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COMPARISON OF PREDICTED AND OBSERVED PLUME TRENDS AT CONTAMINATED SITES
Citation:
Henry, B. M., J. R. Hicks, T. H. Wiedemeier, J. E. Hansen, AND D H. Kampbell*. COMPARISON OF PREDICTED AND OBSERVED PLUME TRENDS AT CONTAMINATED SITES. Battelle's 3rd Int'l. Conf.- Remediation of Chlorinated and Recalcitrant Compounds, Monterey, CA, 05/20-23/2002.
Description:
Over 45 natural attenuation treatability studies (TSs) were performed from 1993 to 1999 by Parsons Corporation (Parsons) for the Technology Transfer Division of the Air Force Center for Environmental Excellence (AFCEE/ERT) in conjunction with researchers from the US EPA - NRMRL. Recommendations for remediation of these sites were based, at least in part, on modeling of natural attenuation processes to predict contaminant trends and plume dynamics. Model-based prediction of an extended time frame for remediation of a site by natural attenuation (e.g., greater than 20 years) was frequently the basis for recommending implementation of more expensive engineered remedial alternatives. Subsequent to completion of the natural attenuation TSs, monitoring data were used to evaluate the effectiveness of natural attenuation at reducing contaminant concentrations in groundwater at 24 sites. Discrepancies between observed and predicted contaminant trends and plume behavior suggested that critical model input parameters may be inaccurate; therefore, the influence of natural attenuation processes may not have been accurately simulated. As a result of these observations, predicted and observed contaminant trends at 14 sites were compared in order to empirically assess the accuracy of some fundamental model input parameters and assumptions, with a goal of making recommendations to improve the accuracy of natural attenuation model predictions. Most of the models developed for the study sites tended to overestimate plume migration distance, source persistence, andor the time required for the benqene, toluene, ethylbenzene, and xylenes (BTEX) and/or chlorinated solvent plumes to attenuate. Models for many sites predicted plume expansion for at least a 10-year period. Actual monitoring data for the majority of these sites indicate stable or receding plumes. This suggests that model simulations for these sites were overly conservative. The conservatism of these models may be attributable to underestimation of natural source weathering rates, overestimation of the mass of contaminant present in the source area, and/or use of overly conservative first-order solute decay rates.