ATRAZINE AND NITRATE TRANSPORT TO THE BRAZOS RIVER FLOODPLAIN AQUIFER

Abstract

The potential for contamination of groundwater and surface water from agricultural chemicals used on riverfloodplains is a serious concern in many parts of the United States. An agricultural research site located near CollegeStation, Texas, was instrumented to determine the fate of agricultural chemicals typically applied to the Brazos Riverfloodplain. Nine well nests were installed in a 33 grid pattern, parallel and perpendicular to the river. Each well nesthas four monitoring wells screened at various depths throughout the aquifer. Ammonium-nitrate fertilizer and theherbicide atrazine were applied to this research site at the time a corn crop was planted in 1994 and 1995. Groundwaterand river samples were periodically collected and tested for nitrate-N, ammonium-N, and atrazine. Increases in nitrate-Nin the groundwater were not observed due to high background concentrations of nitrate-N. Ammonium-N was notdetected in the groundwater above background concentrations (<1 mg/L) due to nitrification of ammonium-N to nitrate-Nin the clay soil. Atrazine was detected in the groundwater 24 days after the second application indicating preferential flowthrough the Ships clay surface layer that was 6 m thick. A pump test that was conducted at the research site just after thesecond atrazine application facilitated the movement of atrazine to a depth of 18 m.