Constructed Wetlands as a Mitigation Strategy to Reduce Pesticide Loads in Agricultural Tailwater
- Author(s): Budd, Robert L.
- Advisor(s): Gan, Jianying
- et al.
Pyrethroid and organophosphate pesticides have been found in numerous waterways throughout the United States. Monitoring studies within the San Diego Creek watershed in Orange County, California, confirmed that runoff from agricultural fields are a primary contributor to pesticide loads within these waterways. As a mitigation option for non-point pesticide pollution, constructed wetlands were investigated in this study to reduce pesticide loading in agricultural tailwaters. Multiple wetlands located along the San Joaquin River in the Stanilaus County were observed over two consecutive irrigation seasons. The wetlands were found to be very effective at reducing pyrethroid concentrations (52 - 100%) and moderately effective at reducing chlorpyrifos concentrations (52 - 61%) under two flow regimes, with loads reduced by 95 - 100% in the outgoing water. Vertical transport of pyrethroids was found to be negligible, and less than 10% of chlorpyrifos was found to leach below 16 cm in the wetland floors. A significant increase in removal efficiencies between seasons within one portion of the wetland was observed, and was attributed to a dramatic increase in vegetation and decreased flow rates. Pyrethroids overwhelmingly sorbed to suspended solids greater than 0.7 µm (62 - 93%), with resulting apparent partitioning coefficients (Kd a) ranging from 1.9 E104 to 3.1 E 105. The freely dissolved concentrations of permethrin represented approximately 26 - 39% of its total mass in water samples with a decreasing trend in concentrations toward the outlet, signaling a decrease in the bioavailable fraction in the outgoing flow. Pesticide concentrations mimicked organic carbon content of the deposited sediment particle fractions. The highest concentrations were associated with larger particles comprised of aggregates of organic and decomposed plant material, which are less susceptible to sedimentation. This observation helps explain why the sediment basin was not effective at removing pesticides from the tailwater. Effective wetland lengths (L1/2) necessary to reduce pesticide concentrations by 50% were estimated to be less than 100 m for cyhalothrin, cypermethrin and permethrin under low flow (0.03 m3s-1), but reaching 267 m under high flow conditions (0.07 m-3 s-1). The degradation studies indicate that the pesticides have the potential for persisting within the wetland system between irrigation seasons. While the half-lives of lambda-cyhalothrin, cypermethrin, esfenvalerate, and permethrin were less than 1 yr (r2 > 0.48) under anaerobic conditions, no detectable dissipation occurred for most pesticides in situ during the dry season. Bifenthrin was found to be relatively stable in all sediments, indicating its potential for prolonged persistence within the wetland systems. The results from this study indicate that constructed wetlands may act as a sink for most pyrethroids, and the removal is achieved through sedimentation and sediment trapping by vegetation and gravity. However, accumulation of pesticides over time, and the associated environmental risks of the accumulated pesticide residues, should be further understood if constructed wetlands are to be widely used as a management practice.