The primary pollutants affecting Bay water quality are nutrients-nitrogen and phosphorus-and sediment. Nutrients spur algae blooms, which block sunlight needed by underwater grasses, a key habitat for waterfowl, fish and crabs. When the algae die, they sink to the bottom and decompose in a process that draws oxygen out of the water, often making huge areas off-limits to most aquatic life. Sediment clouds the water and can smother important bottom habitats, such as oyster beds.

Excess amounts of nutrients and sediments keep the Bay from meeting water quality standards. Those standards, for dissolved oxygen, water clarity and chlorophyll a (a measure of algae) are designed to protect habitats used by fish, crabs and bottom-dwelling worms and clams that form the base of the Chesapeake food web.

Nutrient and sediment reductions have been a cornerstone of Bay restoration efforts since 1987. In 2003, water quality standards were refined and new cleanup goals established to meet them. According to those goals:

  • Nitrogen entering the Bay needed to be limited to 175 million pounds a year;
  • Phosphorus needed to be limited to 12.8 million pounds annually: and
  • Sediment needed to be limited to 4.15 million tons annually.

The states developed plans, known as tributary strategies, to achieve those goals.

According to estimates, states had taken enough actions through 2007 to reduce nitrogen to 262 million pounds (from an estimated 337 in 1985), phosphorus to 18.2 million pounds (from an estimated 27.1 million in 1985), and sediment to 4.76 million tons (from an estimated 5.83 million tons in 1985).

Those figures are under review, using more sophisticated computer models. Initial indications are that more nutrients (but possibly less sediment) are reaching the Bay than previously believed.

Because efforts to meet water quality standards have failed, the Clean Water Act requires that a new cleanup plan, called a Total Maximum Daily Load, be established. Like past goals, a TMDL sets the maximum amount of pollution that the Chesapeake can receive and meet its water quality standards.

The TMDL is being developed by the EPA with input from all jurisdictions in the watershed-Maryland, Virginia, Pennsylvania, Delaware, West Virginia, New York and the District of Columbia. The TMDL, which will be based on new modeling, will require the development of new tributary strategies. Unlike past tributary strategies, the plans developed for the TMDL must provide "reasonable assurance" that the goals will be achieved.

State and EPA officials have said they want the TMDL to be a model for the nation, with more mechanisms to ensure that progress toward the goal is maintained. Although the TMDL requires more planning and documentation than past cleanup plans, it does not provide any new authority to reduce pollution-for instance, the EPA does not have authority to regulate runoff from farms, and a TMDL does not give it that power.

But if the Bay does not reach its water quality standards because of unregulated sources, the EPA could demand additional reductions from sources it does have authority to regulate, such as wastewater treatment plants. Because achieving further reductions from those dischargers would be hugely expensive, it would put pressure on states to establish more effective programs to control other sources of pollution.