Herbicide and nitrate distribution in central Iowa rainfall

Herbicides are detected in rainfall; however, these are a small fraction of the total applied. This study was designed to evaluate monthly and annual variation in atrazine (6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine) alachlor (2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide metolachlor (2-chloro-N-(2-ethyl-6-methylphenyl)-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 the watershed, two wet/dry precipitation samplers were positioned 4 km apart. Detections varied during the year with >90% of the herbicide detections 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 humid day immediately followed by rainfall of <10 mm that washed spray drift from the atmosphere. This was a local scale phenomenon, because the 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 that local scale meteorological processes affect herbicide movement. Yearly atrazine deposition totals were >100 mu g m(-2) representing <0.1% of the amount applied. Nitrate-N concentrations in precipitation were uniformly distributed throughout the year and without annual variation 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 applied from all forms of N fertilizers. Movement and rates of deposition provide an understanding of the processes and magnitude of the impact of agriculture on the environment.