Numerical simulation of hexazinone transport and fate in the vadose zone

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University of New Brunswick
Hexazinone, known commercially as Velpar®, is a common pesticide used in the agriculture industry, and is a potential concern to groundwater quality. In Pennfield, New Brunswick, the chemical has been successfully used to increase production since 1982 on 400 hectares of blueberry fields. This particular site is underlain by a glaciofluvial deposit approximately 30 m thick, composed of highly permeable sand and gravel, with minor clay and silt, and little top soil. To evaluate the potential for certain pesticides to leach to the ground water, the National Water Research Institute has developed a vadose zone simulation model called EXPRES (Expert System for Pesticide Regulatory Evaluations and Simulations). The transport mechanisms of the unsaturated zone are more complex than those of the saturated zone. This complexity leads to numerous assumptions and simplification of equations in these vadose zone models. The major processes responsible for the movement of hexazinone include sorption, biodegradation, dispersion and advection. Vapour diffusion, and erosion from runoff are of little importance in this case because of the nature of the site and chemical. EXPRES employs three models, including LEACHM one of the more physically-based vadose zone models. The LEACHM model was used to simulate conditions at two sites in the Pennfield area. At each location, concentrations from water samples taken from existing shallow wells were assumed to be approximately the concentration at the water table. Over the six month period being observed, the simulation concentrations decreased steadily with time, whereas the measured concentrations remained relatively constant for the same two sites. The difference in trend is considered to be a result of lateral advection of dissolved pesticide in ground water from adjacent soil, which the model does not take into account, and also the difference in the amount of oxygen between the two sources. The most influential input parameters affecting the transport of dissolved hexazinone are the degradation half life, t1/2, the organic carbon partition coefficient, Koc, the saturated hydraulic conductivity, Ks, and the dispersivity, a. The importance of these parameters were evaluated through sensitivity analysis, where varied values were compared to a base case. The base scenario is characterised by: Koc, = 40 L/kg, t1/2 = 30 days for the top soil, 120 days at depth, Ks = 30 m/day, and a = 500 mm. Within the time after spraying, before a period of 8 months,Koc is most influential for values lower than the base case, and dispersivity is the most influential parameter when values are above the base case. After approximately 8 months, Koc and dispersivity are less important, and degradation is most effective in changing the concentration. Ks is the least important variable at any time, and decreases in importance with time.