Once candidate criteria have been derived, and final criteria selected, it is important to be able to clearly communicate how the final criteria are protective of designated uses. This provides regulatory clarity to decision-makers, and the public, so they can have confidence that the final criteria will protect designated uses from nutrient pollution (USEPA 1998). As with estimating candidate criteria, thorough documentation of reasoning organized by clarity, transparency, reasonableness, and consistency is recommended.

Clarity

• Describing the process used in selecting final criteria in a manner that is clearly understandable to and replicable by similarly positioned professionals.

Transparency

• Identifying scientific conclusions separately from policy judgments.
• Articulating differing viewpoints on judgments made.
• Explaining assumptions.

Reasonableness

• Communicating how the scales of the criteria are protective of designated uses, which often exist year-round.  For example, it is important to describe how the magnitude of the pollutant (e.g., 1 mg/L as an average concentration of total nitrogen from a site), the duration of exposure to that pollutant (e.g., 4-month growing season), and the frequency of that magnitude/exposure (e.g., not to exceed 10 percent of all sites observed) protects a given assessment endpoint from which protection of designated uses can be inferred.
• Communicating the likelihood that the estimated criteria will protect the desired designated use(s) and how that protection was inferred through the different components of the NNC development process (e.g., conceptual model, data quality, model development, criteria derivation).
• Communicating the scales of inference being made from the analysis to the population of waters of interest.
• Communicating reasonable alternatives to the decisions made.

Consistency

• Communicating the degree of agreement/disagreement of estimated NNC with other published numeric nutrient thresholds in the scientific literature or state/tribal-promulgated NNC for similar water bodies.

Accounting for these considerations and communicating them to the public creates understanding and confidence in the decisions being made.  It also aids state and federal practitioners tasked with implementing the criteria in other water pollution control programs (e.g., CWA sections 305(b), 303(d), 301), which is an important regulatory component of water quality standards as specified in Title 40 of the Code of Federal Regulations section 131.2.

The Role of Scientific Judgment. NNC development is a deliberative, analytical process conducted by trained scientists.  Throughout the process scientists must make informed decisions based on their experience and level of expertise.  In this way, NNC are the result of an additive decision-making process through which specific choices and decisions can be traced, understood, and reproduced by others.  The process is not a consensus-based endeavor per se, but rather one in which scientists apply, test, and corroborate their expertise.  When performed in adherence to principles of clarity, transparency, reasonableness, and consistency, it provides a scientifically defensible basis for protective criteria.

Independent peer review, another hallmark of rigorous scientific process, can be incorporated into the numeric criteria development process at any point.  EPA recommends peer review because it can assure decision-makers and the public that the criteria were developed in an objective, transparent manner.

Beyond Criteria Derivation

As noted in the introduction of N-STEPS Online, the Clean Water Act authorized states and tribes to establish numeric nutrient criteria that protect designated uses from nutrient pollution. Numeric nutrient criteria can be an effective tool for preventing nutrient pollution by enhancing water quality monitoring programs, reducing management decision ambiguity, and providing stakeholders with an informed perspective on the condition of state and tribal waters.

Once adopted by states and approved by EPA, numeric nutrient criteria and state/tribal water quality standards are implemented in other state Clean Water Act activities. Numeric nutrient criteria facilitate the calculation of water quality-based effluent limits in National Pollution Discharge Elimination System (NPDES) permits to attain nutrient water quality standards and total maximum daily loads (TMDLs) to restore nutrient water quality standards in impaired waters. EPA has developed materials, in coordination with numerous partners, to assist states and authorized tribes with technical and policy information for implementing numeric nutrient criteria.

For more information, please visit:

• EPA’s Nutrient Pollution Policy and Data
• The Association of Clean Water Administrators (ACWA) has developed a companion Nutrient Toolkit page, where state materials—similar in content to the Federal materials on EPA’s toolkit site—will be housed. ACWA’s preliminary state toolkit was developed in partnership with the EPA.