Use of Constructed Wetlands to Remove Pesticides in Irrigation Tailwater.

In areas of intensive agriculture such as the Central Valley of California, pesticides, nutrients, and other contaminants in irrigation return flows (tailwater) contribute to water quality degradation of local streams and tributaries. Given the non-point source nature of such pollutions, flow-through wetlands represent one of the most feasible mitigation options. In this study, we evaluated the effectiveness of two wetlands for their removal of commonly used insecticides including pyrethroids, diazinon and chlorpyrifos throughout an entire irrigation season. Water samples were collected biweekly at multiple locations throughout the wetlands. Accumulated sediments were sampled using sediment traps. Analysis of pesticides showed that both wetlands were highly effective in removing pyrethroid compounds such as permethrin, cypermethrin, cyhalothrin, and bifenthrin. Removal of these compounds was mainly mediated through sedimentation caused by gravity and vegetative filtration. When reductions in both flow volume and concentrations were considered, the seasonal average removal of pyrethroids was 95-100%. Results from this study clearly suggest that flow-through wetlands, when properly designed, are an effective approach for mitigating hydrophobic contaminants in agricultural irrigation tailwater.