Completing TMDLs in Coastal Watersheds: Important Considerations

As required by the Federal Clean Water Act, the Delaware Department of Natural Resources and Environmental Control (DNREC) is responsible for implementing water quality monitoring and assessment activities in the State and also for establishing TMDLs on impaired State surface waters as indicated on the State's 303(d) list. In addition, the State of Delaware is under a court-approved Consent Decree that requires completion of TMDLs for all impaired State waters by 2006. There are 8 river segments in the St. Jones River and 12 river segments in the Broadkill River that are listed on the 1998 303(d) list with the pollutants of concern indicated as nutrients, DO and bacteria. The suspected loading sources originate from nonpoint sources (NPS) except for a few segments in the Broadkill River where the suspected sources are a combination of point source (PS) and NPS loadings. Therefore, TMDLs are being completed that are developing watershed, hydrodynamic and water quality models for the two watersheds to support DNREC in developing nutrient, DO and bacteria TMDLs for these watersheds.The St. Jones River watershed is located in Kent County (area of 90 mi2) and the Broadkill River watershed is located in Sussex County (area of 107 mi2). Both of these watersheds are located along the coastal area of Delaware Bay with tidal lower reaches and upstream freshwater reaches consisting of free-flowing rivers and lakes. Both watersheds have extensive tidal saltwater marshes and sediment-water column interactions that play an important role in the water quality dynamics in the tidal reaches of these rivers. The land use from both watersheds primarily consists of non-urban/residential areas (agriculture, forest and wetland) with 69% of land use in the St. Jones River watershed non-urban/residential and 84% in the Broadkill River watershed.This paper will present the tools used to complete the modeling in each watershed, briefly describe the calibration and validation process and discuss the important issues that need to be considered when completing TMDL modeling in coastal watersheds. These issues will discuss including a sediment flux submodel in the overall model to properly account for the delivery of particulate organic matter (carbon, nitrogen and phosphorus) to the sediments in the tidal reaches of the rivers. This is important for realistically defining the relationship between sediment oxygen demand (SOD) and nutrient fluxes and their effect on dissolved oxygen (DO), nutrient and phytoplankton levels in the tidal reaches. Ultimately, this linkage is important for assessing changes in nutrient, DO and phytoplankton levels when management alternatives are investigated during the allocation and implementation phase of the TMDLs. Other important considerations in the modeling are the location and specification of the downstream tidal boundary condition and water quality interaction with the tidal marshes.