SITE INFORMATION AND CORRESPONDENCE

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`y� ar1 Ile GEOMATRIX Mr. Robert Mihalovich Chevron Environmental Management Company July 12, 2001 Page 4 Estimated Frequency of ORC Replacement For cost estimating purposes, we assumed that ORC would need to be replaced an average of 3 times per year for Alternative 3 —Enhanced In Situ Bioremediation (EISB) (Geomatrix, 2000, p. 10). The frequency of ORC replacement for Alternative 3 was estimated based on the groundwater flux, the oxygen demand required to degrade concentrations of petroleum hydrocarbons in groundwater at the site, and vendor information(i.e., Regenesis Bioremedia- tion Products) regarding the amount of oxygen per quantity of ORC. A summary of these calculations is presented below and a more comprehensive description is presented in Appen- dix A. Based on previous studies at the site,d the groundwater flux is estimated to be about 0.6 cubic feet of water per day(ft3/d) through well MW-6, which is screened in mostly clay and silty sand, and about 6 ft3/d for wells MW-7, MW-8, and MW-12, which are screened in sand and clean sand. The oxygen demand is the specific quantity of oxygen required to degrade a specific quantity of petroleum hydrocarbons. The total oxygen demand for the site was estimated by multi- plying the groundwater flux by the concentration of TPH at each well (i.e., the sum of TPHg and TPHd) and assuming a ratio of 7 pounds of oxygen (3.5 pounds of oxygen at 50 percent efficiency)per pound of TPH. The results of these calculations indicate that between 1 (well MW-8) and 4 (well MW-6) pounds of oxygen per year are required. Based on the information provided by the ORC vendor, we assumed that the ORC "socks" would be placed across the screened interval in each well and would supply 0.75 pound of oxygen per well. Therefore, the frequency of ORC replacement was estimated to be between 1 and 4 times per year based on the previous estimates of oxygen demand for each well. The frequency of ORC replacement is anticipated to become less as the petroleum constituent concentrations in the wells decline with time. Therefore, it was conservatively estimated (from a cost estimation standpoint) that three ORC change outs per year would be required for the estimated remediation duration of 15 years for Alternative 3. This resulted in a cost estimated for Alternative 3 (ORC) of$145,000 (Table 1). If the ORC was to be replaced at a greater frequency than 3 times per year the costs for EISB (Alternative 3) would increase, thus further making NA a more cost effective remedial solution. If the ORC replacement fre- quency is reduced to as little as once per year the estimated cost for EISB is $102,000 (Table 2), which is still greater than the estimated cost of$93,000 for monitored NA. Therefore, a change in the assumed frequency of ORC change out at the site would not result in EISB becoming a more economical remedial alternative. d Well MW-4 was located south of the site;the well was subsequently decommissioned for the City's expansion of Grant Line Road. 1