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I <br /> 4 SITE CONCEPTUAL EXPOSURE MODEL <br /> PSC has prepared a site conceptual exposure model to assess the fate and transport <br /> of petroleum hydrocarbons released at the site The transport of petroleum <br /> hydrocarbons, potential relevant exposure pathways and potential receptors are <br /> discussed in the following sections <br /> 1 <br /> 4.1 Transport <br /> Typical mechanisms of transport for organic compounds in the subsurface <br /> include dispersion (advection), sorption and volatilization Dispersion or <br /> advection is the mechanism that results in the compound migrating laterally or <br />' vertically through the subsurface At this site, petroleum hydrocarbons would <br /> infiltrate through the vadose zone to underlying groundwater occurring at a <br /> depth of about 40 feet The plume of light, dissolved-phased, petroleum <br /> hydrocarbon compounds would then migrate primarily laterally by advection <br /> along the groundwater flow path Migration may be limited by the relative <br /> permeability of the soil and by adsorption onto the soil matrix The plume of <br /> impact eventually reaches a steady state through natural degradation and <br /> attenuation <br /> Volatilization results in mass transfer of the organic compound from the <br /> aqueous phase to the vapor phase At this site, volatilization may occur from <br />' unsaturated soil within the soil source area and from groundwater above the <br /> plume of impact <br /> 4.2 Potential Exposure Pathways <br /> Potential exposure pathways for the gasoline components at this site include <br /> ingestion, inhalation and dermal contact Such contact with petroleum <br /> hydrocarbons in soil or groundwater creates a possible risk to potential <br /> receptors <br /> 4.2.1 Ingestion Pathway <br /> An evaluation of the ingestion pathway assesses the risk of petroleum <br /> hydrocarbons migrating to surface water bodies, drinking water and <br /> non-drinking water supply wells Factors to consider for this pathway <br /> involve the identification of potential receptors, migration potential of <br /> the contaminants of concern, distance and relative location of surface <br /> water bodies and wells, and extent and magnitude of impact <br /> 15 <br /> Remedial Action Completion Report <br /> The Eartligrains Company Facility <br /> 2651 South Airport Way <br /> Stockton California <br /> Prepared for THF EARTIIGRAINS COMPANY Prepared by Philip SLrvices Corp <br /> 11\ENV\62400132\12I382EG\StocktonlSio ktonVRcmedialCompletionRepon doe April 14 2003 <br />