Meaningful Compliance Assessment of Water Quality Standards

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.

For effective water quality management, it is important that resources invested to meet water quality standards are efficiently utilized. This will more likely occur for standards in which the water quality criteria are causally-connected to the designated uses, and the space/time monitoring of the water quality criteria is designed to minimize irrelevant data collection and thus unambiguously assess compliance with the designated use.