How Protective of Designated Use are Nutrient Criteria?

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)