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i i Remedial Action Plan Addendum January 14, 1993 ; 1 1. Southland Store No. 20304, 455 Grant Line Rd., Tracy, CA 2 soil and groundwater quality by HLA revealed that both soil and groundwater had been degraded with hydrocarbon concentrations exceeding state cleanup levels.2'3 In 1989, the USTs and ancillary pumps and pipes were removed. Additional subsurface assessment since 1989 has indicated that remedial action is necessary to meet hydrocarbon concentrations acceptable to the State of Cal'rfomla". i 1.2 Site Conditions i Since 1986, 16 groundwater monitoring wells have been installed at the site, two of which were abandoned during tank removal activities (Figure 2). In 1986, when the leak was discovered, approximately 2 gallons of separate-phase product were balled from monitoring wells adjacent to the tank pit. Since that time no separate-phase product has been measured, although a sheen in wells Immediately adjacent to and east of the tank pit (MW-9 and MW-10) has been intermittently documented. Groundwater monitoring has been conducted on a quarterly basis since 1986. A summary of the soil and groundwater conditions is presented in the following sections. For detailed i information refer to the documents footnoted in Section 1.1. t 1.2.1 Unsaturated Zone i � Lithofogy. The site is underlain by interbedded stiff clays and silty to sandy clays to a depth of at least 15 feet below grade. These soils are well consolidated and are estimated to have low to very low permeability. Hydrocarbon Distribution. Hydrocarbon analyses of soil samples collected during site investigations suggest a preferential distribution of hydrocarbons in the vadose zone soils primarily east and northeast of the former tank pit. The distribution appears to correspond to the direction of groundwater flow, which is generally to the northeast across the site with an isolated easterly = trending trough that passes through the central area of the site. The primary location of s �4 2 Soil as Survey Interim Rego , June 28, 1988 3 Monitorina Well Installation Report, August 25, 1988 4 Problem Assessment Report, January 10, 1989 j R3263A1.5LL GROUNDWATER �h TECHNOLOGY,INC.