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TERRA VAC UTILITY SURVEY The purpose of this survey is to ascertain the potential for existing subsurface utility lines to act i as boundaries or preferential pathways; boundaries that could influence groundwater flow and/or pathways that distribute free phase petroleum hydrocarbons away from the site. On March 28 1997, Terra Vac conducted a survey of utilities proximal to the site. Maps illustrating underground utilities were obtained from Pacific Gas and Electric and the San Joaquin County Department of Public Works. Figure 3 shows the relative position of each of the utilities bordering the site. PLUME ATTENUATION Benzene attenuation in groundwater can proceed by at least three mechanisms; bio-uptake via naturally occurring organisms in vadose and saturated soils; natural half life decay (first order decay); and volatilization from groundwater to soil gas. Because analytical data (reflecting actual field conditions) are available, only bio-uptake will be evaluated in this report. Groundwater samples collected during the fourth quarter 1996 monitoring and sampling event were analyzed for physical properties and chemical constituents that may indicate bio-activity in the substrate (Table 1). Limiting factors that affect bio-activity include the molecular structure of n the contaminant compound, contaminant concentration, bacterial concentration, bacterial species, nutrient concentrations (carbon, oxygen, nitrogen and phosphorous), temperature of substrate and pH (Lyman, W.J., Reidy, P.J., Levy, P., 1992, Mobility and Degradation of Organic Contaminants in Subsurface Environments). Dissolved Oxygen Physical properties affecting biodegradation include dissolved oxygen (DO), oxygen reduction potential (ORP), pH and temperature. Inorganic nutrients include; nitrate and nitrite, ortho phosphate, sulfate and sulfite. fDissolved oxygen in groundwater can be used as an indicator of the metabolism of hydrocarbons. The saturation concentration of DO ranges from 8.56 mg/l at 23 °C to 9.45 mg/1 at 18 'C (Corbit, R.A., 1990; Standard Handbook of Environmental Engineering, Table 6.3, page 6.4). These concentrations are typically confined to the atmosphere/water interface of fresh water. Actual saturation ranges for groundwater may not exist, however, background levels of DO greater than ` i to 2 mg/1 are required to support aerobic bio-activity in groundwater (Groundwater monitoring and Remediation, 1994, A Practical Approach to Evaluating Natural Attenuation of Contaminants in Groundwater, p 165). DO concentrations from the fourth quarter monitoring event are summarized in Table 1. Project 30-0212 April 11, 1997 2