Submitted to: Journal of Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/26/2000
Publication Date: 7/8/2000
Citation: Blanchard, P.E., Lerch, R.N. 2000. Watershed vulnerability to losses of agricultural chemicals: The interplay of chemistry, hydrology, and land-use. Journal of Environmental Science and Technology. 34:3315-3322.
Interpretive Summary: The objective of this study was to determine the relative importance of land-use and hydrology to agricultural chemical contamination of northern Missouri streams. The study encompassed about 20,000 sq. mi. in northern Missouri and southern Iowa with a diversity in soils, hydrology, and land use. In 1994 and 1995, samples were collected at 140 sites from 95 streams and rivers under pre-plant and post-plant conditions. Samples were analyze for common herbicides and dissolved nutrients. Three of the most heavily used corn herbicides, atrazine, cyanazine, and metolachlor, were detected most frequently. Atrazine was detected in at least 90% of pre-plant samples and 100% of post-plant samples, with post-plant levels often exceeding drinking water standards. Nutrient and herbicide contamination showed opposite trends across the study region. Streams draining watersheds with runoff-prone soils had the highest herbicide levels, while streams draining gwatersheds with well drained soils had the highest nitrate levels. From ou data, we developed a conceptual framework to assess watershed vulnerability to agrichemical contamination based on three factors: 1) chemistry of the contaminant; 2) hydrology of a region; and 3)land-use. The major impact of this work will be to promote development of best management practices (BMPs) to protect water quality based on watershed characteristics, rather than a 'one size fits all' approach. Farmers will benefit from this work because development of BMPs will be potentially more effective; thus, minimizing the need for uneconomical herbicide alternatives or punitive water quality regulations.
Technical Abstract: The objectives of this study were to conduct a multi-scale, multi-basin reconnaissance survey of northern Missouri streams to determine the relative importance of land-use and hydrology to agricultural chemical contamination of streams in northern Missouri. In 1994 and 1995, samples were collected from 140 sites on 95 different streams and rivers representing about 52,000 km**2 throughout northern Missouri and southern Iowa with a diversity in soils, hydrology, and land-use. Samples were collected under pre- and post-plant conditions and analyzed for common herbicides and dissolved nutrients. Atrazine, cyanazine, and metolachlor were the most frequently detected herbicides. Atrazine was detected in at least 90% of pre-plant samples and 100% of post-plant samples. Nutrient and herbicide concentrations showed opposite trends across the study region. Streams draining watersheds with runoff-prone soils had the highest herbicide concentrations, while streams draining watersheds with well drained soils had the highest nitrate concentrations. Current data are sufficient to develop a conceptual framework for assessing watershed vulnerability based on three key factors. The primary factor is the chemistry of the compound which determines the potential hydrologic transport pathways. The hydrology of a region is the second factor, as it determines the realtive magnitude of leaching or runoff transport pathways. The third factor is land-use, which includes the intensity of row-cropping within a watershed, the locations within the watershed that are cropped, and the chemical inputs. Management practices intended to improve water quality should be designed in accordance with these factors.