Atrazine retention and degradation in the vadose zone at a till plain sitein central Indiana

The vadose zone was examined as an environmental compartment where signific ant quantities of atrazine and its degradation compounds may be stored and transformed, The vadose zone was targeted because regional studies in the W hite River Basin indicated a large discrepancy between the mass of atrazine applied to fields and the amount of the pesticide and its degradation comp ounds that are measured in ground and surface water. A study site was estab lished in a rotationally cropped field in the till plain of central Indiana . Data were gathered during the 1994 growing season to characterize the sit e hydrogeology and the distribution of atrazine, desethylatrazine, deisopro pylatrazine, didealkylatrazine and hydroxyatrazine in runoff, pore water, a nd ground water. The data indicated that atrazine and its degradation compo unds were transported from land surface to a depth of 1.5 m within 60 days of application, but were undetected in the saturated zone at nearby monitor ing wells. A numerical model was developed, based on the field data, to pro vide information about processes that could retain and degrade atrazine in the vadose zone. Simulations indicated that evapotranspiration is responsib le for surface directed soil-moisture flow during much of the growing seaso n. This process causes retention and degradation of atrazine in the vadose zone. Increased residence time in the vadose zone leads to nearly complete transformation of atrazine and its degradation products to unquantified deg radation compounds. As a result of macropore flow, small quantities of atra zine and its degradation compounds may reach the saturated zone.