Trend Analyses of Atrazine Transport in Goodwater Creek Experimental Watershed.

Climatic variability complicates analyses of environmental effects of agricultural practices, often with variability obscuring the trend signal. Thus, discerning trends is often possible only with long-term databases of hydrology and water quality from experimental watersheds. The objectives of this study were to leverage the long-term data from Goodwater Creek Experimental Watershed (GCEW) to determine trends in atrazine [6-chloro-N²-ethyl-N⁴-(1-methylethyl)-1,3,5-triazine-2,4-diamine] concentration and load from 1992 to 2006. Located within the Central Claypan Region of northeastern Missouri, GCEW encompasses 72 km² of predominantly agricultural land uses, with an average of 21% of the watershed in corn (Zea mays) or sorghum (Sorghum bicolor). Flow-weighted runoff and weekly baseflow grab samples were collected from 1992 to 2006 near the outlet to GCEW and analyzed for atrazine. Using cumulative frequency diagrams and correlation analyses, the results showed no significant time trends for atrazine concentration. Observed trends in daily load were mainly a function of trends in stream discharge. Relative annual loads varied from 0.56 to 14% of the applied atrazine, with a median of 5.9%. Annual variation in loads was a function of the timing of runoff events relative to atrazine application within the watershed, and the magnitude of runoff events was a much less important factor to atrazine transport.