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U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

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Chemical Loads of Nitrate, Atrazine, and Metolachlor in Walnut Creek Watershed near Ames, Iowa, 1991-92


P.J. Soenksen (U.S. Geological Survey, Iowa City, IA 52244), J.L. Hatfield (Agricultural Research Service, Ames, IA 50011), and D.J. Schmitz (Agricultural Research Service, Ames, IA 50011)


Streamflow, stormflow, and tileflow were measured and sampled at numerous sites in the Walnut Creek watershed near Ames, Iowa, to determine the chemical discharges, loads, and yields of nitrate as nitrogen (nitrate-N) and the herbicides atrazine and metolachlor. From April 1991 to September 1992, nitrate-N yield was 58.4 kg/ha (kilograms per hectare), and atrazine and metolachlor yields were 4.5 g/ha (grams per hectare) and 7.3 g/ha, respectively, in streamflow from a 2,540-ha (hectare) subwatershed. Large chemical discharges and increases in cumulative loads were closely related to stormflow. Chemical loads were larger during the 1991 growing season, when soils were wet and stormflow occurred shortly after application of chemicals, compared to loads during the 1992 growing season when stormflow did not occur until several months after application of chemicals. Nitrate-N was transported during periods of base flow throughout the year; there were many periods when the herbicides were not detected.

For July-August 1992, a comparison of loads and yields in streamflow from the subwatershed was made to those in stormflow and tileflow from a 360-ha small basin within the subwatershed. The comparison indicates that most of the chemical load in streamflow was transported by tileflow, and little was transported by stormflow, even during stormflow. There were four periods of stormflow during July-August 1992, but there were none in the preceding 2 months. Nitrate-N yields were 8.0 kg/ha in streamflow, 0.06 kg/ha in stormflow, and 7.9 kg/ha in tileflow. Atrazine yields were 0.42 g/ha in streamflow, 0.010 g/ha in stormflow, and 0.50 g/ha in tileflow. Metolachlor yields were 0.42 g/ha in streamflow, 0.007 g/ha in stormflow, and 0.35 g/ha in tileflow. Comparison of daily yields, one during stormflow and one during base flow, with those from a field tile also indicate tileflow as the primary transporter of agricultural chemicals for this period.

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