C ontroversies have arisen in several states over establishment of ambient water quality standards. At issue is whether states are setting standards at levels that adequately protect public health from pollutants in waterways. Some groups argue that states are adopting overly stringent standards that are unattainable and unaffordable and are being pressured to do so by the U.S. Environmental Protection Agency (EPA). Others contend that the states are failing to protect population groups that consume large amounts of fish, such as members of Indian tribes that have treaty fishing rights. The issue involves complex scientific and technical questions about cancer risk levels and fish consumption rates, among others. States where these controversies have occurred recently include Maine and several in the Pacific Northwest (Washington, Oregon, and Idaho). Similar issues could arise in other states that have large tribal populations or other populations that are exposed to toxic pollutants in streams, lakes, and other ambient waters. In many respects, water quality standards are the fundamental building blocks of the Clean Water Act (CWA; 33 U.S.C. 1253 et seq.). Established by states and approved by EPA, they define a state's water quality goals, and they result in direct requirements for dischargers because states issue enforceable discharge permits based on criteria limits in the standards. They also provide the benchmark against which states identify impaired waters and then develop plans that establish Total Maximum Daily Loads (TMDLs) to attain the standards. Water quality standards consist of narrative and numeric limits on pollutants, designated uses or goals for protection of the waterbody (such as fishing, swimming, or public water supply), and antidegradation policy to maintain and protect existing uses and high-quality waters. In support of standard setting by states, EPA develops risk-based water quality criteria that set a concentration for contaminants in water to ensure that public health and aquatic life will not be harmed. For most pollutants, EPA develops water quality criteria to protect aquatic life and separate water quality criteria to protect human health. The latter are sometimes referred to as the potable water criteria, since they are intended to protect human health from water that is consumed directly or protect human health from exposure to contaminants that may occur as a result of consuming fish. The most recent EPA update of the human health water quality criteria was issued in 2015. It included revisions to 94 existing criteria; EPA now has recommended human health criteria for 122 pollutants. The EPA criteria are recommendations to states—they do not apply automatically, they are not binding on states, nor are they enforceable. Most states use the EPA national criteria as the starting point for developing criteria as part of their water quality standards. They usually are expressed as concentration limits for a pollutant. The states' criteria must protect the designated use of a waterbody and be based on "sound scientific rationale." If a state adopts criteria that differ from EPA's recommended criteria, the state must explain its rationale for doing so, as part of developing enforceable water quality standards that are reviewed and approved by EPA. Following EPA's approval of water quality standards, states establish discharge permit limits to ensure that industrial and municipal sources will not violate criteria in the standards. If EPA disapproves state criteria or determines that revised criteria are necessary, it can issue federal criteria for the state. When that occurs, the federal criteria are the state's water quality standards until such time as EPA approves the state's revised criteria. EPA then withdraws the federally promulgated water quality standards because they are no longer necessary. EPA has not often used its CWA authority to establish federal water quality standards—for example, it promulgated toxic pollutant standards in 14 states and territories in 1992 and toxics standards for waters of the Great Lakes system in 1995 —but doing so has been controversial. An important function of ambient water quality criteria is to manage the risk associated with chemicals that are released into the environment through human activity in such a way that human health is protected. Human health criteria represent the highest concentration of a pollutant in water that is not expected to pose a significant risk to human health. They are set so that fish in a waterbody have levels of targeted pollutants low enough such that when they are consumed by people, or are consumed by people who also are drinking water from the same waterbody, they do not pose unacceptable health risks to individuals. A water quality criterion is calculated as the product of risk-specific toxicity (i.e., the type of health effect—cancer or non-cancer) times exposure. Exposure encompasses multiple factors, such as body weight of individuals, daily intake of fish and water, and bioaccumulation of the pollutant. Over time, the methodology and exposure inputs underlying EPA's national recommended human health criteria have evolved, based on better science, population data, and models. EPA's 2015 Update of Human Health Ambient Water Quality Criteria reflects several revised standard exposure inputs: (1) default body weight of 80 kilograms for adults ages 21 and older (about 176 pounds; previously, EPA's default body weight was 70 kilograms, or about 154 pounds), (2) default drinking water consumption rate of 2.4 liters per day for adults (previously, 2 liters per day), and (3) default fish consumption rate (FCR) for the general population of 22 grams per day (g/d) that is protective of 90% of adults (about ¾ of an ounce; previously, EPA's national default rate was 17.5 g/d, or 0.6 ounce). EPA's national default subsistence FCR is 142 g/d, representing subsistence fishers whose daily consumption is greater than the general population. Fish consumption rates are among the important exposure factors in determining risk level in a criterion, because the more fish that people consume that contain toxic pollutants, the more individuals are at risk for developing cancer and other illnesses. What is considered safe for someone who eats fish once per month might be harmful to someone who eats fish every day. This is important to Indian tribes, who generally eat more fish than average consumers because fish consumption has important cultural and religious significance for tribal members. Consequently, some have long argued that water quality standards should give greater weight to considerations that include FCRs of certain subpopulations, such as Native Americans. Also important is the assumed cancer risk level in a criterion. EPA and other regulatory agencies consider that there is some risk with even the lowest exposure to carcinogens (i.e., there is no threshold exposure below which risk is zero). To establish regulatory criteria for carcinogens, the level of acceptable risk must be determined. Chronic (lifetime) exposure to carcinogenic chemicals is associated with an increased likelihood of developing cancer at some point in an individual's lifetime. This likelihood is sometimes referred to as the incremental excess lifetime cancer risk. That increased likelihood is sometimes referred to as a probability, such as one in 1,000 (expressed as 1 x 10 -3 ) risk above the "background" risk of developing cancer. EPA calculates CWA human health criteria for carcinogenic effects as pollutant concentrations corresponding to lifetime increases in the risk of developing cancer. Because exposure to surface water or other environmental media cannot be risk-free, the challenge is to find some level of risk that most people will find acceptable. For exposure to carcinogens, the risk-based point of departure for many environmental rules has been a risk management decision of selecting a threshold probability of cancer, typically an excess risk of one in 1 million, or 1 x 10 -6 . Risks at this level or lower (e.g., 10 -8 ) are regarded as acceptable, while higher risks (e.g., 10 -3 ) may or may not be considered acceptable, depending on the regulatory program involved. EPA's methodology for developing ambient water quality criteria to protect human health recommends cancer risk levels of 10 -5 (one in 100,000) or 10 -6 as generally acceptable risk management levels to protect the general population and notes that states and authorized tribes can choose a more stringent risk level, such as 10 -7 (one in 10 million), when deriving human health criteria. EPA's methodology also states that the risk to more highly exposed populations (sports fishers or subsistence fishers)—who inherently face greater risk by consuming more fish—should not exceed a 10 -4 risk level (one in 10,000). EPA believes that states have flexibility under the CWA to determine appropriate risk levels in their water quality standards, subject to EPA review and approval or disapproval, but requires that the state has identified the most highly exposed subpopulation and has demonstrated that the chosen risk level is adequately protective of that population. The CWA requires states and authorized tribes to review their water quality standards and revise them, if appropriate, at least once every three years. Increasingly, during the triennial review process, EPA has been encouraging states with populations known to consume large amounts of fish to develop criteria to protect highly exposed population groups and, in doing so, to use local or regional data on FCRs that are more representative of their target population group, in place of a default national value. In addition, the agency encourages states to adopt a cancer risk level of 10 -6 both for the general population and highly exposed groups. Recent controversies between EPA and several states have involved disputes over both the appropriate FCR and cancer risk level used by states in developing their water quality criteria, and the stringency of the resulting ambient water quality standards. Stringent water quality standards, in turn, can result in states issuing permits containing highly restrictive discharge limits that create compliance issues for industrial and municipal facilities. The challenge raised by these controversies is to develop achievable water quality criteria that are protective for the general population and for high-consuming subpopulations, whose risk will be greater, but still acceptable. In 2011, following its triennial review, Oregon adopted revisions to its water quality standards. Initially, Oregon's criteria were based on a fish consumption rate of 17.5 grams per day and 10 -6 lifetime cancer risk level.