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CLEARWATER <br /> r <br /> G R O U P, I N C <br /> Environmental Services <br /> natural attenuation processes rates actually overtake the rate at which contaminants partition into <br /> the dissolved-phase The rate at which contaminants concentrations or mass appears to be <br /> declining is referred to here as the "first-order decay rate" First-order decay rates can be <br /> calculated using either historical contaminant concentrations from individual wells or historical <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-513, 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, <br /> and 6C,) Preliminary first-order decay rates were derived from the slope of each "best-fit" line <br /> The preliminary first-order decay rates for TPHg and benzene in MW-1 are calculated to be <br /> 0 0871/day and 0 2363/day respectively (Figure 6A) The first-order decay rates calculated for <br /> TPHg and benzene also correlate relatively well with published values, which typically range <br /> from 0 1% to 1%/dayl The first-order plot for TPHg and benzene in MW-513 actually indicate <br /> that concentrations of these contaminant species in this well are gradually increasing with time <br /> (Figure 6B) This long-term increase in TPHg (0 0282/day) and benzene (0 021/day) <br /> concentrations in MW-5B may be related to the location of MW-513 downgradient from the <br /> source of contamination, and migration and dispersion of the contaminant plume in the direction <br /> of MW-513 The long-term trend of TPHg concentration in MW-8 is indicated to be slightly <br /> decaying at a rate of 0 003/day, benzene is indicated to be decaying at a greater rate of <br /> 0 1508/day in MW-8 <br /> Physical Processes (Sorption, Volatilization, and Advection) and Indicators <br /> Sorption typically affects only a very small fraction of the plume mass It generally does not <br /> play a role in the ongoing attenuation of the plume, except possibly at the plume edges if the <br /> lBuscheck,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 Ground Water NGWA/API <br /> ZB 178C/1stQtr2004 Monitoring Rpt 7 April 15,2004 <br />