Research Areas > Nutrients > Nutrient Criteria Support Studies > Background: Nutrient Numeric Endpoints

Background: Nutrient Numeric Endpoints

Under the Clean Water Act, states can set narrative or numeric water quality criteria. Nutrient numeric endpoints (NNEs) are a scientifically defensible framework, originally developed by Tetra Tech, Inc., for USEPA Region IX and the California State Water Resources Control Board (SWRCB), that can serve as guidance for adopting narrative criteria for impacts from nutrient over-enrichment (EPA 2006). NNEs have been drafted for streams, rivers, and lakes in California, but this work is just beginning for estuaries.

The NNE Framework

The NNE framework is founded on the concept that biological response indicators are better suited to evaluate the risk of beneficial use impairment, rather than using pre-defined nutrient limits that may or may not result in mitigation of eutrophication for a particular water body. The advantage of the proposed approach is a more robust link to actual impairment of use, rather than an approach that relies on concentration data alone.

The California NNE framework for estuaries is based on three organizing principals:

• Biological response indicators provide a more direct risk-based linkage to beneficial uses than nutrient concentrations alone.

• A weight of evidence approach with multiple indicators will produce NNE with greater scientific validity.

• For many of the biological indicators associated with nutrients, no clear scientific consensus exists on a target threshold that results in impairment.

Beneficial Use Risk Categories

There is no clear scientific consensus on a target thresholds associated with impairment for many biological indicators of eutrophication. To address this situation, the California NNE framework classifies water bodies into the three Beneficial Use Risk Categories. These categories are as follows:

BURC I: This category includes waterbodies in which beneficial uses are sustained and impairment due to nutrients is not exhibited.

BURC II: This category includes waterbodies in which beneficial uses may be impaired; however, additional information and analysis may be needed to determine the extent of impairment and whether regulatory action is warranted.

BURC III: This category includes waterbodies in which impairment due to nutrients is clearly exhibited and regulatory action warranted.

For a given beneficial use designation and indicator(s), the BURC I/II boundary represents a level below which a significant risk of impairment is not present. The BURC II/III boundary represents a level sufficiently high that risk of use impairment is probable. Within BURC II, additional water body-specific cofactors may be brought into the analysis to determine an appropriate target.

Ultimately, the goal is to propose preliminary NNE targets (i.e., BURC thresholds) for each of the biological response indicators using literature sources, monitoring data, and expert opinion. These values may change from among ecoregions within California. BURC thresholds for each biological response indicator can be converted to nutrient concentration targets appropriate for assessment, permitting, and TMDLs by using simple load-response models or more complex dynamic simulation models for biological responses estuaries. Depending on the use, data availability, and economic impact of the decision, other, more detailed and site-specific tools may be appropriate for translating secondary indicator targets to nutrient loading targets.