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TECHNICAL REPORTS

Wetlands and Aquatic Processes

Mitigation Assessment of Vegetated Drainage Ditches for Collecting Irrigation Runoff in California

^a USDA Agricultural Research Service, National Sedimentation Lab., PO Box 1157, Oxford, MS 38655 USA
^b USEPA Region IX, Sacramento, CA 95814 USA
^c Yolo County Resource Conservation District, Woodland, CA 95695 USA
^d Aqua-Science, Davis, CA 95616 USA
^e Univ. of California at Davis, Aquatic Toxicology Lab., Davis, CA 95616
^f California Dep. of Fish and Game, Rancho Cordova, CA 95670 USA

^* Corresponding author (mtmoore{at}ars.usda.gov).

Received for publication April 5, 2007. Widespread contamination of California water bodies by the organophosphate insecticides diazinon and chlorpyrifos is well documented. While their usage has decreased over the last few years, a concomitant increase in pyrethroid usage (e.g., permethrin) (replacement insecticides) has occurred. Vegetated agricultural drainage ditches (VADD) have been proposed as a potential economical and environmentally efficient management practice to mitigate the effects of pesticides in irrigation and storm runoff. Three ditches were constructed in Yolo County, California for a field trial. A U-shaped vegetated ditch, a V-shaped vegetated ditch, and a V-shaped unvegetated ditch were each amended for 8 h with a mixture of diazinon, permethrin, and suspended sediment simulating an irrigation runoff event. Water, sediment, and plant samples were collected spatially and temporally and analyzed for diazinon and permethrin concentrations. Pesticide half-lives were similar between ditches and pesticides, ranging from 2.4 to 6.4 h. Differences in half-distances (distance required to reduce initial pesticide concentration by 50%) among pesticides and ditches were present, indicating importance of vegetation in mitigation. Cis-permethrin half-distances in V ditches ranged from 22 m (V-vegetated) to 50 m (V-unvegetated). Half-distances for trans-permethrin were similar, ranging from 21 m (V-vegetated) to 55 m (V-unvegetated). Diazinon half-distances demonstrated the greatest differences (55 m for V-vegetated and 158 m for V-unvegetated). Such economical and environmentally successful management practices will offer farmers, ranchers, and landowners a viable alternative to more conventional (and sometimes expensive) practices.

Abbreviations: OP, organophosphate • BMP, best management practice • TMDL, total maximum daily load