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MM �41111 WAWK <br />Working To Restore Nature <br />Rd = 1 + Md -M (Equation 4) <br />B <br />Using an estimated porosity of 30%, an estimated bulk density for clay of 1 86 (Peele, 1945), <br />and the calculated Kd values given above, the retardation factor for benzene at the site is <br />calculated to be 106 4, which suggests groundwater beneath the site moves approximately 100 <br />times faster than dissolved benzene. Calculated Rd values for other site contaminants are as <br />follows - <br />Toluene: <br />Rd = 236.6 <br />Ethylbenzene <br />Rd = 534.2 <br />Xylenes: <br />Rd = 534 2 <br />TPHd: <br />Rd = 12215 <br />TPHg <br />Rd = 788 4 <br />Based on these calculated Rd values and the estimated maximum groundwater velocity of 3 <br />ft/yr, dissolved benzene and TPHd beneath the site would migrate at rates of 0.03 ft/yr and <br />0 00025 ft/yr, respectively <br />Biotic Processes <br />Aerobic biodegradation of constituents is widely documented in the literature and is generally <br />modeled as a first order kinetic process with the rate of decay limited by the available oxygen <br />(Borden and Bedient, 1986) Although precise modeling is difficult even for sites where <br />nutrient levels, oxygen levels, site hydraulics, and transport velocities are well known, the <br />changes in contaminant concentration along a flow path due to biodegradation can be analyzed <br />by approximate methods presented in the literature (Domenico and Schwartz, 1990, Borden <br />and Bedient, 1986) The controlling parameters for these models of first order biodecay are <br />the decay constant (degradation rate in percent per day) and the transport velocity Decay <br />constants have been determined at laboratory and field scales for certain gasoline constituents, <br />including benzene. Benzene decay constant values from numerous independent studies <br />reported average values ranging from 0.35-4 0% per day (Kemblowski and others, 1987, <br />Howard and others, 1991). <br />In order to incorporate natural biodegradation into the fate analysis and to calculate the <br />expected downgradient extent of contaminant travel, an analytical transport solution was <br />utilized for one dimensional mass transport involving first order biodecay of benzene under <br />oxygen limiting conditions Borden and Bedient (1986) presented the following equation. <br />Cx = Co e( -m x/v) (Equation 5) <br />1500541RAD2-994 DOC 4 <br />