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CLEARWATER -: G R 0 u j, Environmenlaf Services 4:=.1 2.3.1 Monitored Natural Attenuation ` Natural attenuation of dissolved hydrocarbon plumes is a function of several factors including aerobic and anaerobic biodegradation, volatilization, dispersion/advection, and sorption'. 1- Although all of these factors contribute to actual or apparent removal of contaminant mass from the dissolved-phase, only biodegradation processes were examined in this study as they, particularly aerobic biodegradation processes, tend to be the most rapid processes, and thus have the greatest potential for ongoing remediation through natural attenuation. However, all of the processes are described briefly to provide a general background. The MNA monitoring results follow. 2.3.2 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 =r 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 process2. Advection is responsible for the off-site migration of contaminants in concentrations below the F� 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.e. no retardation). Using the hydraulic conductivity of the water bearing zone, average hydraulic gradient, and { 1McAllister, P.M. and Chiang, C.Y., 1994. "A Practical Approach to Evaluating Natural Attenuation of Contaminants in Groundwater." In Groundwater Monitoring and Remediation, Spring 1994. 2Chiang, 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. Al ZBI78E iSOC System Installation/Start-up 5 Nov,24,20x4