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These concentrations indicate highly degraded volatile components in the gasoline product, with concentrations declining to the east (see Figure 1). The low concentrations of BTEX suggest that natural biodegradation process have operated on this leakage. A relatively flat gradient, together with the clayey sand aquifer strata, appear to have limited migration of the plume. However, the easterly extent of the plume was not observed in this study. Conclusions and Recommendations Soil exploratory borings were used to further define the extent of petroleum contaminants in the vicinity of a former underground storage tank location. Exploratory soil borings and reconnaissance groundwater sampling show that highly degraded TPHG and BTEX occurs downgradient of the former tank and dispenser area, with the highest concentrations at PB-2, and PB-3. Contaminant distribution is indicative of a slowly moving dissolved plume to the east. Cleanup actions of further soil excavation, and soil and groundwater extraction and treatment do not appear to be cost effective for this site's degraded contaminant composition and distribution and pending State guidance. Given the degraded BTEX volatile component chemistry of the dissolved plume, Wright feels that natural biologic action will continue to decompose the gasoline. However, site groundwater monitoring is warranted. Wright recommends that three groundwater monitoring wells be placed upgradient and downgradient of the former tank location at the locations on Figure 1. These wells would be monitored at quarterly intervals to meet the current requirements of the PHS-EHD and evolaving State guidance for underground tank petroleum releases. The data collected in this periodic monitoring program can be used to track hydrocarbons and their degradation in groundwater. We also recommend that monitoring for biologic parameters be added to the monitoring program to estimate biologic activity (i. e., dissolved oxygen, pH, ferrous iron, sulfate and nitrate; Buscheck and O'Reilly, 1995). This will establish a baseline to support the biologic activity inferred from the data collected and gauge selected future biologic monitoring parameters for the site monitoring program. Depending upon the results of the oxygen content of water, Wright may add an oxygen carrier to aid in-situ degradation of the fuel, with the approval of PHS-END. Wright will prepare a monitoring well installation and monitoring workplan for PHS-EHD review prior to starting this work. Limitations This report has been prepared for the specifically for the Haley Flying Services Site at 21000 Paradise, Tracy, CA and was done according to the State and local agency suggested guidance documents for these investigations. The interpretations, conclusions and recommendations made herein are based on the data and analysis for the soil and water samples collected on-site and should be reviewed in the context of the whole report and previous site work. Please note that Haley Flying Services Project No. 4111 Page 6