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iY*� d ua Woodward-Clyde hydrocarbon constituent regardless of depth. As shown, both TPH and BTEX constituents occur in soil gas concentrically around the area of the removed underground storage tanks. This pattern suggests that contaminants moved directly downward from the tanks and ponded either on the groundwater or on a shallow soil horizon. Relatively high TPH, toluene and xylene concentrations observed at sampling point P13a, and high toluene and xylenes concentrations observed at sampling point P11, suggest that the plume is moving generally west. 2.2 aQ xia" WCC subcontracted Datum Exploration and Drilling Company to advance 380 lineal feet of soil borings and install monitoring wells MW-5 through MW-9 in the boreholes at the locations shown on Figure 7. A permit to drill these boreholes was obtained from the San Joaquin Local Health District, a copy of which is included in Appendix B. The borehole locations were selected ?fler reviewing the results of the soil gas investigation with the intent of identifying the do%,lgradient edge of contamination and the direction of groundwater flow. WCC originally intended to advance 5 boreholes to a depth of 60 feel and a sixth borehole to a depth of 80 feet with the understanding that groundwater occurred at a depth of about 40 feet (see WCC 1987). At the time of WCC April 1990 field work, however, the groundwater level occurred almost 15 feet lower than expected. The drilling program was modified by OSA so that WCC Installed 5 wells (MW-5, -6, 7, -8 and, -9) and 1 soil boring (S-5) instead of the original 6 wells. Soil boring S-5 was advanced at the northwest corner of the site adjacent to an open excavation of a waste oil tank that had been recently removed from the site. The total depth of the borehole was 25 feet, the maximum depth of exploration for the monitoring wells was 70 feet. All boreholes were advanced using a truck mounted CME 75 drill rig equipped with 8- Inch O.D. hollow-stem augers. Soil samples were collected at 5-foot intervals and logged according to the Unified Soil Classification System Visual-Manual Procedure ASTM 02488.84 by a WCC geologist. Lags of the materials encountered while drilling are presented In Appendix C. Soils penetrated by drilling consisted primarily of clay, clayey sill and silty clay, with lesser amounts of silt, sandy silt, sand and clayey sand to depths of 52 to 55 feel. From 55 feet to total depth, the soils consisted generally of poorly graded fine grained sand. Groundwater was encountered between 55 and 56 feet below grade. The subsurface distribution of the lithologles encountered is shown on Cross Section A-A', in Figure 8. 5