The U.S. Environmental Protection Agency’s (USEPA) current ambient water quality criteria (AWQC) for cyanide were developed in 1984. Since this time, additional research on cyanide toxicity and analytical chemistry has suggested that cyanide AWQC values need to be updated to reflect the current state-of-the-science and to ensure that the methods for implementing the cyanide AWQC reflect the bioavailable and toxic forms of cyanide. Therefore, the Water Environment Research Foundation (WERF) sponsored a reassessment of the AWQC for cyanide by initiating a thorough review of the current scientific literature on cyanide and conducting additional laboratory studies to further understand cyanide chemistry and toxicity. Consistent with the USEPA’s draft strategy for developing AWQC, this evaluation also considered whether the cyanide AWQC are protective of benthos, threatened and endangered (T&E) species, and aquatic-dependent wildlife. This reassessment of AWQC for cyanide resulted in proposed freshwater acute and chronic criteria of 23 and 4.8 μg CN/L, respectively, which are comparable to the current freshwater acute and chronic criteria of 22 and 5.2 μg CN/L (free cyanide). For saltwater organisms, the reassessment resulted in proposed acute and chronic criteria of 20 and 4.1 μg CN/L (free cyanide), respectively, compared to the current saltwater acute and chronic criteria which are both 1.0 μg CN/L. The difference in the current and proposed saltwater criteria values is attributed to recent cyanide toxicity testing with several crab species in the genus Cancer and the copepod Acartia tonsa, which suggests this genus is not as sensitive to cyanide as previous cyanide toxicity testing indicated. A sediment-based cyanide criterion does not appear warranted if it is assumed that benthic organisms are not inordinately more sensitive to cyanide than the water column organisms that drive the basis for the proposed freshwater and saltwater criteria (the available cyanide toxicity data do not suggest that they would be), and given that bioavailable forms of cyanide are not expected to accumulate appreciably in sediment. Cyanide should thus be effectively regulated based on water column-based criteria and concentrations. Based on a combination of available empirical and estimated cyanide toxicity values for T&E species, the proposed freshwater criteria appear to be protective of most T&E species, but any site-specific modifications that increase the cyanide AWQC for water bodies inhabited by T&E species is not recommended. Finally, the proposed freshwater AWQC for cyanide appear to be adequately protective of aquaticdependent wildlife.
The USEPA’s ambient water quality criteria (AWQC) for cyanide were developed in 1984 (USEPA 1985a). Since then, concerns have arisen that the AWQC for cyanide have been problematic to implement and may not accurately reflect either the toxic forms or bioavailable concentrations of cyanide in water, sediments, and tissues of aquatic organisms. Also, the cyanide criteria typically have been implemented based on total cyanide concentrations rather than the free cyanide concentrations that formed the basis of the criteria calculations (USEPA 1985a, 2002). New knowledge on the toxicity, speciation and measurement of cyanide, and the relative toxicity of bioavailable cyanide species suggest that a re-evaluation of the aquatic toxicity data and chemistry that serve as the basis of the current national criteria is warranted.
In response to these concerns, studies sponsored by the Water Environment Research Foundation (WERF) were undertaken to review and update our knowledge of the aquatic toxicity of cyanide, and to develop recommended updates to the existing national AWQC to enhance the scientific basis of the concentrations and chemical forms of cyanide specified for protection of aquatic organisms (Gensemer, et al. 2006a). This update was conducted according to USEPA guidance for derivation of AWQC for protection of aquatic life (USEPA 1985b), and is based both on an extensive literature review (Clark, et al. 2006, Gensemer, et al. 2006b), and on new toxicity studies conducted to fill key data gaps. Also, consistent with the USEPA’s Draft Strategy: Proposed Revisions to the “Guidelines for Deriving Numerical National Water Quality Criteria for the Protection of Aquatic Organisms and Their Uses”(USEPA 2003b), this re-evaluation of cyanide criteria includes consideration of impacts on benthic (sediment) organisms, wildlife that consume aquatic biota, and possible effects to threatened and endangered (T&E) species to ensure that revised criteria are adequately protective of the entire aquatic ecosystem. It must be emphasized that even though WERF has worked collaboratively with the USEPA on this project, these proposed revised cyanide criteria have not been officially reviewed by the USEPA. Therefore, the criteria recommended below do not necessarily reflect the USEPA’s position with regard to AWQC for cyanide.