VADOSE ZONE TRANSPORT OF PESTICIDES IN THE HIGH PLAINS: EVALUATING GROUNDWATER QUALITY RISKS USING NUMERICAL SIMULATIONS

Evaluation of vadose zone fate and transport of pesticides using numerical models can be an effective tool in managing pesticide environmental risks in regions experiencing water quality impairment such as the High Plains aquifer. The Environmental Protection Agency (EPA), under the 1947 Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) and amendments, maintains responsibility for balancing “adverse environmental effects” against the economic benefits of pesticide use in approving chemical registration and applicator certification. Until recently, evaluations of aquifer vulnerability have historically been more empirical rather than quantitative, limiting assessment of various agricultural scenarios. Using measured data from vadose zone core sections taken from agricultural land in Southwestern Kansas; regional climate data taken from weather stations; and best available literature values (BALVs) for pesticide properties; simulations were performed using HYDRUS-1D to evaluate atrazine and metalochlor transport under varying pesticide application rates and irrigation strategies through a 160 ft. thick unsaturated zone. Modeling results indicate transport to the water table was sensitive to the changes in irrigation practices and differences in the pesticide properties. Comparison of travel times and concentration levels reaching groundwater illustrates what is commonly known based on water quality results throughout the region: Atrazine poses greater water quality threat under all irrigation scenarios in the High Plains as a result of the pesticide’s organic carbon distribution (Koc) and half-life (T1/2). However, the process demonstrates that unsaturated zone modeling can be a highly informative tool in regulating pesticide use and drinking water quality protection based on potential transport risks that could pose “adverse environmental effects.”