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Harding Lawson Associates <br /> area. The tanks were removed in March 1989 and paved over with <br /> asphalt. During excavation, Southland directed HLA to install <br /> ' lateral four-inch Schedule 40 PVC screened piping in the tank <br /> cavity at depths of three feet and six feet below surface for <br /> possible use as soil-vapor and groundwater extraction in the <br /> future. <br /> 2 . Subsurface Conditions <br /> a. Site Stratigraphy, <br /> HLA investigated subsurface conditions at the site <br /> by drilling, sampling, and installing groundwater monitoring <br /> Iwells MW--1 through MW-14, see the Site Plan, Plate 2 . A cross- <br /> section showing subsurface conditions is presented in Plate 3 . <br /> Near-surface soils across the site are generally composed of <br /> stiff clays that extend to between 5 and 15 feet below grade. <br /> The upper clay strata is underlain by a more pervious layer <br /> composed of clayey-silty sands and sandy silts and clays; this <br /> layer varies in thickness from 2 to approximately 12 feet. A <br /> second stiff clay layer of undetermined thickness underlies this <br /> strata. <br /> b. Site Hydrogeology <br /> First groundwater was generally encountered <br /> between three and five feet below grade. Using data collected <br /> during quarterly sampling of September 1991, a potentiometric map <br /> (Plate 4) of the site was developed. This map confirms earlier <br /> ' 4 <br />