Application of Weight-of-Evidence Methods for Transparent and Defensible Numeric Nutrient Criteria
Water quality standards are important for protecting and restoring the condition of lakes, rivers, estuaries, and other water bodies in the United States. Given that nutrient pollution continues to be a widespread problem in aquatic systems, the development of numeric nutrient criteria (NNC) as part of water quality standards is a priority to enhance prospects for managing excess nutrients and their effects. In this report, we complement existing NNC guidance and support to states by discussing weight-of-evidence (WoE) methods that enable rigorous and transparent development and integration of multiple lines of evidence. The NNC development phases (Planning, Problem Formulation, Analysis, and Derivation) align with the Basic WoE Framework steps (Assemble Evidence, Weight Evidence, and Weigh the Body of Evidence). The process is conducted within the context of the WoE core principles of transparency, documentation, and communication.
The following are take-home messages for the role WoE can play in strengthening each phase of NNC development.Planning Phase- Activities undertaken during Planning provide a transparent foundation for developing NNC; transparency is a core principle of WoE. Grouping water bodies during Planning is a process to which WoE could be applied when diverse evidence needs to be combined. Problem Formulation Phase- Selecting endpoints during Problem Formulation is also a process to which WoE could be applied when diverse evidence needs to be combined. Conceptual models developed during Problem Formulation can help inform what evidence should be assembled in the Analysis Phase.Analysis Phase- This phase includes assembling evidence and weighting evidence. Unbiased assembly of evidence is best practice and can ensure NNC are based on transparent data and information of sufficient amount and quality. Weighting evidence by establishing, objectively evaluating, and documenting qualities of that evidence shows how much influence individual evidence will have on overall NNC conclusions. Criteria Derivation Phase- This phase includes weighing the body of evidence by integrating and interpreting evidence, as well as communicating conclusions. Methods for integrating evidence to derive criteria can range from simple to sophisticated; selected methods should be logical, informed by evidence availability and stakeholder needs, and communicated clearly.