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Woodward-Clyde Consuitt,ti 4.0 DISCUSSION As described above, both TPH and/or BTEX were detected in groundwater samples obtained from monitoring wells MW-7, -8 and -9 and in soil samples obtained from borings placed immediately adjacent to the underground storage tanks at the Caltrans Shop 10 facility. Their subsurface distribution is shown on Cross Section A-A', Figure 8. The areal distribution of benzene in groundwater is shown on Figure 10. Because benzene was detected in every well where other hydroc-:bons were detected, its mapped distribution is representative of the general distribution of groundwater contaminants at the site. As shown, these contaminants are present both in soils and groundwater to the wes„ and south-and northwest, of the underground storage tanks. The groundwater contaminants appear to be generally migrating toward the west In the general direction of groundwater flow. A comparison of DOHS Action levels for drinking water and EPA maximum contaminant levels (MCUs) in Table 5 with water quality data summarized in Table 4, indicates that teither benzene, toluene, and ethylbunzene_is.p_resent,jn most wells at the site at concentrations that exceed the designated action level. Wells MW-5 and MW-6 are the only exceptions.. Hydrocarbon contarr k/6010 occurs f predominantly adjacent to or bensath the former.-underg�Aund storage tanks location. A review of their .areal-'and 'subsurface distribution suggests that these contaminants mlg'i.ated generally do_wnward from the leaking tanks) through a thick sequence of silly clays G sand unit that occurs at aepth of about, 5 feet Presently, the edge of the" groundwater plume occurs to the west of well MW-6, east of well MW-5, and just north of well MW-7. The extent of soil contaminalion'is probably outlined by the distribution of soil gas as shown on Figures 4 through 7. ;Total organic-lead was not detected in 20 soil samples or 4 water samples obtained directly beneath the underground storage tanks by WCC in 1987, and therefore is not considered to be a contaminant at the site. 1 13