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acific Gas aftd Environmental Remediation <br /> . 340"0 Crown Ca!;ynn Road <br /> ` S:m R,,.mion,CA EJ bS35s <br /> 925.866.5508 <br /> Fax:J25J'i G.5681 <br /> April 25, 2008 <br /> Ms. Kathy Amaru <br /> Central Valley Regional Water Quality Control Board <br /> 11020 Sun Center Drive #200 <br /> Rancho Cordova, CA 95670 <br /> Re: Estimate of Contaminants Remaining in Soil and Groundwater, <br /> PG&E McMullin Natural Gas Dehydrator Station, San Joaquin County, California <br /> Dear Ms. Amaru: <br /> This letter is being submitted in response to an email request from the Central Valley Regional Water <br /> Quality Control Board (CVRWQCB) for an estimation of contaminants remaining in soil and <br /> groundwater at the McMullin Dehydrator Station (site) in San Joaquin County. The mass of remaining <br /> contaminant is estimated in support of the "Request for No Further Action at the McMullin Dehydrator <br /> Station", which was submitted by CH2M Hill in 2004. <br /> As requested by the CVRWQCB, contaminant data used for the estimation of remaining hydrocarbon <br /> mass were obtained from Table 3 of the "Request for No Further Action at the McMullin Dehydrator <br /> Station". The mass of contaminants remaining in soil is estimated to be 0.40 kg of Total Petroleum <br /> Hydrocarbon in gasoline range (TPHg) and 1.53 kg of Total Petroleum Hydrocarbon in diesel range <br /> (TPHd). The mass of contaminants remaining in groundwater is estimated to be 0.0003 kg of Benzene, <br /> Toluene, Ethyl-benzene, and Xylenes (BTEX), 0.0075kg of TPHg, 0.0075kg of TPHd, and 0.0752kg of <br /> Total Petroleum Hydrocarbon in motor oil range (TPHmo). Table A shows the detailed estimation of <br /> the contaminants remaining in soil, while Table B shows the same information for groundwater. The <br /> approaches to estimate mass of contaminants remaining in soil and groundwater are described below. <br /> Estimate of Contaminants Remaining in Soil <br /> To estimate the mass of hydrocarbon contaminants remaining in soil at the site, the volume of soil, the <br /> concentration of impacted soil, and estimated soil density were determined. Once these data were <br /> compiled, a calculation of remaining hydrocarbon mass was possible. To determine these data, the <br /> following process was followed: <br /> • To represent the lateral extent of impacted soils at the site six (6) two dimensional polygons were <br /> drawn. As shown on Figure 1, each polygon was drawn around an area that: <br /> o contained at least one detection of hydrocarbon(s) in soil, and <br /> o was bounded by non-detect (ND) soil samples. <br /> • To calculate the volume of impacted soil within the two dimensional polygon, assumptions needed <br /> to be made regarding the vertical extent of impacted soil. <br /> o When considering sidewall soil samples, soil volumes were calculated assuming soil <br /> impacts extended from the depth of the polygon's ND boundary line to 16 feet bgs (the <br /> total depth of the 2002 excavation). Furthermore, the slope of the sidewall was <br /> incorporated in the volume calculations. <br /> o When considering excavation floor samples, soil impacts-were assumed to begin at the <br /> total depth of the excavation (16 feet) and extend to 18 feet bgs, an additional 2 feet <br /> Page 1 of 3 <br />