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N� CLEARWATER • G R pU P, I N C Environmental onmental Services occurs at a site which has experienced source removal and/or some active remediation, so that natural attenuation processes rates actually overtake the rate at which contaminants partition into the dissolved-phase The rate at which contaminants concentrations or mass appears to be declining is referred to here as the "first-order decay rate" First-order decay rates can be calculated using either historical contaminant concentrations from individual wells or historical plume mass calculations, if the plume has been delineated for an extended period First-order decay rates for the contaminants beneath this site were calculated using historical monitoring data for wells MW-1, MW-SB, and MW-8 Concentrations of TPHg and benzene for the wells were plotted against time using semi-log technique, and a "best-fit" line was constructed to "normalize" each data set (Figures 6A, 613, and 6C) Preliminary first-order decay rates were derived from the slope of each "best-fit" line The preliminary first-order decay rates for TPHg and benzene in MW-1 are calculated to be 0 0976/day and 0 2447/day respectively (Figure 6A) The first-order decay rates calculated for . TPHg and benzene also correlate relatively well with published values, which typically range from 0 1% to 1%/dayl The first-order plot for TPHg and benzene in MW-513 actually indicate that concentrations of these contaminant species in this well are gradually increasing with time (Figure 613) This long-term increase in TPHg and benzene are 0 0354/day The increasing trend in MW-513 may be related to the location of MW-513 downgradient from the source of contamination, and migration of the contaminant plume in the direction of MW-5B The long- term trend of TPHg concentration in MW-8 is indicated to be decaying at a rate of 0 0074/day, benzene is indicated to be decaying at a greater rate of 0 1463/day in MW-8 Physical Processes (Sorption, Volatilization, and Advection) and Indicators Sorption typically affects only a very small fraction of the plume mass It generally does not play a role in the ongoing attenuation of the plume, except possibly at the plume edges if the plume continues to migrate Sorption generally occurs early in the spill history, as contaminated groundwater contacts clean soils • IBuscheck,T E ,O'Reilly,K T,and Nelson,5 N (1993) Evaluation of Intrinsic Bioremediation at Field Sites ZB178C/2ndQtr2004 Monitoring Rpt 7 August 9,2004