An Introduction to the Total Maximum Daily Load (TMDL) Process

The Total Maximum Daily Load (TMDL) represents the confluence where water quality science, regulatory implementation and oversight, and public interest converge in order to restore and maintain the chemical, physical, and biological integrity of surface waters to support “the protection and propagation of fish, shellfish, and wildlife and recreation in and on the water”, as stated in the Clean Water Act of 1972 (as Amended 2002). The Clean Water Act is the primary federal law that governs and authorizes water quality control activities by the Environmental Protection Agency (EPA), the lead federal agency responsible for water quality management, and the states including Nevada and California.

Section 303 of the Clean Water Act requires states to adopt water quality standards for all surface waters of the United States. Water quality standards consist of two elements: (1) designated beneficial uses of the water body in question (such as drinking, swimming, recreation, fisheries) and (2) water quality criteria that protect the designated uses. Water quality criteria are required to accurately reflect the latest scientific knowledge on the kind and extent of all effects on health and welfare that may be expected from the presence of pollutants in water. Where multiple uses exist, water quality standards must protect the most sensitive use. Section 303(d) lists the waterbodies and associated pollutants that exceed water quality criteria. In Nevada, the Nevada Division of Environmental Protection (NDEP) develops this list.

Section 303(d) requires that the state develop a TMDL for each of the listed pollutants. The TMDL is the maximum amount of pollutant loading that the water body can receive and still be in compliance with water quality objectives. The TMDL is also a plan to reduce loading of a specific pollutant from various sources to achieve compliance with water quality objectives. The TMDL requires public meetings and input from all stakeholders, and must be based on credible science rather than competing ideologies. The TMDL must include an allocation of allowable loadings to point (such as wastewater treatment plants) and nonpoint (such as stormwater) sources, with consideration of background or naturally occurring loadings and a margin of safety. The TMDL must also include an analysis that shows the linkage between loading reductions and the attainment of water quality objectives. The EPA must either approve a TMDL submitted by the state (in Nevada the NDEP) or disapprove the state’s TMDL and issue its own. The goal of the TMDL program is the removal of the targeted pollutant from the 303(d) list and of the causes that led to placement of the pollutant on the list.