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<br /> f'fM EM ASSESSMENT FCPO"
<br /> INCLLOI N4 QUAWMY WAKFMHO WORT
<br /> PIN 1999 East Yvlatnda Avenue,Manteca,Caiifotnia
<br /> fa 2.4.3 Nations Groundwater Associates
<br /> In June 1990, NGA supervised the abandonment of MW-1, which had been damaged during
<br /> the removal of the USTs in September 1989. Monitoring well MW-1A was installed
<br /> i approximately 10 feet east of the former location of MW-1. MW-1A was completed as a 2-
<br /> inch-diameter well to a total depth of about 36'/2 feet.
<br /> Soil samples were obtained from the boring at dep,'hs of 10, 15, 20,and 25 feet below ground
<br /> surface and were analyzed for BTEX, EDC, EDB, and TPH, as gasoline and diesel. Maximum
<br /> concentratio^c of BTEX and TPH in the soil samples from MW-1 were in samples from depths
<br /> of 15 and 20 feet. The sample from 15 feet contained BTEX in concentrations from
<br /> m0. 20 feet t
<br /> 3.3, and 38 ppm, respectively, and TPH,as gasoline, at 1,400 ppm. The sample from 2
<br /> of depth contained BTEX in concentrations of 20, 110, 170, and 65 ppm, respectively, and
<br /> TPH, as gasoline, at 40,000 ppm (fable 2).
<br /> Groundwater sampies from the three on site wells MW-1 A, MW-2, and MW-3 were analyzed
<br /> for BTEX and TPH, as gasoline. The sample from MW-
<br /> 1A contained concentrations of BTEX
<br /> and TPH, as gasoline, at levels of 72, 20, 14, 380, and 4,800 ppb, respectively. EDC was
<br /> found in MW-2 at a concentration of 0.3 ppb. The sample from MW-3 did not contain
<br /> concentrations of petroleum hydrocarbons above the detection limits (Table 3). Results of
<br /> the investigation were reported in Problem Assessment Report (NGA, 1990).
<br /> 2_5 Regional al Geology and Hydrogeology
<br /> The site is located in the Great Central Valley, which is a structural basin bordered to the east
<br /> by the Sierra Nevada Mountains and to the west by the Coastal Range. Surficial sediments
<br /> underlying the site consist of Recent alluvium, which overlies the Victor Formation. The
<br /> Recent and Victor Formation sediments are composed primarily of discontinuous lenses of
<br /> - gravel, sand, and clay.
<br /> Groundwater is present in both confined and unconfined conditions in the basin. In the area
<br /> of the subject site, groundwater occurs in unconfined aquifers, which is characteristic of
<br /> formations composed of alluvial fan, floodplain, and stream channel deposits.
<br /> The direction of regional groundwater flow is toward the Sacramento River Delta from the
<br /> surrounding mountain ranges. Development of pumping wells in the area has created a cone
<br /> of depression in the groundwater surface, which also causes the gradient to be toward the
<br /> central portion of the valley (Department of Water Resources, 1967).
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<br /> 90 o99C.APT
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