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' Page 3, Griii , ion7i94 Geologic Map of California, this sequence is mapped as the Modesto Formation in the site area The Modesto Formation consists of relatively unweathered and non-eroded Iight-brown sand and silt, and was deposited in the late Pleistocene within the last several tens of thousands of years The lower sequence consists of fine-grained to gravelly sand in TB-1, MW-1, and GT-2 but is ' mostly clayey silt in GT-1 This sequence is highly permeable and is water-bearing at a depth of 85 feet at the present time It is separated from the upper sequence by a red-brown, oxidized, 17 consolidated, clayey to sandy zone that drills slowly and probably represents the C horizon r�w (hardpan) of an ancient soil profile (paleosol) This paleosol occurs at a depth of between 55 and 65 feet, and is interpreted as the San Joaquin soil horizon that formed on the Riverbank Formation following its deposition in the late Pleistocene, approximately 100,000 years ago (Figure 3) The base of the Riverbank Formation was not penetrated in the borings,and the thickness of the ' formation at the site is unknown 3.3 Field Evidence of Contamination Detectable hydrocarbons were apparent from about 25 feet in GT-1 and slight hydrocarbon odors were detected to a depth of 85 feet The extruded portion of the sample obtained from the split-spoon sampler exhibited PID readings that ranged from 12 to 72 ppm and averaged 42 throughout a 55-foot interval PID readings in GT-2 were not significant above 50 feet PID readings below this depth ranged from 1 to 57 ppm and averaged about 19 over a 30-foot interval 3.4 Laboratory Results of Soil Samples ' Laboratory analysis of selected samples from GT-1 detected from 98 to 6,500 mg/kg TPH-diesel and 6 5 to 690 mg/kg TPH-gasoline (Appendix B) Also, benzene was detected at a concentration of 0 2 mg/kg at a depth of 50 feet and other aromatic hydrocarbons in the gasoline range were also present at this depth Of the three samples analyzed from GT-2,two had no hydrocarbons above detection limits, and one had 38 mg/kg of TPH-gasoline, 250 mg/kg TPH-diesel and 0 026 mg/kg Xylene 4.0 GROUNDWATER MONITORING 4.1 Well Survey Monitor wells GT-1 and GT-2 were surveyed on September 6, 1994 Monitor well MW-1 was used as the base elevation for comparing groundwater changes s 4.2 Sampling Procedures Quarterly groundwater monitoring took place on September 6, 1994 LJpon arrival at the site, all wells were opened and allowed to equalize with atmospheric pressure Approximately thirty minutes after opening, a Solinst water level meter was used to nieasare the depth to groundwater