Strategies to Address Site-Specific Instream Metals Criteria Development


  • Mary E. Sadler, PE, Saya Qualls, PE - Hazen and Sawyer

A water quality criterion for aquatic life is the level of a pollutant or other measurable parameter that allows for protection of aquatic life in water. A criterion value is the highest concentration of a pollutant in water that is not expected to pose a significant risk to the majority of species in a given environment. The Environmental Protection Agency’s (EPA) national criteria is based on laboratory tests using Ceriodaphnia, Northern Squawfish, and Gammarus species. In the 1980’s, EPA national criterion were expressed as total recoverable metals. In the early 1990s, EPA recommend that water quality criteria be applied as a dissolved metal in lieu of a total metal. Compliance with a dissolved metal criterion is based on EPA derived conversion factors, which represent the fraction of the total recoverable metal that is dissolved. Acute and chronic hardness-dependent conversion factors were developed to convert laboratory-derived total recoverable criteria to dissolved criteria. However, several of EPA’s conversion factors are close to a one to one relationship. Mathematically, a higher percentage of the total recoverable metal is therefore considered to be in the dissolved form.

The current water quality standards in North Carolina are based on total recoverable metals. The Division of Water Resources (DWR) is replacing the total recoverable metals standards with dissolved metal standards. As such, there are many utilities in North Carolina that will be faced with the possibility of having to perform a site-specific metals study to comply with the new application of the dissolved metals criteria. A site-specific study determines a new criterion specific to the stream characteristics at the point of discharge. EPA has approved multiple methods for these studies: Recalculation Procedure with resident species, the Translator Method (e.g., directly measured, linear partitioning, and calculated using EPA coefficients and suspended solids), and the Water Effect Ratio (WER) Method. The WER Method has demonstrated the best results for positive outcomes on site-specific limits for copper and zinc.

This paper will provide an overview of the National Water Quality Criteria Development and an explanation of the various EPA-approved approaches for instream allowable and/or site-specific studies. A discussion of the advantages and disadvantages of each method will also be discussed.

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