand units at <br /> approximately one-hundred fifteen and at approximately one-hundred thirty feet bsg in <br /> borehole CPT-5A. The potential for groundwater contamination in these deeper <br /> permeable units exists, and if detected at significant concentrations must be defined <br /> laterally. <br /> 4. Borehole HP-4, advanced down-gradient of the source area, represents the deepest <br /> borehole where soil samples were collected. Analytical data indicate a significant <br /> increase in concentrations of contaminants of concern between soil samples collected <br /> at sixty-nine feet bsg and seventy-four feet bsg. <br /> Please explain how these data fit into the SCM and are consistent with an adequately <br /> delineated plume. If additional investigation is needed to complete the site characterization, <br /> submit a work plan to the EHD by July 23, 2010, that will address the delineation of <br /> contamination in soil and groundwater at this site. <br /> The site hydrogeological framework consists primarily of fine grained soil, varying mixtures of <br /> silt and clay, from surface grade to depths of approximately one-hundred feet bsg, with <br /> subordinate occurrences of sand to silty sand units of limited known vertical and lateral extent. <br /> Silty sand and silt with subordinate occurrences of sand units extend from approximately one- <br /> hundred to approximately one-hundred seventy-five feet bsg. Depth to groundwater has <br /> varied from approximately twenty-one to forty-seven feet bsg, and the maximum groundwater <br /> gradient direction is toward the southeast. <br /> Soil on the subject site has been found to be impacted by various chemicals of concern, <br /> namely TPH-g, TPH quantified as diesel (TPH-d) and benzene, toluene, ethylbenzene and <br /> total xylenes (BTEX). Intensely impacted soil occurs at depths between fifteen and sixty-two <br /> feet bsg, with less intense impact to depths as great as seventy-four feet bsg. Most of the <br /> intensely impacted soil occurs in fine-grained sediment in the saturated zone. Impacted soil <br /> occurs primarily in the former UST pit where USTs were removed in 1990 and extends <br /> laterally as much as thirty feet in all directions and as much as one-hundred seventy feet <br /> laterally alonq what is thouaht to be an antecedent caoillary frinae zone in the dawn-nrartient <br />