Pesticide Losses in Surface Runoff from Irrigated Fields
Pesticide concentrations were determined in surface irrigation runoff water following the application of pesticides to large fields of cotton, sugar beets, lettuce, alfalfa, onions, or melons in Imperial Valley, CA. The concentrations and total amounts of pesticides in runoff water were dependent upon the characteristics of the pesticides and methods and rates of application. The percentages of the applied pesticides lost in runoff water were generally low with seasonal totals for insecticides in runoff below 1% of the amounts applied. Seasonal losses of the soil-applied herbicides were usually 1 to 2% of the amounts applied. Time elapsed between pesticide application and the irrigation event was inversely related to log concentration, indicating an approximate first-order rate of decrease in concentration with time. The concentration of pesticides in the 0- to 1-cm depth of soil was a good indicator of concentrations and amounts to be expected in runoff water and using the average of three concentrations of methomyl from each peak flow and the flow volume under the peaks resulted in a calculated runoff of 6.1 g/ha methomyl, which was within ± 2% of the integrated estimate using 24 data points.
Other runoff events provided similar comparisons when both the pesticide concentration and the water volume for each peak of water flow was used. Over all, the data indicate that the total pesticide runoff can be quite accurately estimated using only pesticide concentrations from the peak flows. The estimate from the last event in Table 7 is an exception. In this case, prometryn had been recently applied as a post-emergent herbicide with much of the pesticide remaining on the soil surface in the irrigation furrows. In similar situations, when the first runoff water may contain very high concentrations of the pesticide, sampling only peak flows may miss a significant part of the runoff flux and result in underestimates of the total pesticide loss by a considerable amount, such as the -47% in this case.
KeywordsSugar Melon Carbamate Endosulfan Chlorpyrifos
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