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TECHNICAL MEMORANDUM <br /> CH2MHILL <br /> Evaluation of TPH and Benzene Soil Verification <br /> Concentrations, Stockton Former MGP Site <br /> PREPARED FOR: Patricia Sullivan/Pacific Gas and Electric Company <br /> PREPARED BY: Paula Bolio/CH2M HILL <br /> DATE: February 15, 1999 <br /> The Remedial Design and Implementation Plan(RDIP) for the Stockton former <br /> Manufactured Gas Plant (MGP) site includes verification sample target concentrations for <br /> polynuclear aromatic hydrocarbons (PNAs) (samples to be taken in the bottom and sides of <br /> soil source area excavations). The soil source areas that are planned for excavation were <br /> defined by modeling future transport of PNAs in the vadose zone to Zone A groundwater. <br /> After excavation of the soil source areas,modeling of PNAs and benzene showed no future <br /> impact to groundwater. This memorandum documents verification target concentrations for <br /> benzene,TPH-gasoline and TPH-diesel that were developed in response to verbal <br /> comments made by Philip Isorena of the State of California Central Valley Regional Water <br /> Quality Control Board (RWQCB) on January 4, 1999.This memorandum presents <br /> verification target concentrations and documents how these concentrations were developed <br /> for: <br /> • Benzene-in Source Area 3 excavation <br /> • Total petroleum hydrocarbons (TPH) as diesel and gasoline - in all soil source area <br /> excavations <br /> Benzene <br /> The VLEACH model that was used in the Feasibility Study PG&E Stockton Former <br /> Manufachired Gas Plant Site (CH2M HILL,June 1997) for identifying source areas was used <br /> to develop target verification concentrations for PNAs.VLEACH is also being used to <br /> develop the target verification concentration for benzene at the Source Area 3 excavation. <br /> Source Area 3 consists of the tar sump and tar pit and is the only location at the site where <br /> high concentrations of benzene are expected.Development of the target verification <br /> concentration for benzene included a Source Area 3 vadose zone polygon and a <br /> groundwater mixing cell beneath the polygon,as described in the Feasibility Study <br /> (CH2M HILL,June 1997).It is assumed that the excavation of Source Area 3 will result in <br /> clean soil from the surface to 8 feet below ground surface (bgs).The resulting groundwater <br /> concentration beneath this polygon will not exceed the RWQCB's beneficial use <br /> concentration for benzene in water of 0.35µg/L.Conservatively,it is assumed that for 5 feet <br /> beneath the excavation,from 8 to 13 feet bgs,the soil may contain a measureable amount of <br /> benzene. The model predicted that this concentration may be up to 1.8 mg/kg in the 5 feet <br /> of soil beneath the excavation of the tar pit and not adversely impact groundwater. <br /> TPHBBENZENEVERIFICATION MEMO DOG I <br />