In the United States, conflicts can arise between the processes to derive aquatic life criteria (ALC) for chemicals under the Clean Water Act (CWA) and the evaluation procedures used in Endangered Species Act (ESA) consultations on the protectiveness of those criteria for protected species. This paper examines the roots of, and possible reconciliation of, one longstanding conflict over cadmium aquatic life criteria in California. This case study includes 1) an overview of occurrences of cadmium in the aquatic environment, 2) factors affecting toxicity of cadmium to aquatic life, 3) a contrast between the analytical procedures of CWA aquatic life criteria derivation and ESA consultation, 4) quantitative estimates of no-effect concentrations of cadmium for 44 ESA listed species in comparison with updated aquatic life criteria, and 5) concludes with suggestions to update California’s aquatic life criteria for cadmium that would be more protective of sensitive ESA listed species.

A root cause of conflict is the different levels of biological organization that are the focus of CWA and ESA procedures. The CWA ALC are intended to protect diverse ecosystems by protecting at least 95% of the species richness in communities, allowing that it is acceptable for some species in the residual most sensitive 5% of the community richness to be harmed or even locally extirpated so long as they are not societally important species. The ESA is charged with minimizing harm to individual organisms and disallows increasing risk of extinction or impeding recovery of protected species. With cadmium in California, these procedures converge because some of the most sensitive species to cadmium happen to be surrogates for protected species (acute responses of steelhead/rainbow trout, Oncorhynchus mykiss, and chronic responses of threespine stickleback, Gasterosteus aculeatus). The present review concludes that while the superseded 1996 cadmium criteria versions would not be fully protective for up to half of the 44 ESA listed aquatic species in California, the updated 2016 versions would be more protective. Still, the review shows that the updated acute criteria would only fully protect the less sensitive half of the distribution of data for steelhead/rainbow trout sensitivity to cadmium, and the chronic criterion still would not protect the listed threespine stickleback. With a data rich species such as rainbow trout, instead of defining acute criteria using a central tendency statistic such as the geometric mean of multiple test responses, using a lower statistic such as the 10th percentile would ensure that the vast majority of a sensitive, protected species (and all less sensitive species) would be protected. Available data for the stickleback indicate it may be highly sensitive to cadmium, but no threshold can be derived from existing data. Additional testing with cadmium and stickleback would be needed to suggest an alternative, quantitative approach.