HERBICIDE AND NITRATE DISTRIBUTION IN CENTRAL IOWA RAINFALL

Herbicides are detected in rainfall; however, these are a small fracti on of the total applied. This study was designed to evaluate monthly a nd annual variation in atrazine hyl-N'-(1-methylethyl)-1,3,5-triazine- 2,4-diamine) alachlor o-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetami de metolachlor thylphenyl)-N-(2-methoxy-1-methylethyl)acetamide), and NO3-N concentrations in rainfall over Walnut Creek watershed south of Ames, IA. The study began in 1991 and continued through 1994. Within t he watershed, two wet/dry precipitation samplers were positioned 4 km apart. Detections varied during the year with >90% of the herbicide de tections occurring in April through early July. Concentrations varied among events from nondetectable amounts to concentrations of 154 mu g L(-1), which occurred when atrazlne was applied during an extremely hu mid day immediately followed by rainfall of <10 mm that washed spray d rift from the atmosphere. This was a local scale phenomenon, because t he other collector had a more typical concentration of 1.7 mu g L(-1) with an 8-mm rainfall. Variation between the two collectors suggests t hat local scale meteorological processes affect herbicide movement. Ye arly atrazine deposition totals were >100 mu g m(-2) representing <0.1 % of the amount applied. Nitrate-N concentrations in precipitation wer e uniformly distributed throughout the year and without annual variati on in the concentrations. Deposition rates of NO3-N were about 1.2 g m (-2). Annual loading onto the watershed was about 25% of the amount ap plied from all forms of N fertilizers. Movement and rates of depositio n provide an understanding of the processes and magnitude of the impac t of agriculture on the environment.