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10 One sample from the lower Modesto channel sand and one from the upper Riverbank sand were used as substrates for the growth of hydrocarbon-degrading bacteria cultures The cultures were grown un a diesel-motor oil enriched medium for 14 days (Appendix E) The cultures were then analyzed for microbial abundance by a trained microbiologist Both cultures exhibited very poor growth, neither yielding abundances greater than the minimum detection limit attainable (Table 3) The poor response, coupled with the low concentrations of inorganic nutrients and the relatively low TOC values, suggests that mirnm,al natural bnodegredatnon is occurring at this site 5.0 ASSESSMENT OF SOIL AND GROUNDWATER CONTA 11NATION 5.1 Field Evidence of Contamination Hydrocarbon odors were reported in six of the nine borings that have been drilled since assessment of this site began in 1990 Strong diesel odors were noted in TB-2 and GT-4, accompanied by photo- ionization detector (PID) readings in the 200-275 ppm range in the latter In each case, the strong odors were present in the lower Modesto channel sand between 40 and 50 feet Slight or moderate odors were noted at this depth in GT-3 (this boring is termed TB-1/MW-1 in Appendix A), GT-5 and GT-8, with PID readings of 10-60 Slight odors were evident through a longer depth interval in GT-1 (25-80 feet, with PID readings of 10-70) and at greater depths (50-65 feet, with PID readings of 15- 50)in GT-2 No odors or PID readings were observed in GT-6, GT-7, or GT-9 Diesel odors were also evident in water samples collected by hydropunch from GT-4, GT-5, and GT-8 during this phase of investigation Previously, no hydropunch samples had been collected at this site The sample from GT4 was collected at the base of the Modesto channel at a depth of approximately 50 feet and exhibited a very strong odor The samples from GT-5 and GT-8 were collected from the upper Riverbank sand at a depth of approximately 65 feet, and exhibited moderate to strong odors 5.2 Laboratory Evidence of Contamination 5.2.1 Soil Contamination Since the UST's were removed in 1989, a total of 53 soil samples collected from borings have been analyzed for petroleum hydrocarbons Laboratory results for these samples are summarized in Table 4 The results from samples collected in 1990 and 1994 have been discussed in previous reports Discussion here will focus on those samples that were collected during the current drilling phase The laboratory reports for the latter samples are included in Appendix C Based on field evidence of contamination, the four deepest samples from GT4 were selected for laboratory analysis Gasoline-range hydrocarbons were detected in three of these, and diesel-range hydrocarbons were detected in all four Diesel concentrations were much higher than expected, implying that GT-4 was not as near to the fringe of the contaminant plume as had been presumed The highest concentrations (14,600 mg/kg and 14,800 mg/kg)were detected at 40 and 50 feet, in medium- 10