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0 <br /> CLEARWATER <br /> G R O U P <br /> E"i".meata! Services <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-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, 6B, <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 0784/day and 0 2133/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-5B 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 and benzene are 0 0102/day and 0 0078/day The <br /> increasing trend in MW-5B may be related to the location of MW-5B downgradient from the <br /> suspected source of contamination, and migration of the contaminant plume in the direction of <br /> MW-5B The long-term trend of TPHg concentration in MW-8 is indicated to be decaying at a <br /> rate of 0 0036/day, benzene is indicated to be decaying at a greater rate of 0 1089/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 /> plume continues to migrate Sorption generally occurs early in the spill history, as contaminated <br /> groundwater contacts clean soils <br /> Volatilization of dissolved contaminants may play a significant role in the natural attenuation of <br /> volatile organic constituents such as benzene Volatilization can be estimated using a formula <br /> iBuscheck,T E , O'Reilly, K T, and Nelson,S N (1993) Evaluation of Intrinsic Bioremediation at Field Sites <br /> ZB178C/3rdQtr 2004 Monitoring Rpt 7 October 8, 2004 <br />