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ARS Home » Southeast Area » Tifton, Georgia » Southeast Watershed Research » Research » Publications at this Location » Publication #162547


item Wauchope, Robert - Don
item Johnson, Wiley - Carroll
item Sumner, Harold

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/30/2004
Publication Date: 12/1/2004
Citation: Wauchope, R.D., Johnson, W.C., Sumner, H.R. 2004. Foliar and soil deposition of pesticide sprays in peanuts, and washoff/runoff under simulated worst-case rainfall conditions. Journal of Agricultural and Food Chemistry 52(23):7056-7063(2004).

Interpretive Summary: Pesticide loss in runoff water from agricultural fields into streams is the most important source of pesticide contamination of water resources in the U. S. This is the latest report from this laboratory on the use of a large scale rainfall simulator being used to apply 'rain' to a field immediately after pesticide applications'a 'worst case' situation which generates the most possible pesticide loss from the field. By analyzing the runoff water we can determine the amount of pollution that occurs under these conditions and how it is effected by all the factors'soil, crop, pesticide type'that effect the loss. In these experiments we rained on bare soil after two peanut herbicides were applied, then later rained on the when they were fully grown after applying a fungicide and a water-soluble red dye to the peanuts. The results were dramatic: we were able to compare how soil- vs. plant-applied chemicals behaved, and see from the dye now quickly a soluble chemical can be washed off the foliage and into the soil. Results provide basic information on the pesticide runoff process and provide essential pesticide safety evaluation. It also provides insight into how well the simulation experiment itself works: this is important because these experiments are being used by the pesticide industry to provide pesticide runoff information to EPA for registration of products, but they are not being published or reviewed by the scientific community.

Technical Abstract: Runoff losses of ethalfluralin, metolachlor, chlorothalonil and rhodamine WT dye were measured using simulated rainfall on 450 m^22 mesoplots planted in peanut. Ethalfluralin was applied preplant incorporated and metolachlor was applied preemergence on bare soil. Chlorothalonil and rhodamine WT were applied to the peanut canopy at maturity. Rainfall/runoff was simulated 24 h after each chemical application using raindrop irrigation sprinklers, applying 5.5 ± 0.5 mm over a 2 h period to create reasonable worst-case conditions. These rains generated between 3 and 9 mm runoff. Volume-weighted average concentrations of chemicals in runoff were 7, 104, 163 and 179 ug L-1 for ethalfluralin, metolachlor, chlorothalonil, and rhodamine WT, respectively. Total amounts of chemicals lost in individual runoff events were 0.04 ± 0.01, 0.2 ± 0.1, 0.6 ± 0.5 and 0.2 ± 0.1, as percent of amounts applied, respectively. Rhodamine WT formed a vivid red solution and provided visual clues to the dynamics of chemical washoff/runoff, indicating complete washoff from rain-exposed foliage within a few minutes of the beginning of rainfall, and leaching below rain-exposed soil surface within the first 10 minutes of rainfall. However, dye was present at near-constant concentrations throughout the runoff event, indicating that chemical was present in the soil extraction zone throughout. These results confirm that soil incorporation significantly reduces runoff losses, and that a majority of foliar residues can be washable if rainfall occurs within a few days after application. Runoff losses of foliar residues are small relative to Washoff fractions, and may be dependent on runoff timing relative to washoff.