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EVALUATION OF UNSATURATED/VADOSE ZONE MODELS FOR SUPERFUND SITES
Citation:
Nofziger, D. L., J. Chen, AND C. T. Haan. EVALUATION OF UNSATURATED/VADOSE ZONE MODELS FOR SUPERFUND SITES. U.S. Environmental Protection Agency, Washington, DC, EPA/600/SR-93/184, 1994.
Impact/Purpose:
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Description:
Mathematical models of water and chemical movement in soils are being used as decision aids for defining groundwater protection practices for Superfund sites. Numerous transport models exist for predicting movementand degradation of hazardous chemicals through soils. Many of these require extensive input parameters that involve uncertainty due to soil variability and unknown future weather. The impact of uncertain model parameters upon the model output is not known. Model users require an understanding of this impact so appropriate parameters are measured at a site and model prediction uncertainty is incorporated into decisions. This report summarizes research findings that address the sensitivity and uncertainty of model output due to uncertain input parameters. The objective of the research was to determine the sensitivity and uncertainty of travel time, concentration, mass loading and pulse width of contaminants at the water table due to uncertainty in soil, chemical, and site properties for four models, RITZ, VIP, CMLS, and HYDRUS. The models, which are all designed to estimate movement of solutes through unsaturated soils, span a considerable range in detail and intended use. Model parameters investigated include soil properties such as organic carbon content, bulk density, water content, and hydraulic conductivity. Chemical properties examined include organic carbon partition coefficient and degradation half-life. Site characteristics such as rooting depth, recharge rate, weather, evapotranspiration and runoff were examined when possible in the models. Model sensitivity was quantified in the form of sensitivity and relative sensitivity coefficients. The study found that large uncertainty exists in many model outputs due to the combination of sensitivity and high parameter variability. In addition, predicted movement of contaminants was greater when the natural variability of rainfall was incorporated into the model than when only average fluxes were used. This is because major rainstorms that result in large fluxes of water and high leaching are essentially ignored when average flux values are used. The study reaffirms that uncertainty is pervasive in natural systems and that results of modeling efforts presented in a deterministic fashion may be misleading.