INFILTRATION OF ATRAZINE AND METABOLITES FROM A STREAM TO AN ALLUVIALAQUIFER

The infiltration of atrazine, deethylatrazine, and deisopropylatrazine from Walnut Creek, a tributary stream, to the alluvial valley aquifer along the South Skunk River in central Iowa occurred where the stream transects the river's flood plain. A preliminary estimate indicated t hat the infiltration was significant and warrants further investigatio n. Infiltration was estimated by measuring the loss of stream discharg e in Walnut Creek and the concentrations of atrazine and its metabolit es deethylatrazine and deisopropylatrazine, in ground water 1 m beneat h the streambed. Infiltration was estimated before application of agri chemicals to the fields during a dry period on April 7, 1994, and afte r application of agrichemicals during a period of small runoff on June 8, 1994. On April 7, the flux of atrazine, deethylatrazine, and deiso propylatrazine from Walnut Creek into the alluvial valley aquifer rang ed from less than 10 to 60 (mu g/d)m(2), whereas on June 8 an increase d flux ranged from 270 to 3060 (mu g/d)/m(2). By way of comparison, th e calculated fluxes of atrazine beneath Walnut Creek, for these two da tes, were two to five orders of magnitude greater than an estimated fl ux of atrazine to ground water caused by leaching from a field on a pe r-unit-area basis. Furthermore, the unit-area flux rates of water from Walnut Creek to the alluvial valley aquifer were about three orders o f magnitude greater than estimated recharge to the alluvial aquifer fr om precipitation. The large flux of chemicals from Walnut Creek to the alluvial valley aquifer was due in part to the conductive streambed a nd rather fast ground water velocities; average vertical hydraulic con ductivity through the streambed was calculated as 35 and 90 m/d for th e two sampling dates, and estimated ground water velocities ranged fro m 1 to 5 m/d.