Submitted to: Journal of Environmental Science and Health
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 14, 1999
Publication Date: January 19, 1999
Citation: Rahman, A.G., Wauchope, R.D., Truman, C.C., Dowler, C.C. Runoff and leaching of atrazine and alachlor on a sandy soil as affected by irrigation. J. Environ. Sci. Hlth. B31:381-396. 1999. Interpretive Summary: Many pesticides and fertilizers are applied by injection into irrigation water. This results in application in a large volume of water with very dilute concentrations of the pesticide. This might mean that pesticides applied in this way may be washed down into the soil instead of being left on the soil surface. This would make them less "available" for being washed off the soil into rivers and lakes by rainfall. Thus, irrigation might be a way to apply pesticides with less potential for water pollution. Two herbicides, atrazine and alachlor, that have been observed in many rivers and lakes, were applied to the surface of small boxes of packed soil, using a conventional sprayer and irrigation water. The boxes were then tilted and simulated rain was applied to the soil surfaces using a special rainfall simulator which gives extreme, "worst-case" rainfall conditions. We found that "runoff" of the pesticides was decreased by 90% when they were applied in irrigation water, as compared with conventional application. However, the decrease was due not to less pesticide being washed off, but to a dramatic decrease (96%) in the amounts of water that ran off. This result is unexpected and will be investigated further.
Technical Abstract: Rainfall simulation was used with small packed boxes of soil to compare runoff of herbicides applied by conventional spray in 187 L/ha water and by injection into sprinkler irrigation at 64000 L/ha. We hypothesized that larger application water volumes would leach chemicals below the soil surface and thus reduce the amounts of herbicides available for runoff. We esimulated 47 mm of rain falling in a two-hour event 24 hours after application of alachlor and atrazine. Herbicide losses in runoff water were decreased by 90% when applied in irrigation water. However, this difference was due not to a herbicide leaching effect but to a 96% decrease in the amount of runoff water from the irrigated boxes. Only 0.3 mm of runoff occurred from the irrigated boxes while 7.4 mm runoff occurred from the conventionally-treated boxes, even though antecedent moisture was higher in the former. Two possible explanations for this unexpected result tare (a) increased aggregate stability in the more moist condition, leading to less surface sealing during subsequent rainfall, or (b) a hydrophobic effect in the drier boxes. In the majority of these pans, herbicide loss was much less in runoff than in leachate water. Thus, in this soil, application of these herbicides by chemigation would decrease their potential for pollution only in situations where runoff is a greater potential threat than leaching.