United States
Environmental Protection
\r ^1	Agency
Office of Water
EPA 822-F-21-005
August 2021
Fact Sheet: Ambient Water Quality Criteria to Address Nutrient
Pollution in Lakes and Reservoirs
Summary
EPA has published final recommended ambient
water quality criteria to address nutrient pollution in
lakes and reservoirs under Section 304(a) of the
Clean Water Act (CWA). These criteria serve as an
important resource in EPA's ongoing efforts to
support states and authorized tribes in developing
and adopting numeric nutrient criteria into their
water quality standards to protect public health and
aquatic life from the adverse effects of nutrient
pollution in surface waters, including harmful algal
blooms.
States and authorized tribes can adopt these criteria
into their water quality standards or can adopt other
scientifically defensible nutrient criteria. These final
criteria are not a regulation, nor do they impose a
legally binding requirement. These criteria provide
information to develop science-based standards that
are protective of drinking water, recreational, and
aquatic life designated uses of lakes and reservoirs
against the adverse effects of nutrient pollution.
Background
Numeric nutrient criteria provide important tools for
states and authorized tribes to use when managing
the effects of nutrient pollution by providing clear
targets that support the protection and maintenance
of the designated uses of their waters.
In 2000 and 2001, EPA published recommended
numeric nutrient criteria for most lakes and
reservoirs. Since then, scientific understanding of the
relationships between nutrient concentrations and
harmful effects in lakes and reservoirs has increased,
and new data collected from lakes and reservoirs
across the United States have become available.
These new recommended criteria replace the
recommended criteria published in 2000 and 2001
and reflect the latest scientific knowledge, in
accordance with the provisions of CWA Section
304(a).
What is nutrient pollution?
Nutrient pollution is a widespread and costly
environmental and public health challenge. Excess
nitrogen and phosphorus in our waterways degrade
water quality, feed harmful algal blooms, affect
drinking water sources, increase public health risks,
and contribute to costly impacts on drinking water
treatment, recreation, tourism, and fisheries. The
frequency of harmful algal blooms appears to be
increasing, possibly due to interactions between
climate change and nutrient pollution.
What are EPA's recommended nutrient
criteria?
EPA's recommended ambient water quality criteria
for lakes and reservoirs consist of linked models that
states and authorized tribes may use to identify
protective candidate numeric criteria for total
nitrogen and total phosphorus to protect three
designated uses (aquatic life, recreation, and
drinking water supply). These candidate criteria
(consisting of magnitudes generated by the models
and associated duration and frequency components)
can be adopted into state or authorized tribal water
quality standards regulations. For water bodies with
multiple use designations, states and authorized
tribes need to identify candidate criteria to protect
each applicable designated use and then adopt the
candidate criteria that protect the most sensitive
use.

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How were these criteria developed?
The ecological and health protective responses on
which the criterion models are based were selected
by applying a risk assessment approach to explicitly
link nutrient concentrations to the protection of
aquatic life, recreation, and drinking water source
protection designated uses. Statistical stressor-
response models were then used to relate the
ecological and health protective responses to
concentrations of nitrogen and phosphorus. The
models are based on previously published EPA
technical guidance, as well as scientific peer-
reviewed statistical and modeling techniques.
These criteria provide a flexible approach
States and authorized tribes can use the provided
criterion models that are based on national data or
can incorporate local data into the national models
to help develop numeric nutrient criteria that are
protective of designated uses and consistent with
national relationships, while accounting for unique
local conditions.
Modeling expertise is not needed to use the
models
Because EPA has already developed the criterion
models and provided interactive software
applications to apply the models, states and
authorized tribes do not need to develop their own
models. However, states and authorized tribes that
use these models to derive criteria should
understand the science underlying the models. This
will help them correctly specify the parameters
needed to run the models and communicate the
choices they made to derive protective criteria to
stakeholders and EPA.
If states or authorized tribes have local or state level
data and want to use them to derive criteria
reflective of those conditions, some additional
knowledge of stressor-response analyses will be
needed.
How do these criteria affect existing state
nutrient criteria?
The publication of this guidance does not invalidate
criteria previously approved by EPA as effective for
CWA purposes, nor does it compel a state or
authorized tribe to revise current EPA approved and
adopted criteria, Total Maximum Daily Load nutrient
load targets, or nitrogen or phosphorus numeric
values established by other scientifically defensible
methods. EPA encourages states and authorized
tribes to take the opportunity to consider this new
information and determine if revisions are
warranted as part of the state or authorized tribe's
triennial review of water quality standards. States
and authorized tribes could consider using the
recommendations as an alternative to, or as a
supplement for, other scientifically defensible
approaches.
Help is available
EPA stands ready to assist states and authorized
tribes to add their data into the models through the
Nutrient Scientific Technical Exchange Partnership &
Support (N-STEPS) program. For more information,
please see the N-STEPS website at
https://www.epa.gov/nutrient-policy-data/n-steps-
program.
Where can I find more information?
To view the final recommended criteria and for more
information, please visit EPA's website at
https://www.epa.gov/nutrient-policy-
data/technical-support-numeric-nutrient-water-
quality-criteria-development or email Lester Yuan at
yuan.lester@epa.gov.

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