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CLEA ER <br /> G R O U P <br /> Environmental Services <br /> Plume mass calculations if the plume has been delineated for an extended period First-order <br /> decay rates for the contaminants beneath this site were calculated using historical monitoring <br /> data for wells MW-1, MW-4, MW-5B, and MW-8 <br /> Concentrations of TPHg and benzene for the wells were plotted against time using semi-log <br /> technique, and a "best-fit" line was constructed to "normalize" each data set (Figures 6A, 613, and <br /> 6C) Preliminary first-order decay rates were derived from the slope of each "best-fit" line The <br /> preliminary first-order decay rates for TPHg and benzene in MWA are calculated to be <br /> 0 0763/day and 0 2253/day respectively (Figure 6A) The first-order decay rates for TPHg and <br /> benzene concentrations in well MW-4 are calculated to be 0 2422/day and 0 0202/day, <br /> respectively (Figure 6B) The seemingly low rate in MW-4 reflects the relatively low <br /> concentrations reported at the initial sampling (5 7 µg/L) The first-order decay rates calculated <br /> for TPHg and benzene also correlate relatively well with published values, which typically range <br /> from 0 1% to 1%Iday2 The first-order plot for TPHg and benzene in MW-5B actually indicate <br /> that concentrations of these contaminant species in this well are gradually increasing with time <br /> (Figure 6C) This long-term increase in TPHg and benzene concentrations in MW-5B may be <br /> related to the location of MW-5B downgradient from the source of contamination, and migration <br /> and dispersion of the contaminant plume in the direction of MW-5B The long-term trend of <br /> ITPHg concentration in MW-8 is also indicated to be slightly increasing at a rate of 0 0021/day, <br /> however, benzene is indicated to be decaying at a rate of 0 1534/day in MW-8 The increase in <br /> ITPHg in MW-8 may also be related to the location of MW-8 downgradient from the contaminant <br /> source <br /> I <br /> I <br /> Contaminants in Groundwater " In Groundwater Monitoring and Remediation, Spring 1994 <br /> . zBuscheck, T E , O'Reilly, K T , and Nelson, S N (1993) Evaluation of Intrinsic Bioremediation at Field Sites <br /> Proceedings of the Conference on Petroleum Hydrocarbons and Organic Chemicals in Groundwater NGWA/API <br /> IZB 178C RAP 8 May 18,2004 <br />