Regulatory Implementation of the Copper Biotic Ligand Model for Aquatic Life Protection: What Have We Learned and How Are We Doing?

The Biotic Ligand Model (BLM) has formed the numeric basis of the U.S. Environmental Protection Agency’s nationally recommended water quality criteria for copper in freshwater since 2007. Regulatory implementation by states and tribes is now advancing rapidly, with over half of states either allowing or considering use of the BLM and site-specific standards having been adopted in Colorado, Georgia, and Maine. However, many challenges for regulatory implementation remain owing to the additional complexity inherent in the use of the 10-parameter BLM. We believe that the additional complexity underlying the derivation of BLM-based aquatic life criteria does not prevent the use of existing water quality standards application frameworks developed historically on the basis of hardness relationships. This is because water quality standards, permits and 303(d) assessments would still involve solving a model-based criteria exceedance framework, similar to the older one-parameter model using hardness.