Advantages of BLM-based copper freshwater quality criteria
The BLM-based copper freshwater quality criteria update the 2002 hardness-based criteria. While copper toxicity to aquatic life depends on many water quality factors, the previous criteria only considered the effect of hardness. As a result, the WER method—a time-consuming, costly biological assay approach—was often used to develop site-specific water quality criteria for dissolved copper. It is expected that the use of the BLM-based criteria will eliminate the need for costly WER approach.
Adopting the EPA recommendation
The EPA criteria recommendation is not a regulation and, therefore, does not impose legally binding requirements on the EPA, states, authorized tribes, or the regulated community. State and tribal decision makers have discretion to adopt approaches that differ from EPA's water quality criteria recommendations on a case-by-case basis. If a state or tribe opts to implement the BLM-based criteria, they might choose to do so incrementally or on a state- or tribe-wide basis.
Development of the BLM-based criteria
The BLM is based on the understanding that not all forms of copper cause equivalent degrees of toxicity. Some copper forms (for example, Cu²+, CuOH, and CuCl) are more bioavailable and therefore more toxic than copper bound to dissolved organic carbon. Bioavailable copper is copper that is capable of binding with sites in aquatic organisms, known as biotic ligands. The BLM takes into account the water quality parameters that influence the forms of copper present in the water body of interest. The BLM also considers that cations, such as Ca²+ and Mg²+, can compete with copper for biotic ligand sites and therefore mediate toxicity. The BLM is based on the assumption of chemical equilibrium, which may or may not be valid in the site water of interest.
Applying the model
The BLM is implemented using a Windows-based computer program available at http://www.epa.gov/waterscience/criteria/copper/2007/index.htm (BLM2007.exe file). Eleven input parameters are needed to run the BLM program: temperature, pH, dissolved organic carbon, calcium, magnesium, sodium, potassium, sulfate, chloride, alkalinity, and sulfide. The output of the BLM program includes both the criterion maximum concentration (CMC) and criterion continuous concentration (CCC) values. The program sets the CMC equal to the BLM-derived LC50 (also known as the final acute value) divided by two and the CCC equal to the BLM-derived LC50 divided by the final acute-chronic ratio (FACR). The freshwater FACR for copper is 3.22 (unitless). These criteria are applicable only to the specific input conditions used for the BLM program run. Because water quality in the site water of interest varies, the BLM program user is faced with identifying a single appropriate CMC and a single appropriate CCC despite seasonal and daily variation in water quality. EPA has not provided guidance on how to calculate site-specific criteria considering this variability.
Using the new criteria effectively
Whether the BLM would translate into less stringent copper effluent criteria, as compared with the hardness-based criteria, depends on the water quality characteristics of the receiving water. It is expected that the BLM-based criteria will be less stringent in low hardness waters, but possibly more stringent in harder waters. Therefore, wastewater treatment plants discharging into waters with low hardness, especially with high dissolved organic carbon, should consider performing a BLM and proposing alternative copper effluent limits as appropriate. A BLM-based procedure should be considered before a WER-based procedure due to the potential cost savings.
More information
The criteria document, as well as an EPA fact sheet and the Federal Register notice, can be downloaded at http://www.epa.gov/waterscience/criteria/copper/.