Expanding Wetland Mitigation: Can Rice Fields Remediate Pesticides in Agricultural Runoff?

Pesticides are responsible for nearly 1900 water quality impairments in the United States. Impacts of pesticide runoff on aquatic ecosystems can be mitigated by implementing management practices such as constructed wetlands, grass buffers, and vegetated ditches. A new practice currently being examined is the use of rice ( L.) fields for phytoremediation of pesticide-contaminated water. Rice is cultivated on every continent except Antarctica and is the staple food crop of 20% of the world's population. Four flooded 244-m fields (two planted with rice, two left bare) were amended with a mixture of atrazine (CHClN), diazinon (CHNOPS), and permethrin (CHClO) during a one-time simulated storm event, and pesticide concentrations and loads were monitored in water, sediment, and plant samples. The experiment was repeated the following year. Significant differences were noted for mitigation of atrazine and diazinon loads in rice versus bare systems. Overall, atrazine loads in the water of rice systems decreased 85 ± 8% from inflow to outflow, while atrazine loads in the water of bare systems decreased 58 ± 7%. Similar patterns were seen for diazinon (86 ± 4% versus 62 ± 7%), cis-permethrin (94 ± 2% versus 64 ± 12%), and trans-permethrin (97 ± 2% versus 67 ± 14%). All three pesticides were found repeatedly sorbed to plant material in the inflow and outflow areas during the first year, while the second year resulted in much less plant-pesticide contribution to overall mitigation. Further investigation is needed to compare rice's mitigation capacity of different pesticide classes, as well as potential transfer of pesticides to edible seeds.

Diazinon reduction and partitioning between water, sediment and vegetation in stormwater runoff mitigation through rice fields.

BACKGROUND: Contamination of surface waters by pesticides is a concern in the United States and around the world. Innovative mitigation strategies are needed to remediate this potential environmental contaminant. One potential solution is to divert pesticide-laden drainage or surface water through agricultural rice fields. With a hydroperiod, hydrosoil and hydrophyte (rice), these systems serve essentially as a type of constructed wetland. In both summer and fall experiments, diazinon-amended water was diverted through two rice ponds at the University of Mississippi Field Station. Likewise, a non-vegetated control pond was amended with diazinon-laden water. Water, sediment and plant samples were taken spatially and temporally to determine the distribution of diazinon within systems. RESULTS: Outflow diazinon concentrations decreased significantly (P < 0.05) from inflow in both vegetated ponds for both preharvest and post-harvest experiments. Although sorption to rice plants was minimal in the overall mass distribution of diazinon (1-3%), temporal data indicated that diazinon concentrations reached the outflow sediment of the non-vegetated control twice as fast as in either vegetated (rice) system. In both vegetated systems, sediment diazinon concentrations decreased (77 and 100%) from inflow to outflow, while a decrease of <2% was noted in the non-vegetated control. CONCLUSIONS: Diversion of pesticide-contaminated water through rice fields demonstrated potential as a low-cost, environmentally efficient mitigation practice. Studies on these systems are continuing to evaluate the optimal chemical retention time for rice field mitigation, as well as diazinon transfer to rice grain seeds that may be used as a food source.