Challenges for Implementation of Copper Aquatic Life Criteria Using the Biotic Ligand Model: What are We Waiting For?

The USEPA released their latest national guidance for development of copper aquatic life criteria in 2007. These freshwater criteria are derived using the Biotic Ligand Model (BLM) as a replacement for the hardness-based criteria equations currently used in water quality standards by almost all states. The BLM is a computational model that incorporates chemical reaction equations with the binding of metals to organism tissues responsible for causing toxicity (termed the “biotic ligand”, such as a fish gill) to better represent the complex chemical factors that influence copper bioavailability. The BLM generates instantaneous freshwater criteria (acute and chronic) using 10 water quality input parameters: temperature, pH, and concentrations of dissolved organic carbon (DOC), calcium, magnesium, sodium, potassium, sulfate, chloride, and alkalinity. The BLM represents a significant scientific advancement that derives more accurate criteria which are much less likely to be significantly over- or under-protective of aquatic life. BLMs have also been developed for several other metals (e.g., nickel, zinc, and silver); these models will likely form the basis of criteria for metals other than copper in the future. However, only one state has formally adopted the BLM for use in setting water quality standards; a few other states have adopted the BLM as a tool for generating site-specific standards. We will explore the scientific and regulatory challenges that may be forming barriers to adopting BLM-based copper criteria. Scientific challenges include the potentially limited availability of data for all of the BLM parameters and variability in BLM parameter concentrations over time. This temporal variability in water quality is not unique to BLM parameters, but 10 parameters vary in more complex ways compared to hardness. Thus, successful implementation will ultimately require decisions to be made as to the number and location of water quality samples needed to adequately characterize a particular water body. Regulatory challenges include educating stakeholders on the basis of BLM-based criteria, the need for further development and review of implementation guidelines, and addressing developing concerns over the protection of salmonid fishes from copper-induced olfactory impairment. We conclude that these challenges can be met and recommend that states move toward implementation of BLM-based criteria to improve the accuracy of aquatic life protection from copper, thereby minimizing unnecessary 303(d) listings and the need for Total Maximum Daily Loads (TMDLs) where copper impairment is unlikely.