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. Over the past several years, the USEPA has been
assisting the states to adopt/modify nutrient 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. In
addition, 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 indicators of the attainment of designated use.
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 was developed from an ad hoc process of science-based expert judgment.
In my view, we should be cautious in selecting a criterion level simply because
it represents a change/transition point in waterbody response (e.g., transition
to algal, macrophyte, and DO problems). For example, a DO level of zero is
clearly a major transition point for an aquatic ecosystem, but it is unlikely
to make sense as a water quality criterion protective of designated use. The
criterion level should also reflect public values on designated use; good water
quality criterion selection is not strictly a scientific endeavor.
In
Reckhow et al. (2005), structural equation modeling and
expert elicitation were used to quantitatively link candidate water quality
criteria with designated use. This technique was applied to Lake Washington, with designated uses that protect,
for example, salmon and
trout, primary contact recreation, domestic water supply,
wildlife habitat, commerce
and navigation, boating, and aesthetic values. Dr. Eugene
Welch, a Professor Emeritus
at the University of Washington, was chosen as the expert for this study.
Presented with Lake Washington’s
designated-use statement, Dr. Welch identified boating as the most
appropriate nutrient-related designated use to address based on his
technical expertise. He
selected water clarity, the absence of algal scums, odor,
and interference from
aquatic vegetation as desired properties of a “boatable” lake. In addition, the
expert provided a conceptual
model that included chlorophyll a, total phosphorus,
Secchi depth, total
zooplankton, and Daphnia biomass as the key environmental
variables for assessing attainment of the designated use. He
hypothesized that chlorophyll
a would be the water quality variable most closely
linked to the desirable
properties of a boatable lake.
Consider
the approaches of a few other states. In Florida lakes, a numeric chlorophyll
criterion has been set to protect designated use associated with “an imbalance
in the natural populations of the aquatic flora or fauna.” This is similar to
the North Carolina perspective. In addition to the chlorophyll criterion,
Florida also established criteria for nitrogen and phosphorus.
In Texas, PBS&J (2003) conducted a thorough and
thoughtful analysis of nutrient water quality standards in lakes in the Trinity
River Basin. They observed that “the (Trinity River Basin) study reservoirs are
heavily used for recreation, water supply, and support healthy aquatic life
communities. By that measure, all the reservoirs supported their designated
uses.” However, based on the chlorophyll water quality criterion, seven of the
nine reservoirs were not in compliance. Despite this inconsistency, PBS&J “determined
that chlorophyll a was the parameter most directly related to uses, and
that it should be the parameter selected for numerical criteria development.”
While recommending chlorophyll as the best nutrient
criterion, PBS&J pointed out the difficulty in selecting the specific
concentration of chlorophyll to serve as the compliance/noncompliance cutoff. The
figures below from the PBS&J report identify an “optimal range” for the
chlorophyll cutoff for the designated uses in the Trinity River Basin lakes. What
is most striking about these figures is the size of the optimal range and the
insensitivity of designated use to chlorophyll levels within this optimal range.
Why
is this so important? This insensitivity of designated use to chlorophyll
concentration is of particular concern in situations where attainment of a single-number
chlorophyll criterion is expected to be extremely expensive, with little
evidence of designated use improvement. In those situations, it is essential
that water quality improvements (or, designated use improvements/attainment)
justify the costs. This may be the situation in Falls Reservoir (North
Carolina) where, despite little evidence of designated use impairment, compliance
with the 40ug/l chlorophyll criterion is estimated to cost approximately $1
billion.
For much of
my career, I have been a strong supporter of chlorophyll a as the best nutrient
criterion for surface waters in the US. Since chlorophyll integrates the effect
of nitrogen and phosphorus, it effectively serves as a site specific indicator
of eutrophication. However, in my advocacy of chlorophyll as the best nutrient
criterion, I neglected the fact that chlorophyll is not highly correlated with
many common designated uses over the range of chlorophyll levels found in many
US lakes, as demonstrated in the PBS&J figures.
What is the
alternative? Dissolved oxygen is often a water quality criterion for nutrient
enrichment. However, DO can be quite variable in space and time, so monitoring
DO to assess designated use compliance poses challenges. It is an exaggeration
but worth noting that a user survey to assess designated use may be more
cumbersome than DO monitoring for compliance, but at least the user survey is a
direct assessment of what matters most.
Water clarity, typically measured with a Secchi disk (SD), is
another plausible alternative, as the SD depth is a good indicator of aesthetic
appeal. Perhaps, as in Chesapeake Bay, all three (chlorophyll, DO, and SD)
should be nutrient criteria, but I am not sure that this is sufficient. I think
that these three criteria need to be accompanied by a direct assessment of
designated use attainment in situations, like in Falls Reservoir, where
compliance costs are extremely high. This will help ensure that our limited
resources for environmental protection are wisely spent.
References
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.
PBS&J.
2003. Analysis of Use and Nutrient Data on Selected Reservoirs of the Trinity
River Basin. Austin, Texas.
