ARS | Publication request: THE INTERPLAY OF CHEMISTRY, HYDROLOGY, AND LAND-USE IN DETERMINING WATERSHED VULNERABILITY TO LOSSES OF AGRICULTURAL CHEMICALS

Submitted to: International Association of Environmental and Analytical Chemistry
Publication Type: Abstract Only
Publication Acceptance Date: September 13, 1999
Publication Date: N/A

Technical Abstract: In 1994 and 1995, samples were collected from 140 sites on 95 different streams and rivers throughout northern Missouri. Samples were collected under pre-plant and post-plant conditions, and analyzed for alachlor, atrazine, cyanazine, metolachlor, metribuzin, triazine metabolites, and dissolved nutrients. The study area has significant variations in soils, hydrology, and land-use. In the study area, the hydrology is largely determined by the soils. The geographic distributions of nitrates and herbicides in study area streams were different, demonstrating that neither land-use nor hydrology was the primary factor determining the long-term vulnerability of the watersheds to losses of agricultural chemicals. Current data are insufficient for a complete GIS analysis, but are sufficient to develop a useful conceptual framework for assessing watershed vulnerability. The primary factor is the chemistry of the compound, which determines the potential transport pathways for that chemical to be lost from the soil. Compounds such as nitrate or some herbicide metabolites, which are poorly sorbed and slowly degraded, can potentially be leached or lost in runoff. Moderately sorbed compounds, such as atrazine, are more likely to be lost in runoff or degraded within the soil than leached. The hydrology of a region is the secondary factor, as it determines the relative importance of the leaching and runoff transport pathways. The third factor then is the land-use, which includes the percentage of a watershed that is cropped, the locations within the watershed that are cropped, and the chemicals applied. Management practices to improve water quality must be designed in accordance with the dominant problems and transport pathways of a watershed.