If you are involved in any aspect of stormwater management, no doubt you have heard the phrase "impaired waters."  The determination and subsequent listing of a water body as impaired sets off a chain reaction of events affecting everyone who lives, works or recreates near the water body. But what is an impaired water body, and how does a water body get designated as impaired?

It all starts with the Clean Water Act (SWA). States are required to collect water quality data and information on the health of watersheds across the state and relate their actual condition to a slate of designated uses assigned to them based on an assumed level of water quality that should have historically existed in that watershed. Designated uses refer to the goals or intended uses of the water body. Examples include drinking water supply, swimming/recreation, aquatic life support/fishing, and shellfish consumption. The Clean Water Act requires states to compare this data to the state's water quality standards every two years. The resulting impaired waters is a list of waters that violate the water quality standards; that is, they have levels of pollution that are unhealthy for the designated use. This list is called the "303(d) list," based on the section of the Clean Water Act that requires it.

States may use any number of ways to determine whether a water body meets the state's water quality standard. However, federal regulations say states must evaluate "all existing and readily available information" in developing their 303(d) lists, meaning states cannot select what data/information they use and purposely disregard other sources.

In general, once a water body has been added to a state's list of impaired waters, it stays there until the state develops a Total Maximum Daily Load (TMDL) intended to improve water quality, and the EPA subsequently approves it. A TMDL is the calculation of the maximum amount of a pollutant allowed to enter a waterbody so that the waterbody will meet water quality standards for that pollutant. Often referred to as a pollution diet, A TMDL determines a numeric pollutant reduction target (with a factor of safety) and allocates the available load across all the contributors of said pollutant within the watershed. This TMDL is generally at levels lower than the baseline, which makes reductions necessary to return the watershed to acceptable water quality levels and remove it from the impaired list. Under the CWA, the EPA reviews and either approves or disapproves the state's proposed TMDL. If EPA disapproves a state TMDL, the EPA must develop a replacement TMDL.

TMDL's often include pollutants commonly found in stormwater, including sediment, nutrients and heavy metals. Although states are not explicitly required under section 303(d) to develop TMDL implementation plans for every TMDL, many states include an implementation plan with the TMDL to plot a course to success. When developed, TMDL implementation plans often call for more robust stormwater treatment requirements than would otherwise be required by the state, including the use of manufactured treatment devices when they are capable of removing high levels of the targeted pollutants. TMDLs can often necessitate the deployment of stormwater controls in already built-out urban environments where very little space is available to provide stormwater management. The fact that manufactured treatment devices can often be installed under existing impervious areas makes them well suited for retrofit applications in highly constrained urban watersheds grappling with TMDL compliance.