303(d) List: Under Section 303(d) of the Clean Water Act, all states must maintain a list of water bodies that fail to meet water quality standards and update the list every two years. The “303(d) List” identifies a state’s impaired waters so the sources of impairment can be described and corrective actions can be taken. Once a river is on the 303(d) List, it can only be delisted if the river was listed accidentally in the first place or it achieves applicable water quality standards. The Harpeth River has been listed as impaired by phosphorus pollution since 2004 (and for nutrients since 1996), leaving little doubt that the listing was not in error and the river is not on track to achieve water quality standards based on current practices.
TMDL: A TMDL is the maximum amount of a pollutant that a waterbody can receive and still meet state water quality standards, including allocations of specific limits to each of the pollutant’s sources – “waste load allocations” for point sources (e.g. sewer treatment plant discharges) and “load allocations” for nonpoint sources (e.g. agricultural runoff). The calculated TMDL must also factor in a margin of safety and consideration of seasonal variation and critical conditions. The term “TMDL” is used to refer both to the limits set for a pollutant in a river body and to the study conducted to determine those limits.
WLA: Waste load allocations (WLAs) are limits to point sources of pollution, such as wastewater treatment plant discharges. Implementation is relatively straightforward. Point sources that discharge to waters of the United States are regulated by National Pollutant Discharge Elimination System (NPDES) permits, so implementing a WLA tends to be done through NPDES permits. The City of Franklin, for example, has a sewage treatment plant that discharges large amounts of phosphorus into the Harpeth River but (usually) operates within the limits of its NPDES permit. The Harpeth TMDL may result in a WLA that forces Franklin’s sewage treatment plant to obtain an updated NPDES permit with a more stringent phosphorus limit.
LA: Load allocations (LAs) are limits to nonpoint sources of pollution, such as agricultural runoff and unregulated stormwater. Unlike point sources, there is no direct regulatory control over nonpoint sources. Implementing LAs relies on voluntary landowner participation in best management practices (BMPs) to limit nonpoint pollutant discharges into the impaired water. Because LAs do not have the regulatory backing of NPDES permits, compliance cannot be presumed and the EPA instructs that a TMDL should provide “reasonable assurances that nonpoint source control measures will achieve the expected [pollutant] load reductions.” CLICK HERE to learn more about reasonable assurances in the context of the Chesapeake Bay TMDL.