R.F. Cullum

Nutrient mitigation capacity in Mississippi Delta, USA drainage ditches

Eutrophication and hypoxia within aquatic systems are a serious international concern. Various management practices have been proposed to help alleviate nutrient loads transported to the Gulf of Mexico and other high-profile aquatic systems. The current study examined the nutrient mitigation capacity of a vegetated (V) and non-vegetated (NV) agricultural drainage ditch of similar size and landform in the Mississippi Delta. While no statistically significant differences in ammonium, nitrate, or dissolved inorganic phosphorus mitigation between the two ditches existed, there were significant differences in total inorganic phosphorus percent load reductions (V: 36% +/- 4; NV: 71% +/- 4). However, both agricultural drainage ditches were able to mitigate nutrients, thus reducing the load reaching downstream aquatic receiving systems. Further studies examining ecosystem dynamics within drainage ditches such as sediment and plant nutrient partitioning, as well as microbial processes involved, are needed to provide a better understanding of natural nutrient variability, seasonality and flux.

Mitigation of two pyrethroid insecticides in a Mississippi Delta constructed wetland

Constructed wetlands are a suggested best management practice to help mitigate agricultural runoff before entering receiving aquatic ecosystems. A constructed wetland system (180 m x 30 m), comprising a sediment retention basin and two treatment cells, was used to determine the fate and transport of simulated runoff containing the pyrethroid insecticides lambda-cyhalothrin and cyfluthrin, as well as suspended sediment. Wetland water, sediment, and plant samples were collected spatially and temporally over 55 d. Results showed 49 and 76% of the studys measured lambda-cyhalothrin and cyfluthrin masses were associated with vegetation, respectively. Based on conservative effects concentrations for invertebrates and regression analyses of maximum observed wetland aqueous concentrations, a wetland length of 215 m x 30 m width would be required to adequately mitigate 1% pesticide runoff from a 14 ha contributing area. Results of this experiment can be used to model future design specifications for constructed wetland mitigation of pyrethroid insecticides.

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.

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 worlds 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 rices mitigation capacity of different pesticide classes, as well as potential transfer of pesticides to edible seeds.

Ultra Narrow-row Cotton for Erosion Control in Silt Loam Soils

Grass hedges and no-till cropping systems reduced soil losses on standard erosion plots in ultra narrow-row (20 cm) cotton during a four-year study (1999-2002). No-till cotton with grass hedges, no-till cotton without grass hedges, conventional-till cotton with grass hedges, and conventional-till cotton without grass hedges produced four-year average annual soil losses of 1.8, 2.9, 4.0, and 30.8 t/ha, respectively, and produced four-year average runoff amounts of 226, 364, 338, and 738 mm, respectively. The annual ratio of soil loss for no-till ultra narrow-row cotton plots with grass hedges to those without hedges averaged 0.62. The annual ratio of soil loss for conventional-till plots with grass hedges to without hedges was 0.13. Averaged over all plots (with and without grass hedges), no-till plots reduced soil loss from conventional-till plots by 86%. No-till plots without grass hedges had 90% less soil loss than conventional-till plots without grass hedges. Grass hedges effectively reduced soil loss on erosion plots with similar cropping practices as compared to plots without hedges. Other studies of contoured grass hedges on field-sized areas are being conducted to determine their applicability on larger areas with greater concentrations of runoff.