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URS • Mr Harlin Knoll Page 2 San Joaquin County Environmental Health Department ARCO 2093 February 24, 2005 by well graded sand to silty sand to the maximum depth explored Boring logs, the locations of historic generalized geologic cross-sections, and the historic cross-sections are presented in Attachment A r Based on quarterly water-level measurements through the fourth quarter 2004, the inferred groundwater flow direction is northeast at a gradient of 0 002 to 0 005 feet/foot Since monitoring began in April 1987, the groundwater table surface has been relatively stable at approximately 11 to 13 feet bgs Groundwater elevations, gradients, and flow directions are summarized in Tables 1 and 3 Previous Investigations Initial Investigation (December 1986) In December 1986, a 500-gallon waste oil UST was removed from the site by American Environmental Management Corporation Several small holes were noted in the UST upon removal Soil discoloration and petroleum odors were detected in the excavation A single undisturbed soil sample was collected (Tracy #1) under the observation of the San Joaquin Local Health District Twelve days later, two additional soil samples were collected from the floor of the excavation (Tracy #2 and Tracy #3), and two samples were collected from the"clean" soil stockpiled during the tank removal Only Tracy#3 was analyzed at the laboratory Petroleum-related compounds were reported in the Tracy #3 soil sample at the following concentrations benzene 130 parts per billion (ppb) (which is equivalent to micrograms per kilogram [gg/kg] and micrograms per liter [gg/L]), toluene 28 ppb, ethylbenzene less than 1 ppb, and xylenes 30 ppb Soil sample laboratory analytical results for this investigation are presented in Attachment B Problem Assessment(ARM 1987) During April 1987, Brown and Caldwell completed a site investigation for the site The scope of the investigation was to determine the presence or absence of hydrocarbons in the soil and groundwater Four boreholes were drilled, soil samples were collected, and groundwater monitoring wells were installed (ATR-1 thru ATR4) Each well was sampled on April 23, 1987 The maximum reported concentration of total fuel hydrocarbons in the soil was 1,700 milligrams per kilogram (mg/kg) (which is equivalent to parts per million [ppm]), in ATR4 The concentrations of benzene, toluene, and xylenes in + monitoring wells ATR-3 and ATR-4 were above California action levels for drinking water The �+ analytical results for soil and groundwater samples for this assessment are summarized in Attachment C Site Investigation (March 1989) The Regional Water Quality Control Board (RWQCB) directed ARCO to define the extent of hydrocar- bon contamination using exploratory borings and installing additional monitoring wells In March 1989, three boreholes were drilled, and monitoring wells were installed (ATR-5, ATR-6, and ATR-7) Soil samples were obtained during the drilling for laboratory analysis Total petroleum hydrocarbons as K\Wprocess\J5 BP ARCO\02093\Closure Report\Draft of 2093 Closure Report doe