When monitoring reveals that waterways do not meet Delaware’s water quality standards for their intended uses, as defined in Delaware’s Surface Water Quality Standards, they are added to a list of impaired waterways (Delaware’s 303(d) List). For each impaired waterway, the Federal Clean Water Act (CWA) requires states to develop Total Maximum Daily Loads (TMDLs) or other restoration approaches for the pollutants of concern.

The goals of these approaches are to achieve water quality that allows for unrestricted intended usage, including for drinking, agricultural or industrial supply water, recreational purposes and/or for the health of fish and aquatic life and for people who consume them.

A TMDL sets a limit on the amount of a pollutant that can be discharged into a waterbody while still maintaining water quality standards. When numerous sources of a pollutant exist within a watershed, a TMDL is a useful tool for establishing limits and monitoring requirements for discharges.

When sources of the pollutants of concern (POC) are more diffuse and historic in nature, TMDLs are not effective, by themselves, to meet water quality standards. In those cases, other restoration approaches are now recognized by the Environmental Protection Agency as a way to achieve water quality goals more effectively and efficiently.

These other restoration approaches are referred to as Advance Restoration Plans (ARPs) and are intended to be used in advance of developing a TMDL. 

Pollutants and Pathways

Pollutants in Delaware waters include nutrients such as nitrogen and phosphorus, toxic chemicals such as metals, pesticides, and polychlorinated biphenyls (PCBs), bacteria and even excess sediment.

Sources of the pollutants can be wastewater discharges and surface runoff from industrial and agricultural practices in both urban and rural environments. TMDLs and ARPs are used to aid in the reduction of impairment-driving pollutants like these.

Pollutants can originate from both point sources and 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 or urban areas with Municipal Separated Storm Sewer System (MS4) permits.

Nonpoint source pollution originates from many diffuse sources that are harder to pinpoint, including agricultural and urban landscapes, septic systems and runoff from lawns, farms, roads, parking lots, and golf courses.

Setting pollution limits and identifying pollutant sources 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 DNREC Watershed Assessment and Management Section works with partners across the state to identify voluntary and regulatory actions that can be taken in impaired watersheds to reduce pollutant loading. 

TMDLs and ARPs by Watershed

Delaware has been establishing TMDLs for the state’s highest priority impaired waterways since 1998 using data-driven models. Continuous and discrete water quality monitoring data, in addition to permit-required monitoring data, helps DNREC determine whether improvements are occurring.

In 2024, Delaware developed its first watershed-based Advance Restoration Plan (ARP) to identify and address historical sources of toxic chemical pollutant impairments related to fish consumption. This Advance Restoration Plan for The C&D Canal Watershed will serve as a framework for ARPs for pollutants of concern (POCs) in watersheds throughout the state in the future.

The tables below list all of the TMDLs that have been established in Delaware and provides links to technical analysis documents and any 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, the maps at the bottom of the page show the percent reductions in nonpoint source nitrogen, phosphorus and bacteria loads that are required by the TMDLs in order to achieve water quality goals.

• Piedmont
• Delaware Bay
• Chesapeake Bay
• Inland Bays

Related Topics:  advance restoration plans, arp, assessment, clean water, restoration plans, TMDL, total maximum daily load, water quality, watershed, watershed stewardship