Agricultural Pesticides in the Patuxent River, a Tributary of the Chesapeake Bay

Abstract

AbstractThe Chesapeake Bay estuarine drainage area receives the highest pesticide application of any coastal area in the USA, however, large gaps exist in information on pesticide residues entering Chesapeake Bay tributaries. Twelve pesticides, one pesticide degradation product, and one formulation by‐product were measured in the surface waters of the Patuxent River during spring/summer 1995. Pesticide concentrations were compared with river flow and estimated use patterns in the watershed. Atrazine, 6‐amino‐2‐chloro‐4‐isopropylamino‐s‐triazine (CIAT), simazine, metolachlor, and chlorpyrifos were consistently detected with maximum concentrations of 3000, 800, 2700, 70, and 190 ng/L, respectively. Concentrations were highest in the upper watershed, an area where 70% of pesticide usage occurs, and decreased with distance down the river. While tidal dilution is an important removal mechanism, degradation appears to be the main factor leading to decreases in atrazine concentrations. Atrazine levels observed at the mouth of the river correlated more strongly with high river flow events compared with the other analytes. This may be due to a local source of atrazine in the lower Patuxent River. Despite similar use rates in the watershed, metolachlor concentrations were 25 to 40 times lower than atrazine. Overall, triazine herbicides appeared to move quickly into the river and persisted longer in surface waters compared with the acetanilide herbicides. Since land use and agricultural crop patterns in the Patuxent watershed are similar to other western shore Chesapeake Bay tributaries, results of this study may be used as a starting point to predict pesticide fate and transport in this region.