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t „Pombo,Property <br /> Corrective Action Plan <br /> 1. Page: 2 <br /> . Additional definition of vertical and lateral definition of the soil and groundwater <br />' impacts associated with the former underground storage tanks was required by <br /> San Joaquin County Environmental Health in December 8, 1998 letter. No <br /> further assessment of the former underground storage tank area at the southern <br />' potion of the site was required. <br /> 1.2 Scope of Work <br /> ' The Corrective Action Plan presented here consists of preliminary fate and <br /> transport modeling of the identified contaminates of concern and the evaluation of <br /> the effectiveness of four possible mitigation scenarios. Fate and transport <br /> modeling of the volatile aromatic hydrocarbons and methyl tert butyl ether was <br /> conducted using the USEPA model `Bioscreen4." <br /> Corrective action scenarios for low risk closure, excavation of source material, <br /> enhanced aerobic biodegradation, and excavation of source material with <br /> ' enhanced aerobic biodegradation were constructed and the resulting site <br /> conditions modeled with "Bioscreen4." Estimated costs based on the 1995 UST <br /> cleanup Fund Cost Guidelines were calculated for each scenario. <br /> 2.0 Hydrogeologic Characteristics <br /> lie. <br /> 2.1 Beneficial Uses of Surface and Groundwater <br /> ' Beneficial uses of the surface and groundwater in the Tracy Basin of the Central <br /> Valley are Municipal, Domestic, and Agricultural. Imported surface water is <br /> supply by the unlined Westside Irrigation District, Lower Main Canal for flood <br /> irrigation of row crops. Municipal water is supplied by the city of Tracy for the <br /> subdivisions located to the north and east of the site. <br /> tDomestic use of the groundwater is through small water supply wells associated <br /> with private residences and shops. Agricultural uses of the groundwater-through <br /> high volume irrigation supply wells, which are used to augment the surface <br /> ' irrigation <br /> ' 2.2 Physicals and Topographic Features <br /> The Pombo Ranch is located on the southwestern edge of the San Joaquin Delta. <br /> The shallow unconsolidated sediments present at Pombo Ranch are composed <br /> fine-grained overbank and coarse grained channel deposits sourced from <br /> Cretaceous and Tertiary rocks exposed in the Coast Range Mountains to the west. <br /> ' The soils at Pombo Ranch are mapped by the US Soil Conservation Service as <br /> being composed of the Capay Series Soils, which are interpreted as forming in <br />