The U.S.
Environmental Protection Agency has recommended an ecoregion-based national
strategy for establishing nutrient criteria. The importance of nutrient criteria
is evident from the Clean Water Act’s required listing of impaired waters under
Section 303(d); state water quality standard violations due to nutrient
overenrichment are a leading cause of surface water impairment. Clearly, a
sound scientific basis is needed for the many costly total maximum daily loads (TMDLs)
that will be required.
Eutrophication-related
water quality standards and criteria already widely exist. For example, most
states have dissolved oxygen criteria intended to be protective of designated
uses that are impacted by oxygen depletion, resulting from nutrient-enhanced algal
production. Additionally, some states have adopted nutrient or chlorophyll criteria;
for example, North Carolina has a chlorophyll a criterion of 40 ug/l.
However, criteria like the North Carolina chlorophyll criterion were set years
ago using informal judgment-based determinations; the EPA’s new strategy reflects
a recognition that more analytic rigor is needed given the consequences of TMDL
decisions.
State water
quality standards are established in accordance with Section 303(c) of the
Clean Water Act and must include a designated-use statement and one or more water
quality criteria. The criteria serve as measurable surrogates for the narrative
designated use; in other words, measurement of the criteria provides an
indication of attainment of the designated use. Additionally, violation of the
criteria is a basis for regulatory enforcement, which typically requires establishment
of a TMDL. Thus, good criteria should be easily measurable and good predictors
of the attainment of designated use.
This latter
basis for criteria selection – that they must be good predictors of the
attainment of designated uses, is the motivation for the analysis described in
Reckhow (2005). I believe that the best criterion for eutrophication-related
designated use is a measurable water quality characteristic that is also the
best designated use predictor. In addition, I believe that there are
alternative and arguably better ways to define the criterion level than through
reference to least impacted waterbodies expected to be in attainment of designated
use. Rather, because it is an enforceable surrogate for designated-use
attainment, the level of the criterion should be chosen on the basis of
societal values, which should reflect the realities of society’s tradeoffs between
environmental protection and cost. Beyond that, selection of the level of the
criterion should realistically take into consideration natural variability and
uncertainty in predicting water quality outcomes, both of which imply that 100%
attainment in space/time is not a realistic basis for a water quality standard.
Designated
uses evolved from the goals of the Clean Water Act. As part of the water quality
standard for a regulated water body, they are typically expressed as brief
narrative statements listing the uses that the waterbody is intended to support,
such as drinking water, contact recreation, and aquatic life. Water quality
criteria must then be chosen as measurable quantities that provide an
indication of attainment of the designated use. Finally a criterion level (and possibly
the frequency and duration) must be selected as the cutoff point for
nonattainment.
Traditionally,
the task of setting criteria has involved judgments by government and university
scientists concerning the selection of specific water quality characteristics
and the levels of those characteristics that are associated with the designated
use. For example, consider the North Carolina chlorophyll a criterion of
40 ug/l, which was established in 1979. This criterion applies to Class
C waters, which are freshwaters with use designations of secondary recreation, fishing,
and aquatic life support. To establish this criterion, the NC Division of
Environmental Management examined the scientific literature on eutrophication
and then recommended a chlorophyll criterion level of 50 ug/l to a panel
of scientists for consideration. After reviewing a study of nutrient enrichment
in 69 North Carolina lakes, the panel responded that 40 ug/l reflected a
transition to algal, macrophyte, and DO problems and thus represented a better choice.
Following public hearings, 40 ug/l was adopted as the chlorophyll water
quality criterion. The 40 ug/l criterion developed from an ad hoc
process of science-based expert judgment. In my view, we should avoid selecting
a criterion level simply because it represents a change/transition point in
waterbody response (e.g., transition to algal, macrophyte, and DO problems).
The criterion level should also reflect public values on designated use; good water
quality criteria selection is not strictly a scientific endeavor.
The current
U.S. EPA approach for nutrient criteria development is a similar mix of science
and expert-judgment. In 1998, the President’s Clean Water Action Plan directed the EPA to
develop a national strategy for establishing nutrient criteria. The resultant
multiyear study produced a set of documents and recommended criteria based on
ecoregions and waterbody type. Specific modeling methodologies were proposed to
aid in the extrapolation of reference conditions and to assist managers in
setting loading allowances once nutrient criteria have been established. In addition,
enforcement levels for the proposed criteria were based on “reference
waterbodies” perceived to reflect essentially unimpacted or minimally-impacted
conditions.
In
principle, standard setting should be viewed from the perspective of decision
making under uncertainty, involving interplay between science and public
opinion. The determination of designated uses reflects public values, both in the
statements in the Clean Water Act and in the waterbody-specific statement of
designated use. The selection of the criterion is a choice based largely on
science. Selection of a good criterion, one that is easily and reliably
measured and is a good indicator of designated use, is largely a scientific determination.
However, determination of the level
of the criterion associated with the attainment-nonattainment transition ideally
requires the integration of science and values. Natural variability and
scientific uncertainty in the relationship between the criterion and the
designated use imply that selection of a criterion level with 100% assurance of
use attainment is generally unrealistic. Accordingly, scientific uncertainty
and attitude toward risk of nonattainment should be part of the criterion level
decision. Therefore, the decision on a criterion level might be addressed by
answering the following question: Acknowledging that 100% attainment is impractical
for most criteria, what probability (or, perhaps, what percentage of
space-time) of nonattainment is acceptable? EPA guidance
addresses this question by suggesting that 10% of samples may violate a
criterion before a waterbody is listed as not fully supporting the designated
use. Analytically, this question may be answered by integrating the probability
of use attainment (for a given criterion level) and a utility function
reflecting water quality costs and benefits. The criterion level associated with
the highest expected utility might then be chosen. Realistically, this decision
analytic framework is prescriptive; it guides us toward what ought to be done,
but it almost certainly exceeds what actually will be done.
An
additional consideration that was discussed in NRC (2001) is where in the
causal chain from pollutant source to designated use should a water quality
criterion be placed? Referring to the figure (taken from NRC 2001), the
NRC panel recommended that the preferred “location” should be in the “human
health and biological condition” box. If instead, the pollutant loading or
waterbody pollutant concentration box was selected, there would be additional
hidden uncertainty in the causal chain (in the figure) to designated use. This
hidden uncertainty can be reduced by selection of a criterion as close as
possible to designated use.
In Reckhow
et al. (2005), we addressed the process of numeric water quality criteria
setting from the prescriptive basis that criteria should be predictive of
designated use and from the pragmatic basis that risk of nonattainment should
be acknowledged and therefore considered when setting a level or concentration.
Thus, from a prescriptive standpoint, a good criterion should be an easily
measurable surrogate for the narrative designated use and should serve as an
accurate predictor of attainment. Correspondingly, from a pragmatic
perspective, natural variability and criterion-use prediction uncertainty will likely
result in some risk of nonattainment; thus the selection of a criterion level
for the attainment-nonattainment transition realistically should be based on an
acceptable probability of nonattainment. Furthermore, the selection of the
acceptable probability is a value judgment best left to policy makers informed
by scientists. To illustrate how this could be accomplished, Reckhow et al.
(2005) used structural equation modeling to quantify the relationship between
designated use and possible water quality criteria. This identified the best
predictor of designated use, which would become the water quality criterion.
This result can then be presented to decision makers for selection of the
criterion level associated with the
acceptable risk of nonattainment. Given the estimated number of nutrient-related
TMDLs required, and the costs/benefits of addressing these ambient water
quality standard violations, it is clear that the choice of water quality
criteria for eutrophication management and nutrient TMDLs has significant
consequences. Thus a rigorous procedure, like that described in Reckhow et al.
(2005), should be considered for establishment of nutrient criteria.
NRC. 2001. Assessing
the TMDL Approach to Water Quality Management; National AcademyPress:
Washington, D.C.
Reckhow, K.H. G.B. Arhonditsis, M.A.
Kenney, L. Hauser, J. Tribo, C. Wu, K.J. Elcock, L.J. Steinberg, C.A. Stow,
S.J. McBride. 2005. A Predictive
Approach to Nutrient Criteria. Environmental
Science and Technology. 39:2913-2919.
(https://www.researchgate.net/publication/7814574_A_predictive_approach_to_nutrient_criteria?ev=prf_pub)
