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<br /> 4. SITE CONDITIONS
<br /> 4.1 STATUS OF PETROLEUM HYDROCARBONS IN THE SOIL AND
<br /> GROUNDWATER
<br /> To date,24 vadose and groundwater monitoring wells have been installed on and off the site to
<br /> investigate the extent of petroleum hydrocarbons in soil and groundwater and to provide for soil
<br /> and groundwater extraction. Boring logs for all existing wells are in Appendix B.
<br /> Because of the persistent drought in California, all wells installed in 1987 and 1988 in the inves-
<br /> tigated area are dry or nearly dry,except MW8,MW9,and MW 10.
<br /> 4.1.1 Soil
<br /> The highest concentrations of petroleum hydrocarbons have been found in soil samples collected
<br /> from the area of the former tank field (V4),downgradient of the tank field(V2 and RW 1),and
<br /> further downgradient to RW4(Table 1,Figure 3). The petroleum hydrocarbon content has been
<br /> lower in soil samples collected in an upgradient direction (P1,P2, and IW1) but increased again
<br /> at MW8. Concentrations of TPH ranged up to 12,000 mg/kg in samples collected from V4.
<br /> Vertically,the petroleum hydrocarbons are spread from a depth of 20 feet at the former tank field
<br /> to at least 40 feet below the surface(V3 and RW4). Table 1 summarizes the analytical results of
<br /> documented soil samples,and the vertical distribution of petroleum hydrocarbons is shown in
<br /> Figures 4 and 5. As shown in these figures,most of the hydrocarbons,except in the intmediate
<br /> vicinity of the former tank field,are entrapped below the water table. With the declining water
<br /> table over the last few years,some of the product has come into the vadose zone and appears as
<br /> LPPH in wells MW3, MW6, RW3, and, most recently, V4, Copies of laboratory analytical
<br /> results of soil sampling of wells in January, February, and April 1991 are included as
<br /> Appendix D.
<br /> 4.1.2 Groundwater
<br /> Groundwater has been sampled twice in 1991, First on 30 January and more recently on 2 May
<br /> 1991. In January only six wells (IW 1,RW 1, RW2,RW4,MW10, and MW12) were sampled,
<br /> The other wells were eitherdry(MWl-MW9), not y,-.t installed(MWI1, V1, V2, and V4), in-
<br /> stalled for purposes other than groundwater monitoring(Pl, P2, 1W 1, and V3),or, like RW3,
<br /> having LPFH above the water table. The highest concentration of TPH, 170,000 ttg/L, was
<br /> measured in samples of groundwater collected from RW4. Benzene in water samples collected
<br /> from this weli was measured at 3(),000 gg/I.• No concentrations of petroleum hydrocarbons
<br /> greater than the detection limit were found in samples collected from the downgradient well
<br /> MW 12. In samples collected from other wells,TPH concentrations ranged from 60,000µg/L
<br /> (MW 10)to 3,500µg/L in IW I. 'I'lle results are cumulatively summarized with analytical results
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