Pesticide retention by buffer strips receiving simulated runoff containing different sized sediment
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Infiltration water and sediment mass retained are the two key processes for pesticide mass retention by buffer strips from agricultural runoff, based on the review of 106 published articles. Estimates, based on average published data for runoff volume and sediment mass retention, show that the average pesticide retention is 46, 51, and 70 % for the three sorption classes (Koc<100, 1001000, respectively). Source area to buffer area ratios ranging between 10:1 to 50:1 are more practical and effective under field applications of buffer strips. Buffer strips have an upper area where larger particles settle and a lower area where runoff containing fine particles passes through. Rainfall-runoff experiments were conducted on 1.0 m wide x 5.6 m long switchgrass buffer strips to measure pesticide mass transport through buffer strips receiving runoff containing different sized sediment under steady-state rainfall intensity of 6.35 cm/h. Twenty four strips were used to provide three replications each of the sediment type treatments of fine sand, fine aggregates, clay-sized particles, and no sediment; and two treatments of flow convergence represented by source area to buffer area ratios of 10:1 and 30:1. Atrazine, chlorpyrifos, and linuron were used in the experiments at the label recommended rates using field formulations. When receiving runoff mixed with fine sand, buffer strips retained 73% and 53% atrazine, 87% and 80% chlorpyrifos, and 81% and 54% linuron for the two area ratios of 10:1 and 30:1 respectively. The corresponding numbers, when receiving runoff mixed with fine aggregates, were 72% and 54% atrazine, 87% and 71% chlorpyrifos, and 76% and 58% linuron respectively for the two area ratios. Switchgrass buffer strips retained, on average, 70.1% and 49.2% atrazine, 83.0% and 57.6% chlorpyrifos, and 71.2% and 50.4% linuron, respectively for the two area ratios of 10:1 and 30:1 when receiving simulated runoff containing clay-sized particles. Linuron data presented in these experiments is an estimate and readers are cautioned when interpreting linuron data. Results were significantly different for atrazine when the two area ratios were compared for all three sediment types. Results for chlorpyrifos and linuron were not significantly different between the two area ratios indicating the strips performed equally well under both flow conditions in case of sediment type fine sand and fine aggregates. In case of clay-sized particles, results for atrazine and linuron were significantly different for the two area ratios indicating flow convergence can impact atrazine and linuron retention by buffer strips. In case of fine sand, outflow from the buffer strips showed some re-entrainment of sediment from previously deposited sediment, buffer strip soil, or erosion at the exit point, which needs to be further investigated. Infiltration and sediment retention were the key processes for pesticide retention in case of fine aggregates, whereas infiltration alone was the key process in case of fine sand and clay-sized particles. VFSMOD-W, embedded with the empirical linear-additive pesticide mass retention model was used to predict atrazine, chlorpyrifos and linuron retention by the switchgrass buffer strips studied in the experiments. Saturated hydraulic conductivity (Ksat) of the switchgrass soil was the key parameter in calibrating the model to the experimental conditions, indicating type of buffer strip vegetation and timing of calibration data collection are important factors. Predicted pesticide mass retention results indicate that the performance of buffer strips receiving runoff from farm fields containing large proportions of fine sand or clay-sized particles needs to be further investigated.