SITE INFORMATION AND CORRESPONDENCE

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GEOMATRIX Mr. Robert Mihalovich Chevron Environmental Management Company July 12, 2001 Page 3 decay constant (k) can be approximated using an exponential equation (Wiedemeier, et al., 1996).b Ct= Coe-kt Where: Ct=concentration at time t Co =initial concentration k= first-order decay constant (day-'). Existing benzene concentrations from monitoring wells MW-6, MW-12, MW-7, and MW-8 (from 1991 through 2001) were plotted on log-linear graphs vs. time (days), as shown on the attached Figure 2 (Geomatrix, 2000; Figure 7). An exponential regression trend line of the data was plotted on each graph(using Microsoft® Excel). As shown on Figure 2, the expo- nential equations generated from these plots ranged from: y=4E+18e-O.001x(for MW-6) to y=2E+46e o.0029x (for MW-8). Using the equation from MW-6 as an example: y=4E+18e o.00u Where: y axis=benzene concentration at time t, Ct 4E+18 =represents an initial concentration' at Day 0, Co 0.001 =first-order decay constant (total attenuation rate), k(day-) x axis =time, t(day) Time (x) and benzene concentration(y) are input parameters from groundwater monitoring data collected at the site over the past 10 years. The attenuation rate (k) is the slope of the trend line estimated by exponential regression. Based on exponential regression of the data, the first-order decay constants (i.e., the range of rates of declining concentrations of benzene in groundwater) are estimated to be 0.1 percent per day (0.001/day at MW-6) to 0.29 percent per day(0.0029/day at MW-8). Comment 3: Estimated Frequency of ORC Replacement and Remediation Duration using ORC The estimated frequency of ORC replacement was based on site data and vendor information regarding the oxygen content of ORC. The remediation duration using ORC was estimated to be similar to that of NA based on its limited effectiveness due to site constraints. A brief description of this evaluation is provided below. b Wiedemeier,T.H.,et. al, 1996,"Approximation of Biodegradation Rate Constants for Monoaromatic Hydrocarbons(BTEX)in Ground Water," Ground Water Monitoring&Remediation,Vol.XVI,No. 3, Summer 1996. It should be noted that the representative initial concentration(Co)is estimated based on backward projection of the trend line and does not represent actual site conditions.