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i <br /> JZZAWMA <br /> June 21, 1993 Working To Restore Nature <br /> ARCO Service Station No 2130 <br /> I <br /> measured in January 1992 at depths ranging from approximately 61 to 63 feet bsg, these <br /> depths decreased during 1992 to 64 to 66 feet bsg Between October 1992 and April 1993, <br /> the ground water has risen approximately 5 feet to depths ranging from 59 to 61 feet bsg <br /> At several locations beneath the site(borings/wells B-17/AS-1,B-21/MW-6 and B-22/MW- <br /> 7), <br /> -22/M -7), a clayey silty unit of unknown thickness exists beneath the sand and silty sand at depths <br /> starting at 71 to 74 feet bsg The direction of ground water flow during 1992 was <br /> 0 consistently to the southeast. The interpreted subsurface conditions in the area of the site <br /> subject to air sparging is shown on geologic cross-section A-A' (Plate 3) The trace of the <br /> geologic cross-section is included on Plate 2 <br /> N Residual Uldrocarbons in Soil <br /> Results of previous investigations revealed residual hydrocarbons in vadose zone and <br /> p • capillary fringe soil at concentrations greater than 100 parts per million (ppm) at depths <br /> ranging from 15 to 61 feet bsg Residual hydrocarbons in vadose zone soil (defined as the <br /> depths from 15 feet to 40 feet bsg) appear limited to beneath and slightly adjacent to the <br /> existing UST's. Soil containing residual hydrocarbons in this region exists predominantly <br /> north and south of the existing tanks as evidenced by TPHg detected in borings B-1 and B-3 <br /> Residual hydrocarbons in vadose zone soil that appear to have had historical contact with <br /> shallower ground water surfaces and the present capillary fringe zone (defined as the depths <br /> i from 40 feet to 61 feet bsg) exist beneath and west of the tanks Based on the analytical <br /> data, it appears that the majority of the TPHg in soil above the ground water surface exists <br /> beneath and to the general vicinity of the UST's <br />• <br /> Dissolved Hydrocarbons in Ground Water <br /> Dissolved hydrocarbons were detected in ground water beneath the site during previous <br /> investigations Based on results of ground-water samples collected in January 1993, TPHg <br /> 30003-8 <br /> 5 <br />