• provide, wherever attainable, water quality for the protection and propagation of fish, shellfish, and wildlife, and recreation in and on the water (fishable/swimmable conditions); and
• consider the use and value of state waters for public water supplies, propagation of fish and wildlife, recreation, agriculture and industrial purposes, and navigation.

The Idaho water quality standards program is a joint effort between DEQ and the U.S. Environmental Protection Agency (EPA). DEQ is responsible for developing and enforcing water quality standards that protect beneficial uses such as drinking water, cold water fisheries, industrial water supply, recreation, and agricultural water supply. The EPA develops regulations, policies, and guidance to help Idaho implement the program and to ensure that Idaho's adopted standards are consistent with the requirements of the Clean Water Act and relevant regulations. The EPA has authority to review and approve or disapprove state standards and, where necessary, to promulgate federal water quality rules (such as the Idaho Bull Trout Rule).

A water quality standard defines the designated beneficial uses of a water segment and the water quality criteria necessary to support those uses. Water quality standards are important because they help to protect and restore the quality of the Idaho's surface waters. Associated criteria may be numeric (i.e., not to exceed some concentration) or narrative. Narrative criteria are sometimes referred to as "free from" criteria, as they often state that the water body must be "free from" something (e.g., free from nuisance aquatic growths).

A water quality standard consists of three basic elements (The "ABCs of water quality"):

• Antidegradation: an antidegradation policy to maintain and protect existing uses and high quality waters.

• Beneficial Uses: designated uses of the water body (e.g., recreation, water supply, aquatic life, agriculture),

• Criteria: water quality criteria to protect designated uses (numeric pollutant concentrations and narrative requirements), and

The state also has the prerogative to create general policies that address implementation issues such as low flows, variances, and mixing zones.

Antidegradation describes policies designed to maintain water quality even if it exceeds levels necessary to support beneficial uses. The state antidegradation policy and implementation procedures must be consistent with the components detailed in 40 CFR 131.12.

The federal water quality standards regulation requires Idaho to establish a three-tiered antidegradation program. Idaho's antidegradation policy is contained in IDAPA 58.01.02, Section 05.

• Tier 1 maintains and protects existing uses and water quality conditions necessary to support such uses. Where an existing use is established, it must be protected even if it is not listed in the water quality standards as a designated use. Tier 1 requirements are applicable to all surface waters. Waters that just meet the numeric criteria are tier 1.

• Tier 2 maintains and protects "high quality" waters – water bodies where existing conditions are better than necessary to support Clean Water Act "fishable/swimmable" uses. Water quality may be lowered in tier 2 waters, but only with public review of the social and economic tradeoffs. In no case may water quality be lowered to a level that would interfere with existing or designated uses. Most of the waters in Idaho are likely tier 2.

• Tier 3 maintains and protects water quality in outstanding resource waters (ORWs). Except for certain temporary changes, water quality cannot be lowered in such waters. Outstanding resource waters generally include the highest quality waters of the United States. However, the ORW classification also offers special protection for waters of exceptional ecological significance, such as those that are important, unique, or sensitive ecologically. Decisions regarding which water bodies qualify to be ORWs are made by states and authorized tribes. Though several waters in Idaho have been considered for ORW recognition, none have been legislatively approved.

A water quality standard defines the water quality goals for a water body or portion thereof, in part, by designating the use or uses to be made of the water. The designated beneficial use of a water body must consider its actual use, the ability of the water to support in the future a use that is not currently supported, and the basic goal of the Clean Water Act that all waters support aquatic life and recreation where attainable. Idaho must designate its uses accordingly.

A designated use is a beneficial use assigned to a specific water body in Idaho water quality rules.  The Clean Water Act requires Idaho to recognize existing uses, which are uses that are/were actually attained in a water body on or after November 28, 1975, whether or not they are designated uses. Idaho presumes most undesignated waters will support cold water aquatic life and either primary or secondary contact recreation. These are termed presumed uses. Designated, existing, and presumed uses must all be protected.

In designating uses, Idaho takes into consideration the use and value of the water body for public water supply; for protection of fish, shellfish, and wildlife; and for recreational, agricultural, industrial, and navigational purposes. While there may be competing beneficial uses in a river or a stream, federal law requires DEQ to protect the most sensitive of the beneficial uses.

Idaho evaluates the suitability of a water body for the uses based on the following:

• physical, chemical, and biological characteristics;
• geographical setting and scenic qualities; and
• economic and public values.

Read more on the beneficial uses identified in Idaho's water quality standards and determining beneficial use support status.

DEQ has established water quality criteria (IDAPA 58.01.02; See Section 200) that identify specific benchmarks that describe the quality of water needed to support the uses listed above. These criteria can be numeric (parameter-specific) or narrative. Numeric criteria are use-specific, while narrative criteria are general, applying to all waters irrespective of use. Together, the numeric and narrative criteria must contain sufficient parameters or constituents to protect the designated use.  Read more about traditional water quality measures.

Numeric criteria are required where necessary to protect designated uses. Individual numeric criteria are based on specific data and scientific assessment of adverse effects. Numeric guidelines assign numbers that represent limits and/or ranges of chemical concentrations, like oxygen, or physical conditions, like water temperature. A typical numeric statement for an aquatic life criterion usually contains a concentration and averaging period (acute – short term or chronic – long term). For example, for toxics, a one-hour averaging period applies for an acute (short term) concentration, while a four-day average applies for a chronic (long term) concentration. The criteria are values that should rarely be exceeded if uses are to be supported.

Idaho has numeric criteria for temperature, dissolved oxygen, pH, turbidity, bacteria, ammonia, and a list of 121 toxic substances (both metals [including mercury] and humanmade organic chemicals). The criteria values and the applicability of the criteria to a specific water body depend upon the designated use of the water. Temperature, dissolved oxygen, pH, turbidity, and ammonia criteria have been developed to protect aquatic life (fish as well as the aquatic insects they feed upon). Bacteria criteria are designed to protect human health and are applied to waters designated for recreation use. Toxics criteria are developed for protection of both aquatic life and human health.

Surface waters of the state shall be free from excess nutrients that can cause visible slime growths or other nuisance aquatic growths impairing designated beneficial uses. Read more about narrative water criteria.

DEQ's water quality criteria may not always reflect the toxicity of a pollutant in a specific water body. They represent only a limited number of the natural and humanmade chemicals that exist in the environment that may pose a threat to designated or existing beneficial uses. Therefore, it may be prudent in some water bodies to develop new water quality criteria or modify existing criteria through site-specific analyses that will effectively protect designated and existing beneficial uses. Some site-specific criteria have been established for specific Idaho water bodies to reflect local environmental conditions. Site-specific criteria are allowed by regulation and are subject to EPA review and approval. As with all water quality criteria, site-specific criteria must be based on sound scientific principles in order to protect the designated use.

The following are acceptable conditions for developing site-specific criteria:

• Resident species of a water body are more or less sensitive than those species used to develop a water quality criterion.
• Biological availability and/or toxicity of a pollutant have been altered due to differences between the physicochemical characteristics of the water in a water body and the laboratory water used in developing a water quality criterion (e.g., hardness, temperature, or pH).
• Seasonal changes to the physicochemical characteristics of a water body affect the biological availability and/or toxicity of a pollutant (seasonally dependent site-specific criteria).
• Existing ambient water quality is not protected with statewide water quality criteria.
• Other factors or combinations of factors that may warrant modifications to the criteria.

DEQ's procedures to derive site-specific criteria can be found in IDAPA 58.01.02, Section 275.01(h). See examples of site-specific criteria used in Idaho.

The designated use of a water body may be changed if it is shown that it is not attainable. A use attainability analysis (UAA) is a structured scientific assessment of the beneficial uses a water body could support, given application of required effluent limits and implementation of cost-effective and reasonable best management practices. Use attainability analysis flow charts show how the process works (pdf 25 kb, 2 pages).

A UAA is required anytime a state or tribe designates a use that does not include the "fishable/swimmable" goals of the Clean Water Act or changes a use to one that would apply less stringent criteria than the current use. If a use is designated that does not include the "fishable/swimmable" goals, that use designation and the rationale behind it need to be revisited every three years to see if circumstances have changed.

Waters must be protected for the most sensitive of their uses. Most streams have overlapping or competing uses. When this is the case, the most sensitive use is that which requires better water quality (the use with the most stringent criteria). If an existing use is "better" than the designated use, the existing use must be protected and that use must become the designated use. Uses that currently exist, or have existed since November 25, 1975, cannot be removed or downgraded.  Read more about UAAs.

Lower Boise UAA Decision:

Link to EPA's Decision Letter, November 29, 2004 (EPA Publication, pdf 16 kb, 2 pages)

Link to EPA's Technical Justification for UAA Decision, November 29, 2004 (EPA Publication, pdf 312 kb, 59 pages)

View the Bucktail Creek UAA (DEQ Publication, February 2002, pdf 1.4 mb, 25 pages).  Bucktail Creek UAA appendices listed below:

Bucktail Creek UAA Appendix 1 (pdf 1.8 mb, 6 pages)

Bucktail Creek UAA Appendix 2 (pdf 1.7 mb, 30 pages)

Bucktail Creek UAA Appendix 3 (pdf 6.0 mb, 91 pages)

Bucktail Creek UAA Appendix 4 (pdf 2.8 mb, 42 pages)

DEQ is required to review its water quality standards at least once every three years.  This process, called triennial review, is required by Section 303(c) of the Clean Water Act and must include the opportunity for the public and EPA to review the standards.  The review is intended to ensure that standards meet public concerns, reflect new scientific and technical information, and follow EPA guidelines.  Read more about the Triennial Review.

Natural background conditions exist when there is no measurable difference between the quality of water now and the quality of water that would exist if there were no human-caused changes in the watershed. Since human-caused changes don't always affect all aspects of water quality, it is possible for water to be considered natural for one parameter but not another. For instance, it would be possible for a water body's temperature to be unaltered by humans, but at the same time be polluted with a humanmade toxic substance.

Water quality standards in most states include a provision that allows for water quality to exceed numeric criteria due to natural background conditions of the water body. Inclusion of such provisions is recognition that water quality laws are about control of pollution, and that pollution is human-caused alteration of water quality. Idaho has had such provisions in its water quality rules since April 2000 (IDAPA 58.01.02.200; See Section 09).

When pollutants are humanmade chemicals, such as pesticides or solvents, the evidence of pollution is clear. But in controlling water quality and protecting beneficial uses we also pay attention to natural characteristics of water, such as sediment, nutrients, dissolved oxygen, and temperature, which can be adversely altered by human activity. Even some toxic materials, such as metals and ammonia, occur naturally. This complicates pollution control, because for these natural constituents of water it becomes necessary to distinguish between the natural background concentration and the increase in concentration due to pollution. Read more about natural background conditions.

Wastewater discharges from industrial, municipal, or other sources may be permitted so long as quality of the water body receiving the discharges is still protected. However, it is not always necessary to meet water criteria within the discharge outfall to protect the integrity of the water body as a whole. Sometimes it is appropriate to allow a mixing zone as an area of transition from a higher effluent concentration to a lower ambient concentration. Concentrations within this zone may exceed criteria so long as the area is kept relatively small.

A mixing zone is a defined area or volume of a receiving water surrounding by or adjacent to a wastewater discharge where the receiving water, as a result of the discharge, may not meet all applicable water quality criteria standards. It is a place where wastewater mixes with receiving water for dilution and not as a place where effluents are treated. Read more about mixing zones.

Water quality standards are periodically revised and the revisions are submitted each year to the Idaho State Legislature for approval. Any change to the rules requires a formal rulemaking process according to the Idaho Administrative Procedures Act, which includes an opportunity for public comment. Notices of new rulemaking are announced in Idaho's monthly Administrative Bulletin, as are the status of rulemaking and links to existing rules. Opportunities for public involvement also arise in application of water quality standards in:

• TMDLs
• Reporting water quality status
• 401 certification

Each of these has its own public comment periods.