Runoff and leaching of atrazine and alachlor on a sandy soil as affected by application in sprinkler irrigation

Rainfall simulation was used with small packed boxes of soil to compare runoff of herbicides applied by conventional spray and injection into sprinkler-irrigation (chemigation), under severe rainfall conditions. It was hypothesized that the larger water volumes used in chemigation would leach some of the chemicals out of the soil surface rainfall interaction zone, and thus reduce the amounts of herbicides available for runoff. A 47-mm rain falling in a 2-hour event 24 hours after application of alachlor (2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)-acetamide) and atrazine (6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2, 4-diamine) was simulated. The design of the boxes allowed a measurement of pesticide concentrations in splash water throughout the rainfall event. Initial atrazine concentrations exceeding its' solubility were observed. When the herbicides were applied in 64,000 L/ha of water (simulating chemigation in 6.4 mm irrigation water) to the surface of a Tifton loamy sand, subsequent herbicide losses in runoff water were decreased by 90% for atrazine and 91% for alachlor, as compared to losses from applications in typical carrier water volumes of 187 L/ha. However, this difference was not due to an herbicide leaching effect but to a 96% decrease in the amount of runoff from the chemigated plots. Only 0.3 mm of runoff occurred from the chemigated 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 are (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.