Natural Gradient, Surface-Applied Tracer Test at the Ohio Management Systems Evaluation Area

A tracer test was conducted from spring 1991 to winter 1995 to calculate travel times of agrichemicals surface applied in 1991 through a 1.0- to 5.4-m thick vadose zone to an unconfined aquifer. Bromide was applied on a 2.6 hectare agricultural plot coincident with the application of nitrogen, atrazine, and alachlor in the corn phase of a corn-soybean rotation in May 1991. Soil-water and ground-water samples were collected from lysimeters, soil-core extractions, and wells. Transport velocities in the soil based on the conservative behavior of bromide as observed in breakthrough curves from lysimeters were between 60 and 80 cm/year. Because the test occurred under natural recharge conditions, transport velocities varied over the five-year period depending on natural variations in precipitation and the associated transient infiltration rates. The center of mass of the bromide pulse intercepted the water table four years after application. Bromide remained in soil above the water table six years after application. There were no detections of alachlor, atrazine, or the two degradation products of atrazine, desethylatrazine and deisopropylatrazine at the water table during the six-year period of monitoring. This suggests that despite the ability of conservative species to travel to the water table at this site, the pesticides were attenuated in the vadose zone.