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 run off of herbicides applied by conventional spray and injection into sprinkle -irrigation (chemigation), under severe rainfall conditions. It was hypothe sized 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 wate r throughout the rainfall event. Initial atrazine concentrations exceeding its' solubility were observed. When the herbicides were applied in 64000 L/ ha of water (simulating chemigation in 6.4 mn irrigation water) to the surf ace of a Tifton loamy sand, subsequent herbicide losses in runoff water wer e 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, t his difference was not due to an herbicide leaching effect but to a 96% dec rease in the amount of runoff from the chemigated plots. Only 0.3 mm of run off occurred from the chemigated boxes while 7.4 mm runoff occurred from th e 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 les s in runoff than in leachate water. Thus, in this soil application of these herbicides by chemigation would decrease their potential for pollution onl y in situations where runoff is a greater potential threat than leaching.