| Popis: |
Pesticide contamination of ground water has traditionally been considered to be due primarily to spills, and to improper storage, disposal and application practices. There is increasing evidence, however, that normal agricultural practices can also result in low-level, non-point source contamination of ground water via the downward migration of pesticides through the soil profile (Agriculture Canada, 1990). Although this type of contamination is usually well below Canadian drinking water guidelines, there are growing public concerns over potential health hazards related to long-term exposure to low levels of pesticides (Agriculture Canada, 1990). Consequently, there is a need to determine how important and widespread low level non-point source pesticide contamination of ground water might be what the controlling soil, land use and weather factors are and which agricultural practices are required to mitigate and control this type of pollution at acceptable and sustainable levels. Essential steps in obtaining this information include identification of the primary mechanisms controlling pesticide movement through the soil profile, and development of the capability to characterize and predict the pesticide movement in space and time with acceptable accuracy. Accordingly, the objectives of this study are: (i) to develop a methodology for redicting, characterizing and quantifying pesticide migration rates through the soil profile, and ii) to conduct a test and evaluation of the methodology by applying it to atrazine migration through the soil profiles of the Grand River watershed in Southern Ontario, Canada. The herbicide atrazine was chosen for study because it is the most commonly used pesticide for corn production in Southern Ontario and because atrazine residues have been detected in the ground waters of many agricultural watersheds in the Great Lakes basin, particularly where there is some combination of high atrazine usage, intensive agriculture, high precipitation, coarse textured and other highly permeable soils, high water tables, and sloping topography (Millette and Torreiter, 1992). The watershed scale was chosen (rather than the field scale, for example) because it is a natural landscape unit because the most readily available and complete soil and weather databases are applicable on the watershed scale and because the watershed provides a convenient basis for estimating agrochemical loadings to the Great Lakes. The Grand River watershed (Fig. 1) was chosen as a test case because it is one of the largest in Southern Ontario (. 680,000 ha) it contains a large range of soil textures with a complexity of distribution that is typical for the region the primary land use is field crop production using standard agricultural practices and "normal" rates of pesticide usage (Shelton et al., 1988) and it empties directly into the Great Lakes (Lake Erie). These features make this watershed ideal for testing and demonstrating the methodology. In addition, the results obtained for the Grand River watershed should be "characteristic" of the entire Southern Ontario region. Ground water contamination in this study is defined as non-zero values of predicted annual mass loading of pesticide to the 90 cm depth in the soil. Non-zero mass loading was used, rather than pore water concentrations above a specified threshold, because of the need to estimate the quantities and distributions of all pesticide additions to ground water, not just the "high level" additions. The 90 cm depth was selected because it reflects the mean tile drain depth in Southern Ontario, as well as the primary rooting depth for most field crops. It was assumed that if a pesticde reached this depth, it would not be intercepted by tiles and roots, but continue to percolate downward and eventually enter the ground water. |
