EFFECTS OF SAMPLING STRATEGIES ON ESTIMATES OF ANNUAL MEAN HERBICIDE CONCENTRATIONS IN MIDWESTERN RIVERS

The effects of 10 sampling strategies on estimates of annual mean conc entrations of the herbicides atrazine, alachlor, and cyanazine in sele cted midwestern rivers were tested. The accuracy of the strategies was computed by comparing time-weighted annual mean herbicide concentrati ons calculated from water samples collected from 17 locations on midwe stern rivers, with simulated annual mean concentrations calculated for each sampling strategy, using Monte Carlo simulations. Monthly sampli ng was the most accurate strategy tested. The U.S. Environmental Prote ction Agency requires quarterly sampling for municipalities using surf ace water as a source of drinking water. Due to the seasonality of her bicide occurrence and transport, quarterly sampling underestimates ann ual mean herbicide concentrations in over 40% of the simulations. Thre e of the strategies tested showed that, relative to quarterly sampling , a more accurate representation of annual mean concentrations could b e obtained by sampling more frequently during spring and early summer runoff and assuming zero herbicide concentration during late summer an d winter months.