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CLEARWATER G R O U P • Environmental Services 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, 6C, and 6D) 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 0763/day and 0 2253/day respectively (Figure 6A) The first-order decay rates for TPHg and benzene concentrations in well MW-4 are calculated to be 0 2422/day and 0 0202/day, respectively (Figure 6B) The seemingly low rate in MW-4 reflects the relatively low concentrations reported at the initial sampling (5 7 µg/L) 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%/day2 The first-order plot for TPHg and benzene in MW-5B actually indicate that concentrations of these contaminant species in this well are gradually increasing with time (Figure 6C) This long-term increase in TPHg and benzene concentrations in MW-513 may be related to the location of MW-5B downgradient from the source of contamination, and migration and dispersion of the contaminant plume in the direction of MW-5B The long-term trend of • TPHg concentration in MW-8 is also indicated to be slightly increasing at a rate of 0 0021/day, however, benzene is indicated to be decaying at a rate of 0 1534/day in MW-8 The increase in TPHg in MW-8 may also be related to the location of MW-8 downgradient from the contaminant source 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 Volatilization of dissolved contaminants may play a significant role in the natural attenuation of volatile organic constituents such as benzene Volatilization can be estimated using a formula Contaminants in Ground Water " In Ground Water Monitoring and Remediation, Spring 1994 2Buscheck,T E , O'Reilly,K T , and Nelson, S N (1993) Evaluation of Intrinsic Bioremediation at Field Sites Proceedings of the Conference on Petroleum Hydrocarbons and Organic Chemicals in Ground Water NGWA/API ZB178C/4Q03 Monitoring Rpt 8 January 29,2004