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ovv <br /> CLEARWATER <br />. G R O U P <br /> Env-ro...ntal $rrv:crs <br /> Hydrocarbon First-Order Degradation Rates (Methodology) <br /> When natural attenuation or biodegradation is occurring within a plume, a reduction of <br /> hydrocarbon concentrations or mass is usually observed over time Natural attenuation usually <br /> occurs at a site which has experienced source removal and/or some active remediation The <br /> process or rate with which hydrocarbons degrade often occurs according to a first-order kinetics <br /> equation The first-order degradation rate can be determined by evaluating the change of either <br /> hydrocarbon concentrations from individual wells or total plume mass with time When natural <br /> attenuation or biodegradation occurs, the degradation rates overtake the rate at which the <br /> petroleum hydrocarbons are being released from the sorbed-phase into the dissolved-phase <br /> First-order degradation rates for the petroleum hydrocarbons beneath the protect site are <br /> estimated by using historical monitoring data obtained from groundwater monitoring wells MW- <br /> 1, MW-513, and MW-8 These groundwater morutoring wells were chosen based on their location <br /> relative to the hydrocarbon plume (1 a up gradient, near the core and down gradient <br /> respectively) Concentrations of TPH-g and benzene detected in the samples collected from these <br /> wells were plotted against time as a semi-log function. A degradation rate was determined by <br /> fitting a first-order kinetic equation to the plotted data <br /> Hydrocarbon First-Order Degradation Rate(Results) <br /> The estimated first-order degradation rates for TPHg and benzene in MW-1 are 0 122 per day <br /> and 0 219 per day, respectively (Figure 6) The semi-lag plot for TPHg and benzene in MW-513 <br /> indicates that specifically these concentrations in this well are either stabilized or slowly <br /> decreasing with time (Figure 7) MW-513 is located down gradient of the former UST excavation <br /> area, the observed trend in MW-5B suggests that residual hydrocarbons may exist in the soil, or <br /> center of the plume, which is still located up gradient of MW-5B The observed TPHg <br /> concentration trend in MW-8 (Figure 8) is similar to that observed in MW-5B The estimated <br /> degradation rate is 0 0006 per day However, benzene shows a decreasing trend with an <br /> ZB178I 4 of 10 May 2006 <br />