For the more information about water resources in the National Park Service, please visit https://www.nature.nps.gov/water/.
Hydrographic & Impairment Statistics Standards
Water quality standards are central to implementation of the water quality-based control program mandated by the Clean Water Act (CWA). Three elements of water quality standards include:
- Designated use classifications
- Numerical and/or narrative water quality criteria
- An antidegradation policy
The CWA requires all States to establish use classifications for all waterbodies within the State, e.g., public drinking water supplies, propagation of fish and wildlife, recreational purposes, industrial, and other uses. Water quality criteria are numerical descriptions of the physical, chemical, and biological characteristics of waters necessary to support the designated uses.
The antidegradation policy as promulgated by the Environmental Protection Agency (EPA) in the Code of Federal Regulations at 40 C.F.R. Part 131.12 acts as a key portion of States' water quality standards by requiring States, at a minimum, to include provisions for the management of water quality in accordance with the following "Tiers":
- Tier 1: Includes the provisions to protect existing uses of water in the State, which constitute the absolute floor or minimum level of protection that must be provided all waters.
- Tier 2: Applies to waters whose quality exceeds that necessary to protect "fishable/swimmable" goals of the CWA. Management of these waters must attempt to keep them at existing quality. Degradation may be allowed if it cannot be avoided for social or economic development reasons, but only after public review has occurred.
- Tier 3: Applies to Outstanding National Resource Waters (ONRWs) where ordinary use classifications and supporting criteria may not be sufficient or appropriate. ONRWs are frequently considered the highest quality waters of the United States, but may also include waterbodies that are of "exceptional recreational or ecological significance," as stated under 40 C.F.R. Section 131.12(a)(3) of the antidegradation policy. ONRWs are afforded the highest level of protection under the antidegradation policy. Existing water quality must be maintained and protected, and only activities that cause short-term and temporary degradation may be allowed.
An additional concept of a "Tier 2½" waterbody was developed by States out of a concern that the Tier 3 provision was too restrictive of social and economic development. A Tier 2½ waterbody, which the EPA does accept, offers more protection than a Tier 2 waterbody without the strict prohibition against the lowering of water quality found in the Tier 3 provision.
The National Park System encompasses some of the most sensitive, pristine, and significant aquatic resources in the United States. Many of these aquatic resources have been afforded the protection of Tier 2½ or Tier 3 ONRW status.
Section 303(d) Waterbodies
Waterbodies that fail to comply with standards are compiled by States into a list, commonly referred to as "303(d) lists" after the section of the CWA which contains the requirement, for submittal to the EPA. The EPA approves the list only if it meets applicable requirements. Waterbodies on an approved 303(d) list require the establishment of a total maximum daily load (TMDL). A TMDL specifies the amount of a particular pollutant that may be present in a waterbody, allocates allowable pollutant loads among sources, and provides the basis for attaining or maintaining water quality standards.
Last Updated: April 25, 2011