MODELING PESTICIDE TRANSPORT IN SUBSURFACE DRAINED SOILS

Transport of chemicals through the soil profile and crop root zone, an d the discharge from subsurface drainage lines into surface water can be a significant source of water pollution. This study measured and si mulated subsurface drain outflows and atrazine loads in the subsurface drains from a field site in Ottawa, eastern Ontario, Canada, during 1 988 and 1989. Corn was grown for silage at the site, and an H-flume wi th an automated water sampler was used to monitor temporal changes in quantity and quality of subsurface drain outflows from a 14-ha field s ite. A drainage simulation model, DRAINMOD, was combined with a chemic al transport model, GLEAMS, to simulate the chemical transport of atra zine through the soil into the subsurface drain outflow. The model was calibrated with 1989 field data and compared to measured 1988 data. T he calibrated DRAINMOD model predicted subsurface drain outflows for 1 988 with a coefficient of determination of 0.40 and a standard error o f the estimate, S(y/x), of 0.09 mm. Measured Atrazine concentrations e xceeded 6 mug/kg on one occasion, but simulated Atrazine concentration s did not exceed 2.5 mug/kg. The model underpredicted atrazine mass in the subsurface drain outflows. Spring underpredictions of atrazine ma ss discharge was due to underprediction of subsurface drain flows whil e fall underpredictions were due to underpredictions of atrazine conce ntrations. Effects of temperature on atrazine half life and adsorption constant may be partially responsible for these results. This integra ted model of chemical transport with the drainage simulation model pro vides a useful tool for studying chemical transport through the soil a nd crop root zone into surface waters.