When monitoring reveals that waterways do not meet Delaware’s water quality standards, they are reported on a list of impaired waterways (303(d) List). For each impaired waterway, the Federal Clean Water Act (CWA) requires states to develop Total Maximum Daily Loads (TMDLs) for the pollutants of concern. A TMDL sets a limit on the amount of pollution that can be discharged into a waterbody such that water quality standards can still be met. A non-scientific definition for TMDL could be “pollution limit.”

Pollutants in Delaware waters are often chemicals, such as nitrogen and phosphorus from fertilizer runoff and wastewater, but TMDLs could also be set for other pollutants such as bacteria, sediments, or even heat – anything that can injure a waterway’s natural health. Pollutants can come from specific point sources or from nonpoint sources. Point sources are discrete sources of pollution, and include facilities that have a National Pollution Discharge Elimination System (NPDES) Permit, such as municipal and industrial wastewater treatment plants.

Nonpoint source pollution travels over and through the ground from many diffuse sources, including human activities on agriculture and developed lands like septic systems and runoff from lawns, farms, parking lots, and golf courses.

TMDLs consist of three parts: a wasteload allocation (WLA) for point sources, a load allocation (LA) for nonpoint sources, and a margin of safety (MOS): TMDL = WLA + LA + MOS

TMDLs by Watershed

Delaware has been establishing TMDLs for our State’s impaired waters since 1998. Monitoring and other types of data are used to develop models that can predict how water quality will change under a variety of pollutant loading scenarios. These models help us determine TMDL levels that will achieve water quality standards.

The tables below list all of the TMDLs that have been established in Delaware and provides links to technical analysis documents and the resulting regulations.

Each tab includes the watersheds in a major drainage area. Each watershed listed is numbered and shown on the map on this page.

In addition, Maps 2, 3, and 4 at the bottom of the page are provided to show reductions in nonpoint source nitrogen, phosphorus, and bacteria loads as required by the TMDLs in order to achieve water quality goals.

• Piedmont
• Delaware Bay
• Chesapeake Bay
• Inland Bays

Setting pollution limits is just the first step toward improving water quality.  Once the pollutant limits are established, efforts must be taken to reduce the pollutant loads from point and nonpoint sources. The Watershed Assessment Section works with Tributary Action Teams to identify voluntary and regulatory actions for each impaired region of the state. Collectively, these actions are called pollution control strategies (PCS) and they are designed to achieve TMDLs and water quality standards. In addition, other types of watershed plans and strategies have also been developed across the state.

Related Topics:  assessment, clean water, TMDL, total maximum daily load, water quality, watershed, watershed stewardship