DEQ has established water quality criteria that identify specific benchmarks that describe the quality of water needed to support beneficial uses. These criteria can be numeric 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 measures or constituents to protect the designated uses of a water body.  Some of the most common measures of water quality are listed below, with an explanation of why they are important to the health or utility of a water body.

Ammonia is quite toxic to aquatic life. While some ammonia is naturally occurring, elevated ammonia levels can result from water polluted with nitrogen-containing organic waste, such as domestic sewage. The ammonia criteria depend upon pH and temperature of the water.

Bacteria are measured to determine the relative risk of swimming (contact recreation). These bacteria originate from the wastes of warm-blooded animals; their presence indicates that pathogens from these wastes may be reaching a body of water from inadequately treated sewage, improperly managed animal waste from livestock, pets in urban areas, aquatic birds and mammals, or failing septic systems. Idaho's bacteria criteria are numeric.

Nutrients in water include various forms of the chemical elements nitrogen and phosphorus—the same materials we apply as fertilizer to our lawns, gardens, and farms to foster the growth of plants. They have the same effect in water as they do on land, encouraging the growth of aquatic plants such as algae (floating or attached to rocks) and rooted macrophytes (e.g., water lilies).

Without nutrients water would be sterile and not support aquatic life. Adding nutrients can be acceptable, and even beneficial at times, as they increase the productivity of a water body. However, if we inadvertently add too much nutrients to our waters the growth of aquatic plants becomes excessive. This is known as cultural eutrophication.

Visually this can change the clarity and desirability of the water. More importantly, the plants eventually will die and their decay uses up oxygen dissolved in the water. Excessive aquatic plant growth is a leading cause in the depletion of oxygen needed for fish. The root cause, however, is too much nitrogen and phosphorus. Because they accumulate many things, including nutrients, lakes and reservoirs are most prone to cultural eutrophication.

Currently, Idaho's criterion for nutrients is narrative; DEQ is working on a national EPA initiative to develop numeric nutrient criteria. The most common measures of nutrients are nitrate-nitrite nitrogen and total phosphorus, but much is often learned by measuring other forms such as total inorganic nitrogen, organic nitrogen, or soluble reactive phosphorus.

A water's pH is a measure of the balance between hydrogen ions, which are acidic (sour) and hydroxide ions, which are basic (bitter). A perfect balance of the two is at a pH of 7. Most aquatic organisms prefer a pH of 6.5 to 9. The pH of water, like temperature, is a fundamental controlling property that affects many other chemical constituents (e.g., dominant form of ammonia and solubility of metals) as well as important biological processes such as the level of permeability of fish gills and amphibian skins (that is, how well gases can flow through the gills/skin, allowing the fish/amphibians to breathe).

  Sediment

In flowing waters there is a dynamic balance between the supply of sediment from natural erosion and the energy of the moving water that carries and redistributes the sediment load. That balance determines the very character of many streams and their suitability for various forms of aquatic life.

Many human activities disturb the ground and thus accelerate erosion. Concentrated runoff, such as in roadside ditches, can also accelerate erosion. This increases the load of sediment in a stream and often degrades a stream's ability to sustain aquatic life. Spawning gravels are particularly vulnerable to degradation by deposits of fine sediment—sand and silt—that fill the spaces between the larger gravel. This reduces living space for eggs and recently hatched fish and, in extreme cases, suffocates the eggs.

Idaho's criterion for sediment is narrative. There are many ways to measure sediment and many complexities in determining how much is too much. For more information, refer to the Guide to Selection of Sediment Targets for Use in Idaho TMDLs (DEQ Publication, June 2003: pdf 908 kb, 84 pages).

  Temperature

  Toxic Substances

A toxic substance is any substance, material, or disease-causing agent, which, upon exposure, ingestion, inhalation, or assimilation into any organism, will cause death, disease, malignancy, genetic mutation, or other abnormalities in affected organisms or their offspring. Toxic criteria exist for protection of aquatic life and human health. Idaho's WQS contain many numeric criteria for toxic substances, but many known toxic substances have no numeric criteria. Thus, there is also a narrative criterion for toxics substances to protect against the adverse effects of a vast array of substances whose toxicity is either unknown or insufficiently quantified to specify numeric criteria. Read more about toxics.

• Metals: High concentrations of metals such as cadmium, mercury, and lead pose a threat to aquatic life, drinking water supplies, and human health. Eating fish contaminated with metals can cause these toxic substances to accumulate in human tissue, posing a significant health threat. Metals also pose a threat to livestock and aquatic life. Potentially dangerous levels of metals and other toxic substances are identified through chemical analysis of water, sediment, and fish tissue.

• Mercury: Mercury is toxic to both aquatic life and humans, but its toxicity is primarily a human health concern. Inorganic mercury occurs naturally due to its presence in rocks and soils. It is slowly released through erosion and weathering into surface waters. Most of the mercury in surface waters remains inorganic, but in favorable environments (low pH, low dissolved oxygen, and high organic matter, such as are found in the bottoms of lakes, marshes, and wetlands), some of it is converted to a much more toxic organic form—methylmercury. Methylmercury tends to accumulate in the tissue of fish, and thus is of particular interest from a human health standpoint. Read more about mercury.

In April 2005 Idaho adopted a fish tissue methylmercury criterion to protect individuals that may eat fish from Idaho surface waters. This criterion of 0.3 milligrams methylmercury per kilogram of fresh weight fish is based on protecting an adult consumer who eats on average of 17.5 grams of fish per day—about one, 8-ounce meal every other week. 

A fish tissue criterion to protect water quality is a new thing for Idaho, and in fact a new thing under the Clean Water Act anywhere in the nation. It represents a fresh approach that is more closely tied to the effects on human health than the traditional approach of controlling pollutant discharge. This requires new techniques for applying Clean Water Act programs such as National Pollution Discharge Elimination System permits and total maximum daily loads. Mercury adds its own unique challenges as it is toxic in very low concentrations that have traditionally been hard to detect, air deposition is often a major source of deposition to surface water, and aerial pollution sources can be regional or beyond Idaho's control. The Implementation Guidance for the Idaho Mercury Water Quality Criteria (pdf 1.7 mb, 212 pages) was developed to address these issues and support Idaho's adoption of a methylmercury criterion in its water quality rules.

> Link to fish advisories currently impacting water bodies in Idaho on the Idaho Department of Health and Welfare Web site.

• Organics: There are many toxic substances that are humanmade, carbon-based chemicals. These are collectively called "organics," and include household and agricultural pesticides, many solvents, and other industrial chemicals. Polychlorinated biphenyls (PCBs), for example, are industrial chemicals that are toxic and probably carcinogenic. Although banned in the United States in 1977, PCBs remain in the environment, and they accumulate in fish and human tissues when consumed.

Pharmaceuticals and personal care products (PPCPs) are emerging pollutants of concern.  PPCPs can affect reproductive and developmental processes in fish and wildlife. Contaminants include metals, synthetic chemicals, hormones, pesticides, cleaning agents, and pharmaceuticals. It is believed that PPCPs enter surface waters primarily through end-use rather than manufacturing, either by excretion or disposal by flushing. Low concentrations of PPCPs have been found in the Boise River, which was part of a 2002 USGS study on pharmaceuticals, hormones, and other organic wastewater contaminants in surface water.  The most common PPCPs found in the Boise River were steroids, nonprescription drugs, and insect repellant.

> Link to more information on PPCPs on EPA's Web site. 

Turbidity is a physical property of water that describes its clarity: "cloudy" waters are turbid. Turbidity is due to the scattering of light by small particles. These particles can be inorganic (e.g., silt and clay) or organic (e.g., algae). Turbidity can cause several problems. The reduction in light penetration can reduce photosynthesis and thus the productivity of water. More commonly, the mere reduction in clarity degrades the aesthetic appeal of water to people, as well as makes it harder for fish that rely on vision to catch their prey.  Finally, if turbidity due to silt and clay persists for too long, it can lead to problems for fish by abrading gill surfaces and depositing sediment that can smother eggs or reduce gravel pore space essential to spawning success.