SITE INFORMATION AND CORRESPONDENCE_FILE 2

Home Browse Search

'Chapin Brothers, Inc. - 3 - 5 January 2011 1766 Monte Diablo Ave., St(- ..ton San Joaquin County intrusion, gross contamination or direct contact to Site workers. Land use (auto repair shop) is not expected to change in the foreseeable future. However, lead in soil was reported at 1,900 mg/kg in one shallow (2' bgs) soil sample taken outside of the building in a grassy area along the eastern edge of the property during soil vapor sampling in May 2010. The soil was resampled at three locations approximately 3' away at the same depth around the initial sample, and lead in soil was found to be elevated (49 to 150 mg/kg) above background lead levels for Stockton (20 to 40 mg/kg per SJCEHD verbal communication), but an order of magnitude below the initial lead sample. The consultant stated soil contamination from lead was limited in extent. Past service station practices with disposing of old automotive batteries may be responsible for the lead results, although no visual evidence of batteries or lead was reported as found in the soil samples. Sensitive Receptor Survey There are six public supply wells located within 2,000 feet of the site. The closest public supply well (inactive) is 1,200 feet west of the Site. One domestic well is located 250 feet to the north of the Site. Groundwater monitoring and grab groundwater sampling indicates the plume has not traveled more than 200' and 500' offsite to the west and southwest, respectively, and less than 100' to the north. Based on the limited lateral extent of the groundwater plume, the public supply and domestic wells are not threatened by the groundwater plume. The California Water Service Company, which serves public water in Stockton, stated in writing on 2 March 2010 that they do not plan to use the groundwater within the next 100 years. Site Geology and Hydrogeology Site soils are described as gravels, sands, silts and clays to 40' bgs, the maximum extent of investigations. Boring logs and geologic cross sections show a large fine sand layer is located between 10' and 30' bgs to the west and north of the USTs, and the sand varies in thickness from 5' (west of the USTs to the southwest across Buena Vista Ave.) to 15' (under the USTs and to the north across Monte Diablo Ave.). Between 1990 and 2010, depth to water varied from 8' bgs to 21' bgs. The groundwater gradient varied from 0.0007 ft/ft/ to 0.016 ft/ft. Measured groundwater flow direction, predominantly towards the north, varied from southeast to northwest. However, the concentration gradient for TPI-Ig was biased towards the west and southwest (highest offsite concentrations), possibly indicating an earlier than 1994 to 2010 groundwater gradient in those directions prior to groundwater monitoring, or a change in elevations of the monitoring wells. Residual Mass Calculation The mass of TPHg removed from soil by over-excavation and SVE was not reported in the available documents. The mass of TPI-Ig removed from groundwater by AS and GWP&T was reported as approximately 957 lbs. The consultant estimated TPI-Ig remaining in soil as approximately 5,733 lbs. and TPI-Ig remaining in groundwater as 2.9 lbs. The consultant estimated that Water Quality Goals will be reached Site wide by 2035.