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Nave %4001 P A G E 2 ERM used sample analytical data from the monitoring events completed prior to implementation of enhanced in situ bioremediation with Oxygen Releasing Compounds (ORC). ERM applied First Order Kinetics to estimate the rate constant for natural biodegradation, as described below. The purpose of these calculations was to predict the time for COCs attenuating under natural biodegradation to attain their respective MCL/Target Concentration. To do this, a natural biodegradation constant was needed. It is well documented that natural degradation processes can be modeled using the First Order Kinetics equation (EPA Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Ground Water, Appendix C- Data Interpretation and Calculations, EPA/600/R-98/128, September 1998). The mathematical equation representing First Order Kinetics is the following: C„ = Coe-kr Where n = number of samples in the data set t = time in days Co = initial concentration of constituent, mg/L C„ = constituent concentration taken after "t" days. The equation can be rewritten as: In Ca = kt C„ Using this form of the First Order Kinetics equation, a plot was constructed with"ln(Co/Cn)" on the y-axis and "t" on the x-axis. A straight-line equation was fitted to the plotted points using linear regression to estimate the slope of the equation of the line,which is k, the natural biodegradation constant. The k estimates are presented in Table 1 (Site #1) and Table 2 (Site #2) and summarized in Table 3. Figures 1 through 3 show the plotted values and the regression line. Well MW-5 (Site #1) and MW-1A (Site #2) have historically had high concentrations of COCs and currently (as of 22 July 1999) have the highest site-specific reported values for benzene and TPH-g (Tables 1 and 2). Therefore, the natural biodegradation constants estimated for MW-5 and MW-1A were used in the predictive calculations for TPH-g and benzene at Site #1 and Site #2, respectively.