HIS Standards

Water quality standards (WQS) are central to implementation of the water quality-based control program mandated by the Clean Water Act (CWA). Three elements of WQS include:

  1. Designated use classifications
  2. Numerical and/or narrative water quality criteria
  3. 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' WQS 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.

CWA Section 305(b)

Waterbodies that have been assessed are assigned a category based on the guidelines set forth by the CWA Section 305(b). Multiple categories were created under Section 305(b) to help states standardize water quality reporting. Below is an explanation of each 305(b) category (for more details, check out the EPA's Guidance for 2004 Assessment, Listing, and Reporting Requirements Pursuant to Sections 303(d) and 305(b) of the Clean Water Act).

1 These waters meet all applicable WQS and support all designated uses.
2 Data collected for Category 2 waters support some designated uses, but not all. More information is needed to make a determination on the attainment of other applicable WQS.
3 There is insufficient evidence to make a determination for the attainment of any applicable WQS and designated uses.
4 Waters within category 4 do not meet water quality standards (WQS) and have one or more violations of designated uses. Category 4 waters, however, are different from category 5 because they do not require the establishment of a Total Maximum Daily Load (TMDL). Waters that fall within this category are typically subcategorized into either category 4a, 4b, or 4c.
4a Waters within category 4a have USEPA approved TMDLs established for all applicable WQS. The USEPA advises that all approved TMDLs should be implemented as soon as practicable to ensure the attainment of WQS within the projected time stipulated in the TMDL.
4b Waters within category 4b require "other pollution control" to be set forth by the State rather than the development of a TMDL to attain all applicable WQS. States must demonstrate that best management practices can be applied to address all major pollutant sources and achieve WQS within a reasonable timeframe.
4c Waters within category 4c do not require the establishment of a TMDL because the impairment is not caused by a pollutant. Category 4c waters are impaired due to the presence of pollution, which is defined by the Clean Water Act as "the man-made or man-induced alteration of the chemical, physical, biological, and radiological integrity of water" (Section 502(19)). Pollution occasioned by pollutants (and vice versa) does require the development of a TMDL to address the underlying pollutants. Examples of pollution that do not involve pollutants may include flow alterations and elevated temperatures.
5 Only waters within category 5 are included in State 303(d) lists. These waters are impaired and do not meet specified designated uses due to the presence of one or more pollutants. Waters remain in category 5 until all violations of WQS and designated uses are addressed by a USEPA approved TMDL and/or some other delisting factor stipulated by the USEPA occurs.

It is important to note that some states have created their own subcategories to supplement those provided by CWA Section 305(b). While these subcategories often render more information regarding the basis for category assignment (ex. the Ohio Environmental Protection Agency has implemented a Category 5h, which represents impaired waterbodies based on historical evidence that cannot be verified at the present time), HIS only reports 305(b) status using the categories stipulated within the CWA Section 305(b).

CWA Section 303(d)

Waterbodies that fail to comply with WQS 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 WQS.

Last updated: October 4, 2021