United States
                      Environmental Protection Agency
   ice of Water
Mail Code 4305T
September 2013
         Guiding Principles on an Optional Approach for
Developing and Implementing a Numeric Nutrient Criterion
         that Integrates  Causal and Response Parameters

     The purpose of these guiding principles is to offer clarity to states about an optional approach
     for developing a numeric nutrient criterion that integrates causal (nitrogen and phosphorus) and
     response parameters into one water quality standard (WQS). The EPA recognizes that
     developing numeric values for both nitrogen and phosphorus may present challenges
     associated with the temporal and spatial variability, as well as the ability to tie them to
     environmental outcomes. As the EPA's understanding of nutrient science progresses, these
     guiding principles may be updated or supplemented with additional information.

     States are, as always, encouraged to work closely with their EPA counterparts when developing
     numeric nutrient criteria, particularly if they are interested in following the approach outlined in
     these principles.


     These guiding principles do not impose legally binding requirements on the EPA, states, or the
     regulated community, nor do they confer legal rights or impose legal obligations upon any
     member of the public. The Clean Water Act (CWA) provisions and EPA regulations described in
     this document contain legally binding requirements. These guiding principles do not constitute a
     regulation, nor do they change or substitute for any CWA provision or EPA regulation.

     I.   Applicability

             1.  The integrated approach described in these guiding principles applies only to WQS
                for the nutrients nitrogen and phosphorus.
             2.  These guiding principles apply when states wish to rely on response parameters to
                indicate that a designated use is protected, even though  a nitrogen and/or
                phosphorus level is/are above an adopted threshold. If a state prefers to apply
                causal and response parameters independently, the principles in II.C will not apply.
             3.  States interested in this approach should have a biological assessment program
                that confidently measures biological responses and other nutrient-related response
                parameters through a robust monitoring program to account for spatial and
                temporal variability to document the effects of nutrient pollution. This will allow the
                state to have the capability to: a) identify shifts in multiple biological  assemblages
                (e.g., periphyton, benthic macroinvertebrates, fish) along a gradient of
                anthropogenic stress that can be tied to designated uses, and b) quantify the
                relationship  between nitrogen and phosphorus concentrations and measures of
                biological assemblage response.

II.   Criterion science and expression

     A. Protectiveness

        1.  A criterion must protect the designated use of the water, and states should clearly
            identify the use(s) they are seeking to protect. Where a criterion is intended to
            protect multiple designated uses, states must ensure that it protects the most
            sensitive one (40 CFR 131.11(a)).

        2.  Numeric values for all parameters must be set at levels that protect uses (i.e.,
            before adverse conditions that will require restoration).
        3.  In order to comply with 40 CFR 131.10(b), states must ensure that WQS provide
            for the attainment and maintenance of the WQS of downstream waters.

     B. Sound science rationale

        1.  Documentation supporting the criterion should identify all applicable nutrient
            pathways, addressing all potential direct and indirect effects (e.g., as identified in a
            conceptual model that outlines the effects of nutrient pollution). Documentation for
            the criterion should describe which pathways are and are not accounted for and
        2.  It is important to select biological response parameters that are consistent with the
            Agency's definition of assessment endpoints in the Ecological Risk Assessment
            Guidelines. Assessment endpoints should be relevant to management goals (e.g.,
            protect and maintain aquatic life) and should be sensitive to the stressor of interest
            (e.g., increased nitrogen and phosphorus concentrations). Appropriate biological
            response parameters will directly link nutrient concentrations to the protection of
            designated uses.
            Indicators that are most indicative of nutrient pollution in streams are intensively
            measured total phosphorus and total nitrogen, measures of primary productivity
            (e.g., benthic chlorophyll a, percent cover of macrophytes), measures of the algal
            assemblage  (e.g., algal assemblage indices), and measures of ecosystem function
            (e.g., continuously monitored pH and dissolved oxygen).1 On the other hand,
            reliance on higher trophic level  indicators designed to measure general biological
            condition (fish or invertebrates) may not be adequately sensitive or diagnostic of
            nutrient pollution. Therefore, these general higher trophic level indicators may be
            used in a suite of response variables but should not  be the predominant or sole
            indicator of nutrient pollution.
            The EPA recommends the use of one or multiple of these ideal response  indicators
            when deriving a combined criterion. This criterion should demonstrate the
            sensitivity of the response indicator(s) to increased nutrient concentrations and
            quantify how these nutrient-response linkages will achieve the goal of protecting
            and maintaining aquatic communities.

            Appropriate type and quantity of response parameters may vary by state,
            ecosystem, and waterbody type.

        3.  It is important to have sufficient data to allow the development of quantitative
            relationships (e.g., via regression models). Sufficient data can also inform the
1This conclusion resulted from a 2013 EPA-hosted scientific workshop held in Washington, DC.

            selection of reference sites for deriving a criterion using the reference condition
            approach and calibration of mechanistic models.

        4.  States should clearly and thoroughly document in their WQS (or supporting
            documentation)—for public review and submission to the EPA—how the criterion
            was developed and the technical aspects of their biological assessment protocols
            (including the assessment endpoints). This will ensure reproducibility,
            transparency, and defensibility. (See 40 CFR 131.6(b), 131.20(b)).2

     C. Expression of the Criterion

        1.  In order to ensure that states evaluate causal and response components as one
            standard when determining whether a segment is meeting any applicable WQS for
            purposes of CWA §303(d)(1)(A) and 40 CFR  130.7, causal and response
            parameters must be combined into one criterion.

        2.  All causal and response parameters should be expressed numerically.
        3.  Duration and frequency components for all parameters should be included in the
            criterion in the state's WQS.

        4.  The criterion should be expressed in a way that clearly establishes the water
            quality goal that applies for permitting, assessment/listing,  and total maximum daily
            load (TMDL) decisions. However, the criterion should not include provisions or
            conditions (e.g., minimum sample size) that restrict its use for any CWA
            implementing program, including permitting, assessment/listing, or TMDL activities.

            If a state identifies a scientifically defensible range of numeric values for the
            response parameters above which impairment of designated uses is known and
            below which designated uses are protected, the state should transparently identify
            and include as part of the criterion the decision framework it will use when
            waterbody conditions are within that range.
        5.  The criterion should be constructed in a way that integrates causal parameters and
            a suite of response parameters; clearly states the desired ambient condition of, or
            level of protection for, the waterbody; and allows for a transparent and reproducible
            assessment/listing decision. The criterion should make the following situations
            a.  If all causal and response parameters are met, then the water quality criterion
               is met and the waterbody is meeting its designated uses.
            b.  If all response parameters are met, but one or more of the causal parameters is
               exceeded, then the criterion is met and the waterbody is meeting its designated
            c.  If a causal parameter is exceeded and any applicable response parameter is
               exceeded, then the criterion is not met and the waterbody is not meeting its
               designated uses.
            d.  If a causal parameter is exceeded and data are unavailable for any applicable
               response parameters, then the criterion is not met and  the waterbody is not
               meeting its designated uses.
2 For example, the submittal should have sufficient detail so that the public and the EPA can understand and evaluate the
appropriateness of the metrics, QA/QC of data, thresholds for determining biological attainment/nonattainment, evaluation of
upstream/downstream conditions, and how trends in biological and chemical quality will be considered, when available.

           e.  If a causal parameter is not exceeded but an applicable response variable is
               exceeded, then the criterion is not met and the waterbody is not meeting its
               designated uses (in this scenario, further investigation may be warranted to
               determine if nutrient pollution is the cause).

III.  Implementation

     A. Section 303(d) Assessment and Listing

        1.  The CWA Section 303(d) assessment methodology should be consistent with the
        2.  CWA Section 303(d) requirement that states identify water quality-limited
           segments still requiring TMDLs where pollution controls are not stringent enough to
           implement any WQS still applies.

        3.  If a causal parameter is significantly exceeded but no response parameters are
           exceeded, then the state should pursue additional studies to determine whether
           site-specific criteria are appropriate.

        4.  States should have a process for monitoring response parameters downstream
           when assessing upstream conditions.

     B. Permitting

        1.  States should develop NPDES permitting implementation procedures to ensure a
           consistent application of the criterion.

        2.  NPDES permits must contain limits for any pollutants or pollutant parameters that
           are or may be discharged at levels that will cause, have reasonable potential to
           cause, or contribute to an excursion above any WQS. (40 CFR 122.44(d)(1)). Such
           limits must be sufficiently stringent to achieve all applicable WQSs. Under this
           approach, where reasonable potential exists,  permit writers must include limits in
           permits to achieve the WQS and, in doing so, should develop water quality-based
           effluent limits based on the numeric nutrient causal parameters.