The two primary components of a water quality standard are
the designated (beneficial) use and the water quality criterion (criteria). The
criterion serves as an easily-measurable indicator of designated use attainment.
Thus an effective water quality standard must have a criterion that causally
relates to designated use, and a criterion level that best discriminates
between attainment and nonattainment of the designated use. All of this seems
self-evident.
An important consideration that generally is not considered
is the space/time for which designated use is relevant. An example of this is a
“swimmable” designated use when a waterbody is covered with ice. This
hypothetic example is obvious, but are there others that are not so obvious?
Yes, there are.
Consider a dendritic reservoir that has a nutrient TMDL and
the state agency is monitoring for compliance based on the water quality
criterion. In this case, the designated use is “swimmable and warm water
fishery” primarily for recreation. For this beneficial use, common water
quality criteria include dissolved oxygen and chlorophyll a.
If it is causally determined that the fishery responds to
chlorophyll levels in spring, but not to DO levels in winter, then the monitoring
design for compliance with these water quality criteria should explicitly
consider these temporal aspects. This might, for example, result in an intense
monitoring of spring chlorophyll and no winter monitoring of DO.
From a spatial perspective in this dendritic reservoir,
there may be reservoir segments (or discrete basins) where these designated uses
are irrelevant. For example, this might be the case for shallow embayments that
are subject to drought-related periods of surface area shrinkage and sediment
exposure. In locations where this is the case, water quality criteria monitoring
may be unnecessary; this also might be a situation where site-specific water
quality criteria should be considered for different segments of the reservoir.
