ecological modelling 200 ( 2 0 0 7 ) 149–159 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/ecolmodel Modeling pesticide fate in a small tidal estuary Annette M. McCarthy a , Jerad D. Bales b , W. Gregory Cope a,* , Damian Shea a a North Carolina State University, Department of Environmental and Molecular Toxicology, Box 7633, Raleigh, NC 27695-7633, USA b U.S. Geological Survey, North Carolina Water Science Center, 3916 Sunset Ridge Road, Raleigh, NC 27607, USA article info Article history: Received 15 August 2005 Received in revised form 9 June 2006 Accepted 4 July 2006 Published on line 28 August 2006 Keywords: Pesticide Atrazine Metolachlor EXAMS Modeling Water quality Estuaries abstract The exposure analysis modeling system (EXAMS), a pesticide fate model developed by the U.S. Environmental Protection Agency, was modified to model the fate of the herbicides atrazine and metolachlor in a small tidally dominated estuary (Bath Creek) in North Car- olina, USA where freshwater inflow accounts for only 3% of the total flow. The modifications simulated the changes that occur during the tidal cycle in the estuary, scenarios that are not possible with the original EXAMS model. Two models were created within EXAMS, a steady- state model and a time-variant tidally driven model. The steady-state model accounted for tidal flushing by simply altering freshwater input to yield an estuary residence time equal to that measured in Bath Creek. The tidal EXAMS model explicitly incorporated tidal flushing by modifying the EXAMS code to allow for temporal changes in estuary physi- cal attributes (e.g., volume). The models were validated with empirical measurements of atrazine and metolachlor concentrations in the estuary shortly after herbicide application in nearby fields and immediately following a rain event. Both models provided excellent agree- ment with measured concentrations. The steady-state EXAMS model accurately predicted atrazine concentrations in the middle of the estuary over the first 3 days and under-predicted metolachlor by a factor of 2–3. The time-variant, tidally driven EXAMS model accurately predicted the rise and plateau of both herbicides over the 6-day measurement period. We have demonstrated the ability of these modified EXAMS models to be useful in predicting pesticide fate and exposure in small tidal estuaries. This is a significant improvement and expansion of the application of EXAMS, and given the wide use of EXAMS for surface water quality modeling by both researchers and regulators and the ability of EXAMS to interface with terrestrial models (e.g., pesticide root zone model) and bioaccumulation models, we now have an easily-accessible and widely accepted means of modeling chemical fate in estuaries. © 2006 Elsevier B.V. All rights reserved. 1. Introduction Estuaries are important and productive ecosystems that support a variety of living resources. These resources are extremely vulnerable, especially at key life stages, to the adverse effects of the many pollutants delivered to the estu- ary by tributary rivers and drainage from lands adjacent to ∗ Corresponding author. Tel.: +1 919 515 5296; fax: +1 919 515 7169. E-mail address: greg cope@ncsu.edu (W.G. Cope). the estuary. Many estuaries, particularly along the southeast Atlantic and Gulf coasts of the United States, are surrounded by agricultural lands upon which a variety of pesticides may be applied. Moreover, rapid urbanization continues to occur in the coastal zone, and pesticide occurrence in streams draining urban areas typically exceeds that of streams draining agricul- tural lands (U.S. Geological Survey, 1999). Atmospheric depo- 0304-3800/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ecolmodel.2006.07.013