Susceptibility of Indiana watersheds to herbicide contamination

An index of watershed susceptibility to surface water contamination by herb icides could be used to improve source water assessments for public drinkin g water supplies, prioritize watershed restoration projects, and direct fun ding and educational efforts to areas where the greatest environmental bene fit can be realized. The goal of this study is to use streamflow and herbic ide concentration data to develop and evaluate a method for estimating comp arative watershed susceptibility to herbicide loss. United States Geologica l Survey (USGS) concentration data for five relatively water soluble herbic ides (alachlor, atrazine, cyanazine, metolachlor, and simazine) were analyz ed for 16 Indiana watersheds. Correlation was assessed between observed her bicide losses and: (1) a herbicide runoff index using GIS-based land use, s oil type, SCS runoff curve number, tillage practice, herbicide use estimate s, and combinations of these factors; and (2) predicted herbicide losses fr om a non-point source pollution model (NAPRA-Web, an Internet-based interfa ce for GLEAMS). The highest adjusted R-2 value was found between herbicide concentration and the runoff curve number alone, ranging from 0.25 to 0.56. Predictions from the simulation model showed a poorer correlation with obs erved herbicide loss. This indicates potential for using the runoff curve n umber as a simple herbicide contamination susceptibility index.