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*Awl <br /> MEMORANDUM <br /> September 7, 1995 <br /> Richmond, California <br /> SESOIL Vadose Zone Model <br /> Former Chevron Statlon#9-7780 <br /> 3246 East Fremont Street <br /> Stockton,California <br /> Mr. Brett Hunter: <br /> San Ramon, CA <br /> This memorandum addresses the request made by the San Joaquin County Public Health <br /> Services, Environmental Health Division (PHS/EHD)Technical Review Committee (TRC)for a <br /> fate and transport analysis of the remaining residual soil contamination and its' potential threat to <br /> contaminate groundwater at the site.The American Petroleum Institute Decision Support System <br /> (APIDSS)for exposure and risk assessment contains the vadose zone fate and transport model <br /> SESOIL.The APIDSS SESOIL model was used to model the migration of the residual soil <br /> contamination in the soil column to determine if this contamination would migrate 45 feet into the <br /> groundwater directly beneath the site (location of Receptor for this SESOIL model).Groundwater <br /> is currently approximately 60 feet below ground surface (bgs), and the highest groundwater levels <br /> recorded at the site were 45 feet bgs. Benzene,toluene, ethylbenzene, xylene and naphthalene <br /> (as a surrogate for TPH-gasoline) are the contaminants modeled for this site. The parameters, <br /> assumptions, calculations used as inputs to this model are presented for discussion purposes. <br /> Two SESOIL scenarios were modeled for this site: 1) Practical Scenario: Maximum soil analytical <br /> results from site monitoring wells and soil borings were modeled and 2)Conservative Scenario: <br /> Maximum soil analytical results from the bottom of the UST excavation (18 foot samples)were <br /> modeled. The results of the two modeled SESOIL scenarios indicate that the residual soil <br /> contamination at the site would not migrate to groundwater located 45 feet beneath the site. <br /> Therefore, based on the results of the SESOIL modeling performed for this site, it is <br /> recommended that the PHS/EHD Technical Review Committee proceed with closing this site. <br /> SESOIL Model Input Parameters <br /> The attached APIDSS Data Requirements Section outlines the input parameters that were used <br /> to model the soils at this site. The input parameters will be discussed in the order that they are <br /> presented on the attached output sheets. <br /> 1) Chemicals Modeled <br /> The BTEX constituents were chosen to be modeled because they are the regulated <br /> primary chemicals of concern at the site. TPH-gas can not be modeled as a single <br /> compound because it is a complex mixture of many chemicals. For the purposes of this <br /> model, naphthalene was used as a surrogate representing the TPH-gas detected in the <br /> site soils. <br /> 2) SESOIL Model Control/Climate Parameters <br /> The model was simulated for 50 years through 2 soil zones (upper silty clay zone and <br /> lower silty sand zone) at the site. The surface temperature was modeled as 16 degrees <br /> C,with a 0.001 cm/day evapotranspiration (negligible). Rainfall was modeled at 35 <br /> v <br />