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(Figure 6) , suggesting that the channel deposits are elongated in a general northeast-southwest direction. This is consistent with ' the geologic history of the region, which is characterized by frequent development and abandonment of fluvial channels draining northeast-southwest from the Sierra Nevada. The geometry of the channel deposits probably plays an important role in controlling groundwater flow and the migration of the plume of contaminated groundwater, but does not appear to exert major control over the geometry of the vadose zone contaminant plume (see below) . 4.2 Field Evidence of Contamination Slight to strong hydrocarbon odors were detected by the field geologist during drilling, and volatile organic vapors were measured in one or more samples from all borings. Readings were E generally low in the upper 40 feet of most borings, but exceeded 1, 000 ppm in some samples from B1, B4 , and B5 (see Appendix A) . Below 40 feet, readings in excess of 1000 ppm were recorded in all borings. 4 .3 soil Sample Analysis Petroleum hydrocarbons were detected in samples from all borings (Table 2) . The highest concentration of TPH-g (4 , 329 mg/kg) was detected in B1 at a depth of 25 feet; a secondary peak concentration (2 , 705 mg/kg) was detected in B5 (MW-14) at a depth of 40 feet. Concentrations of hydrocarbons decrease upward from 25 feet. For this reason, no contamination was detected in the shallow GL borings that were drilled in 1986 . Contamination also decreases downward below 40 feet; below 55 feet, hydrocarbons were detected only in B4 and B6 . 4 . 4 Groundwater sample Analysis When MW-14 was sampled on May 28 , no free product was present but TPH as gasoline was detected at a concentration of 3 , 100 Ag/l. All BTE&X compounds were also detected. When re-sampled by Sun Tech on June 30, TPH was detected at a concentration of 8 , 800 gg/l. BTE&X were also detected at higher concentrations than during the May ' sampling. TPH and BTE&X were also detected in both EW-1 and MW-11, but not in MW-6, -7 , -10, -12 , or --14 . The results of the June monitoring event are shown in Table 3 . 4 . 5 Groundwater Elevation and Gradient The depth to first groundwater ranges from 57 feet north of the site to 59 feet southwest of it. Figure 8 is a groundwater gradient map, based on groundwater elevation data provided by Sun Tech from the June monitoring event. Groundwater flow is slightly west of south in the area. The gradient is somewhat flatter north of Oak Street than to the south of it. The map does not incorporate data from MW-14 , which has not been surveyed as of this date. I 4