Use of rhodamine WT to facilitate dilution and analysis of atrazine samples in short-term transport studies

Because pesticide transport studies often require many samples, and certain pesticide analytical methods have small detection ranges, repeat analyses of samples that are outside of this range can be costly. Dye-tracers often have a lower analysis cost than pesticides and can be visibly diluted to within detection range. We wanted to see if we could predict the dilution factor necessary for atrazine analysis by using a visible dye-tracer, rhodamine WT. Rhodamine WT and Aatrex-41 atrazine (2-chloro-4-ethylamino-6-isopropylaminoazine) were surface applied to large, intact soil cores taken from three replications of U-Sr no-till, 2-yr no-till, and 12-yr moldboard plow treatments. Cores were then rained on for Ih with an oscillating-head rainfall simulator at intensity = 50 mm h(-1). Based on rhodamine WT concentrations, runoff, leachate, and soil extract samples were diluted to near mid-atrazine-detection range, on a logarithmic scale (0.9 mu g L-l), for atrazine analysis by enzyme-linked immunosorbant assay (ELISA). Only 6% of runoff and leachate samples (n(total)=262) needed additional dilution and only 0.5% of soil extract samples (n(total)=174) needed additional dilution. No predictions resulted in overdilution of samples, Rhodamine WT concentration was shown to be an effective predictor of dilution factor necessary for atrazine analysis in short-term transport through this soil.