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UNCERTAINTY IN LEACHING POTENTIAL OF NONPOINT SOURCE POLLUTANTS WITH APPLICATION TO A GIS
Citation:
Hantush*, M M., Z. Zhang, M. A. Marino, AND E. Hoque. UNCERTAINTY IN LEACHING POTENTIAL OF NONPOINT SOURCE POLLUTANTS WITH APPLICATION TO A GIS. In Proceedings, Water: Lessons of World Development, Universities Council on Water Resources 1999 Annual Meeting & Water & Environmental Resources Management: Focus on Asia & the Pacific, International Water Resource Economics Consortium, 6th Biennial Meeting, Kamuela, HI, June 29 - July 02, 1999. Universities Council on Water Resources, Carbondale, IL, 21-36, (1998).
Impact/Purpose:
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Description:
This paper presents a stochastic framework for the assessment of groundwater pollution potential of nonpoint source pesticides. A conceptual relationship is presented that relates seasonally averaged groundwater recharge to soil properties and depths to the water table. The analytical relationship shows a linear association with the soil saturated hydraulic conductivity, and predicts less recharge for shallower water tables. A modified index model for pesticides mass emissions in the soil is developed, which accounts explicitly for biochemical degradation in the mobile and immobile soil-water zones and losses by volatilization and crop uptake. The index model shows that the effect of nonequilibrium transport on the residual mass emissions is dependent on ratio of the mass transfer coefficient and the first-order degradation rate in the immobile phase. The stochastic framework utilizes first-order approximations of the mean and variance of each of the recharge and the residual mass emissions in the soil. Soil and chemical properties and related environmental factors, which affect the fate and transport of pesticides and the depth to the water table, are modeled as random variables. The environmental-fate models are integrated with a GIS, and the stochastic framework is applied to assess potential nonpoint-source vulnerability of shallow groundwater to the pesticide dicamba in Mid-Atlantic coastal plain agricultural watersheds. It is shown that recharge estimates on the basis of the SCS abstraction method resulted in lower expected dicamba concentrations in groundwater, than when leaching is based on the conceptual model. This may be attributed to the fact that in the former, leaching is averaged over the soil hydrologic groups, whereas in the latter, leaching reflects the spatial variability of the soil types and environmental factors, including the depth to the water table. In the analysis, biochemical degradation of dicamba is ignored below the root zone and the vulnerability results were based on a single application of 2.5 lb/acre. The first-order estimated of the variance of dicamba mass emissions and estimated groundwater concentrations, were of order of magnitude of their respective means and greater. While some of the mean groundwater concentrations showed significant residual levels of dicamba, these values however should be viewed as subject to high uncertainty. Given the expected uncertainties in the input data and model errors, regulatory decisions and environmental land-use planning should take into account estimates of the uncertainties (e.g., variances) associated with predictions of groundwater vulnerabilities.