Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 1, 2003
Publication Date: March 1, 2004
Citation: McConnell, L.L., Harman-Fetcho, J.A., Hagy, J.D. 2004. Measured concentrations of herbicides and model predictions of atrazine fate in the Patuxent River Estuary. Journal of Environmental Quality. 33:694-704.
Interpretive Summary: Estuaries are the interface between rivers and ocean waters. They represent some of the most diverse and productive ecosystems in the world. The fate of pesticides in the estuarine systems is poorly understood. The constant movement and flushing of the estuary by saline water makes predictions of chemical exposure and persistence very difficult. The Patuxent River is a tributary of the Chesapeake Bay, and the lower part of the river is an estuary. The Patuxent River watershed contains approximately 20% agricultural land usage with the largest crops being corn and soybeans. In this study, herbicides used on these crops were measured in surface waters at four locations spanning the estuary on forty days from April to July 1996. Temporal trends in herbicide concentrations were found to reflect the timing of agricultural practices in the watershed and concentrations were highest at the up-river sites decreasing towards the mouth of the estuary. The measured concentration values were used in a "box model" where the estuary is segmented into boxes of known volumes. The model results suggest that 93% of a major herbicide, atrazine, enters the estuary upstream. Results also indicated that significant degradation of this chemical (48%) occurred in the turbid, shallow upper estuary region and that 31% was released to the Chesapeake Bay. These results provide further evidence for protecting the shallow wetland areas in agricultural regions and for using constructed wetland areas as "reactors" for mitigating agricultural runoff waters before they are released to streams and rivers.
The environmental fate of herbicides in estuaries is poorly understood. Estuarine physical transport processes and the episodic nature of herbicide release into surface waters complicate interpretation of water concentration measurements and allocation of sources. Water concentrations of herbicides and two triazine degradation products, 6-amino-2-chloro-4-iospropylamino-s-triazine (CIAT) and 6-amino-2-chloro-4-ethylamino-s-triazine (CEAT), were measured in surface water from four land-based sites on forty days from 4 April to 29 July 1996 in the Patuxent River estuary, part of the Chesapeake Bay system. Atrazine was most persistent and present in the highest concentrations (max = 1290 ng/L). Metolachlor, CIAT, CEAT, and simazine were frequently detected with maximum concentration values of 610, 1100, 760, and 490 ng/L, respectively. A physical transport model utilizing measured freshwater inputs and salinity was used to interpret atrazine concentrations in the context of estuarine water transport. The estimated half-life of atrazine in the turbid, shallow upper estuary was 20 d, but was much longer (100 d) in the open, flowing lower estuary. Although most (93%) of the atrazine entered the estuary upstream via the river, simulations suggested additional inputs directly to the lower estuary. The total atrazine load to the estuary was 71 kg with 48% loss by degradation and 31% exported to the Chesapeake Bay. Atrazine persistence in the estuary is directly related to river flows into the estuary. Low flows will increase atrazine residence time in the upper estuary and increase degradation losses.