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1 4.4 Groundwater Elevation and Gradient The groundwater gradient and flow direction were calculated from the elevation data shown in Table 1 The present gradient is 0 042 ft/ft, with flow oriented to the northwest (Figure 4) ' TABLE 4 ANALYTICAL RESULTS of WATER SAMPLES ' µg/L November 1, 1993 ' Sample ID TPH as Ethyl- Gasoline Benzene Toluene benzene Xylene MW-1 1300 40 0 6 17 38 MW-2 31000 560 1700 1600 6300 ' MW-3 ND 19 ND ND ND µg/L = parts pei billion (ppb) ND = None Detected I ' 5 0 CONCLUSIONS AND RECOMMENDATIONS I 5.1 Vertical Fxtent of Soil Contamination I In ge'n'eral, hydrocarbon contamination at this site is limited in vertical extent to a I narrow zone between 30 and 45 feet in depth, corresponding to the capillary fringe ' and upper few feet of the satuiated zone Ethylbenzene was detected at low concentrations in boring 133 at 30 and 35 feet, and also at 40 feet in monitoring well MW-1, along with benzene and xylene The absence of detectable hydrocarbons at tshallower depths in these borings indicates that the contamination plume is more limited within the vadose zone than within the saturated zone This suggests that contamination has spread by migration within groundwater I E ; I 1The upper limit of contamination is shallower between MW-1 and boring Bl, where i *drocarbons were detected at 25 and 30 feet in MW-2 This suggests that the point ; ' of release of the hydrocarbons was neat the west side of the tank cavity (near MW- 2), bather than in the center (near B1) Figure 3'shows the estimated extent of the I I contamination plume in cross section I 11 'll I 1 In V I n III i I� Illl I II I I 1 . I I III I i I 'I t I'I I I '[ilf!1 EI I' III 11 e 1 !I ! .1 I II E'I I I ' �_ _ I`tl If _I I v I � � I � _I fF k 1 IIIlI I !I tl I 1 II I!I �!' i n II1l 1111 t1 I'FI ll1 I Iv 411 v I I If it H