Agricultural pesticides in the Patuxent River, a tributary of the Chesapeake Bay

The Chesapeake Bay estuarine drainage area receives the highest pesticide a pplication of any coastal area in the USA, however, large gaps exist in inf ormation on pesticide residues entering Chesapeake Bay tributaries. Twelve pesticides, one pesticide degradation product, and one formulation by-produ ct were measured in the surface waters of the Patuxent River during spring/ summer 1995, pesticide concentrations were compared with river now and esti mated use patterns in the watershed. Atrazine, 6-amino-2-chloro-4-isopropyl amino-s-triazine (CIAT), simazine, metolachlor, and chlorpyrifos were consi stently detected,with maximum concentrations of 3000, 800, 2700, 70, and 19 0 ng/L, respectively. Concentrations were highest in the upper watershed, a n area where 70% of pesticide usage occurs, and decreased with distance dow n the ri rer. While tidal dilution is an important removal mechanism, degra dation appears to be the main factor leading to decreases in atrazine conce ntrations. Atrazine levels observed st the mouth of the river correlated mo re strongly with high river flow events compared with the other analytes. T his may be due to a local source of atrazine in the lower Patuxent River. D espite similar use rates in the watershed, metolachlor concentrations were 25 to 40 times lower than atrazine. Overall, triazine herbicides appeared t o move quickly into the river and persisted longer in surface waters compar ed with the acetanilide herbicides. Since land use and agricultural crop pa tterns in the Patuxent watershed are similar to other western shore Chesape ake Bay tributaries, results of this study may be used as a starting point to predict pesticide fate and transport in this region.