In 1998, significant portions of the Chesapeake Bay and its tidal tributaries within Virginia were listed as impaired, meaning they were identified as not meeting water quality standards. 

Areas of the Bay and tidal rivers within Maryland, Delaware and the District of Columbia are also on the federal list of impaired waters. The main pollutants contributing to this designation are nitrogen, phosphorus and sediment. 

Despite significant efforts from the Chesapeake Bay Program, the water quality goals under the Clean Water Act have yet to be met. Because these waters remain on the impaired waters list, EPA required the development of a Total Maximum Daily Load (TMDL), which is essentially a "pollution diet," in order to protect the Bay.

Based in part on the Watershed Implementation Plans (WIP) developed by the Bay watershed states and the District of Columbia, the TMDL addresses all segments of the Bay and its tidal tributaries that are on the impaired waters list. 

As with all TMDLs, a maximum aggregate watershed pollutant loading necessary to achieve the necessary water quality standards has been identified. This is divided among all of the Bay states and their major tributary basins, as well as by major source categories of wastewater, urban storm water, septic, agriculture and air deposition.

The TMDL was issued by EPA on Dec. 29, 2010, and is available on EPA's Chesapeake Bay TMDL website.