Distribution, Sources and Fate of Atrazine in a Sandy-Till Aquifer

A field study was initiated around an atrazine-contaminated private dairy-farm well to investigate sources and extent of contamination from atrazine and its metabolites in the shallow glacial till aquifer and to relate the distribution of the contaminants to the ground-water flow system. Area landuse is dairy farming with typical rotations of corn and alfalfa; soils are predominantly silt loams. The shallow groundwater system is dominated by downward vertical hydraulic gradients and water from the till aquifer discharges to the bedrock aquifer. Where downward gradients exist it is possible to predict travel time (TT) and mean horizontal distance travelled (MHD) to vairous maonitoring wells. The MHDs to many wells are small, and atrazine residue concentrations result from pesticide application and handling nearby. According to the Darcy TT estimates, once water reaches the water table, it takes decades to reach some of the deeper wells. Tritium-interpreted ages suggest that at least part of the water at contaminated wells is > 7 yr old even in wells screened near the water table. Such long travel times indicate that responses of groundwater quality to atrazine-use restrictions will be slow.