Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 19, 1995
Publication Date: N/A
Interpretive Summary: Observations of herbicides in rainfall has prompted concern about the impact of farming practices on the quality of rainfall. A study was undertaken in the Walnut Creek watershed of central Iowa to evaluate the deposition of atrazine, alachlor, metribuzin, and metolachlor, common herbicides used in Iowa, and nitrate-nitrogen in precipitation events. Two specialized samplers that are designed to capture precipitation were located about 4 km apart in the central part of the watershed. Water samples were collected after each storm as well as the amount of herbicides and nitrate analyzed with chemical analysis procedures. Herbicides were most frequently detected in the rainfall during the spring months around the time of application. These amounts were very low and only one time did the concentration exceed a contaminant limit. This occurrence was due to spray drift from the field under a condition of extreme fog and rain. All of the other times the concentrations are only above the detection limits. The amount of herbicide deposited onto the soil is very small and insufficient to cause any problems. Understanding the transport processes of chemicals into the atmosphere will lead to improved application methods that reduce these losses.
Herbicides detected within precipitation are considered to be a large amount of the total applied and represent a negative impact of agricultural practices on environmental quality. This study was designed to evaluate the monthly and annual variation in the atrazine, alachlor, metolachlor and nitrate-nitrogen concentrations in precipitation over Walnut Creek watershed in central Iowa. Two wet/dry precipitation samplers were positioned 4 km apart. Rainfall samples were collected for every event that produced a minimum of 250 ml. A separate tipping bucket rain gage was used to measure rainfall totals. Almost 90% of the herbicide detections occurred from April through early July. Concentrations varied among events from nondetectable amounts to concentrations of 1 ug/L. The largest concentration recorded in one of the collectors was 154 ug/L which occurred when atrazine was applied during an extremely humid day immediately followed by a small precipitation event of less than 10 mm which washed the spray drift from the atmosphere into the collector. This was a local scale phenomenon. Variation between the two collectors suggests that local scale meteorological processes within the watershed affect herbicide movement into the atmosphere. Yearly atrazine deposition totals were over 200 ug/m**2, representing less than 1% of the amount applied within the watershed. Variation among the years in the deposition totals of atrazine, alachlor, and metolachlor suggest that deposition rates are a function of timing of rainfall relative to application. Deposition rates of NO3-N were approximately 1.2 g/m**2. Annual loading from precipitation onto the watershed was about 25% of the amount applied from fertilizers.