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CLEARWATER G R Q U P Environmental 5eroices £ to be 0 24%/day and 0 02%/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 decay rates derived for TPHg and benzene in MW-5B actually indicate that concentrations in this well are increasing with time (Figure 6C) However, a qualitative evaluation of TPHg concentrations in well MW- 5B suggests they are relatively stable with seasonal concentration fluctuation The first-order decay rates for TPHg and benzene concentrations in well MW-8 were calculated to be 0 011/o/day and 0 16%/day, respectively (Figure 6D) The trends in contaminant concentration reduction over time suggest that a combination of natural attenuation processes is likely occurring within the plume 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 which takes into account the observed mass loss (contaminant decay rate), however, volatilization has been demonstrated to account of only 5% of benzene mass loss at a site with relatively favorable conditions for this process3 Advection is responsible for the off-site migration of contaminants in concentrations below the detection limit To evaluate the extent of a plume attenuated through advection, the dissolved constituents are assumed to move at the same velocity as the groundwater (i a no retardation) Using the hydraulic conductivity of the water bearing zone, average hydraulic gradient, and average effective porosity, an approximate volume of hydrocarbons moving off site at concentrations at or below the detection limit can be estimated using Darcy's Law However, this process typically accounts for a relatively small percentage of apparent contaminant mass loss at most sites Biodegradation Processes and Indicators During biodegradation, microbes utilize electron acceptors to convert hydrocarbons to carbon dioxide, water, and biomass In aerobic degradation, the electron acceptor is dissolved oxygen (DO) In anaerobic degradation, compounds other than DO are used as electron acceptors The reactions that yield the most energy take precedence over those reactions that yield less energy 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 Chenucals in Ground Water NGWA/API 3Cluang, C Y et al(1989) Aerobic Biodegradation of Benzene,Toluene,and Xylene in a Sandy Aquifer-Data Analysis and Computer Modeling Groundwater Vol 27, No 6,p 823-834 ZB178C/2Q03 Monitoring Rpt 6 September 15,2003