Movement of Simazine in Runoff Water from Citrus Orchard Row Middles

In California, pre-emergence herbicide residues have been measured in runoff water from citrus orchards that resulted from winter rainfall. This study measured the effect of rainfall on the redistribution of herbicides within a citrus orchard and the effect that shallow mechanical incorporation had on residue movement. Simulated rainfall treatments were applied to plots within a citrus orchard where simazine was applied only to row middles. Simazine movement in runoff water was compared between middles that were either undisturbed, the normal orchard practice, or subject to shallow mechanical incorporation. In undisturbed middles, simazine concentration in runoff water collected from the first of two simulated rain events averaged 0.87 mg L; simazine concentration in runoff water from a second event applied 1 week later averaged 0.40 mg L. Shallow mechanical incorporation of row middles decreased runoff water volume from the first simulated rain event by approximately 50% with simazine concentration decreased to 0.14 mg L; runoff water volume was unaffected at the second rainfall event but simazine concentration remained low at 0.07 mg L. Total simazine mass removed from both events, which also accounts for mass recovered in furrow soil, was estimated at 13.1% of the amount applied to row middles in undisturbed plots compared to only 2.1% in mechanically disturbed middles. We conclude that ambient rainfall is unreliable for incorporation of pre-emergence herbicides into orchard soil with low infiltration, and that shallow mechanical incorporation should be tested under commercial citrus growing conditions. Abbreviations: ELISA, enzyme-linked immunosorbent assay; TMB, tetramethylbenzidine. Published in J. Environ. Qual. 27:488-4494 (1998).