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WWRC 97-05t
Fate of Nitrate and Aldicarb Under Irrigation Management


Environmental contamination by agrochemicals (fertilizers and pesticides) due to overuse of irrigation water is a national concern, particularly where flood irrigation has been the conventional water application method. Therefore, there is a need to assess other management approaches that can potentially reduce leaching of agrochemicals; the use of drip irrigation may offer such an alternative. The objective of this study was to evaluate nitrate (fertilizer) and aldicarb (pesticide) movement through soils under flood and drip irrigation practices. A transport and irrigation model, CHAIN_IR, was utilized to assess the leaching potential of these two solutes. With flood irrigation, higher concentrations of both nitrate and aldicarb were found deeper in the soil profile as compared to drip treatment, thus, suggesting a greater leaching of chemicals under flooding. In addition to the downward flow, a lateral movement of both nitrate and aldicarb was observed and the results suggested that under drip, fertilizers and pesticides would be available for a greater extent around the point of application, whereas under flood irrigation, there would be run-off loss of the agrochemicals. Nitrate concentration at an internal point (x = 0, z = 35) inside the soil profile increased curvilinearly with time from 0 to 17 g/cm3 within 7.5 d, thus, indicating leaching of the added nitrate from the surface (x = 0, z = 0). Concentrations of aldicarb decreased exponentially following first order kinetics with a specific rate constant (K) of 0.25/d and half-life period (t0.5)2.8 d. Two daughter compounds, aldicarb sulfoxide and sulfone, were also predicted to be formed following a logarithmically increasing function and a zero order linear reaction (K of 0.008g/cm3/d), respectively. Correlations for all these equations were significant at the 1% probability level. The contour maps for soil water pressure head (h) distribution at 0.5 d post-irrigation period, indicated a greater percolation of the water-front under flood irrigation as compared to the drip irrigation, which would lead to less leaching of the contaminants with drip practices. The overall results of this study suggest that application of drip irrigation is a better alternative for developing best irrigation management practices that can diminish leaching of agrochemicals and control overuse of water.

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