United States
Environmental Protection Agency
ice of Water
Mail Code 4305T
PA-820-F-13-039
September 2013
Guiding Principles on an Optional Approach for
Developing and Implementing a Numeric Nutrient Criterion
that Integrates Causal and Response Parameters
Purpose
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.
Disclaimer
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.
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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
why.
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.
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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
clear:
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
uses.
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.
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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
criterion.
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.
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