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_=>—CKD <br /> CLEARWATER <br /> G R O U P, I N C <br /> Environmental 5erveces <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 /> which takes into account the observed mass loss (contaminant decay rate), however, <br /> volatilization has been demonstrated to account of only 5% of benzene mass loss at a site with <br /> relatively favorable conditions for this process2 <br /> Advection is responsible for the off-site migration of contaminants in concentrations below the <br /> detection limit To evaluate the extent of a plume attenuated through advection, the dissolved <br /> constituents are assumed to move at the same velocity as the groundwater (i e no retardation) <br /> Using the hydraulic conductivity of the water bearing zone, average hydraulic gradient, and <br /> average effective porosity, an approximate volume of hydrocarbons moving off site at <br /> concentrations at or below the detection limit can be estimated using Darcy's Law However, <br /> this process typically accounts for a relatively small percentage of apparent contaminant mass <br /> loss at most sites <br /> SUMMARY AND CONCLUSIONS <br /> Results from this quarterly monitoring event appear consistent with those from previous <br /> groundwater monitoring activities, and indicate that the contaminant plume is gradually decaying <br /> and is migrating Known contaminant concentrations are highest in the vicinity of monitoring <br /> well MW-513 The contaminant concentrations detected in MW-5B in the current quarter (i e , <br /> 21,000 ug/L TPHg) exceed those of the previous sampling (3,100 ug/L TPHg) by nearly an order <br /> of magnitude It is Clearwater's opinion that the recent "spike" in contaminant concentrations in <br /> MW-5B may be related to high sample turbidity, which was observed at the time of sample <br /> collection on March 16, 2004 High turbidity could have allowed adsorbed contaminants to have <br /> 2Chiang,C Y et al (1989) Aerobic Biodegradation of Benzene,Toluene,and Xylene in a Sandy Aquifer-Data <br /> Analysis and Computer Modeling Groundwater Vol 27,No 6,p 823-834 <br /> ZB178C/1stQtr2004 Monitoring Rpt 8 April 15,2004 <br />