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' C,eologcc al7`ecknus!n . Page 7 Initial Site Conceptual Model Project No. 1030.2 June 18, 2003 The test results showed the following: 1. MW-1 has a high level of alkalinity compared to MW-2 and MW-3. Elevated levels of alkalinity are an indicator of benzene biodegradation, either aerobic or anaerobic. 2. The average nitrate as nitrogen constituent in MW-2, 3 and 4 is 15.7 mg/L. MW-1 has 12-mg/L indicating nitrate depletion, which is consistent with biodegradation in an ' anaerobic environment. 3. Redox potential has been consistently lowest in MW-1 suggesting MW-1 is near the ' center of the plume (Table 5). o Redox potential measured over three years suggest the site conditions are in an anaerobic state. ' u Redox potential measurements from the last two quarters suggest that there is a reaction halo near MW-2, MW-3 and MW-4. 4. The groundwater temperature at the site ranges between 65° and 75°F. ' 5. The measured pH in the water table wells range between 7.3 and 7.6. The test results showed the following trends when comparing the contaminated zone to background conditions. 1. Alkalinity of the contaminated zone was enriched compared to background. This may be secondary evidence of aerobic biodegradation activity. 2. Nitrate of the contaminated zone was depleted compared to background. 3. Sulfate and total iron was inconclusive. 4. Oxidation-reduction potential of the contaminated zone is lower compared to background. 5. The measured pH in the water table wells range between 7.3 and 7.6. This pH range is ' conducive to maintaining aerobic biodegradation activity. 6. Temperature was slightly elevated in the impacted area. 4.6 Contaminant Mass Estimate Calculations Figure 4 shows a contour map of TPH-G soil contamination as interpreted by soil collected ' during various drilling events. The contamination appears to cover an area of approximately 3.000 W. Figure 5 is a cross section showing the estimated boundaries of TPH-G contamination, in the subsurface, used to calculate the mass of contamination in the ' soil. Figure 6 is a contour map of TPH-G groundwater contamination as interpreted by analytical data from quarterly groundwater monitoring events. The preliminary mass balance calculation, quantifying the amount TPH-G remaining at the -- site, is included as Table 6. Approximately 550 gallons of gasoline remain in the soil and ' approximately 2 gallons remain in groundwater at the site. 1