Use of SSDs in the USA – endangered species and water quality criteria
Environmental Protection Agency, USA
Species sensitivity distributions are used in the United States (US) in the development of national ambient water quality criteria (AWQC), with site-specific and numeric modifications to protect sensitive taxa including threatened and endangered species. The US Environmental Protection Agency (EPA) first used SSDs constructed of acute toxicity values in 1978, with formal guidance issued in 1985 for computing 5th percentile hazard concentrations (HC5) from SSDs constructed of at least 8 families with acceptable toxicity data. Additional minimum data requirements (MDRs) include acceptance of only North American species and specific taxa diversity requirements that have limited the development of AWQC to only 47 chemicals. EPA is currently considering alternative approaches for developing SSD-based AWQC, with the recognition that species composition appears to affect HC5 estimates for aquatic species more than differences in geography or habitat of the assemblage. The protectiveness of SSD hazard concentrations used in endangered species risk assessment remains a concern because of uncertainty in sensitivity compared to standard test species. In a recent study, the relative sensitivities of US federally listed and non-listed aquatic species were compared for a broad range of chemicals. The SSD, HC5s and HC1s were lower than 97 and 99.5% of all endangered species mean acute LC50s, indicating that the use of SSDs as distribution-based risk assessment and criteria development approaches can be generally protective of listed species. A recent US National Academy of Sciences report suggested SSDs should be applied in endangered species risk assessments as an alternative to general uncertainty factors. This presentation gave an overview of US applications of SSDs in AWQC development and listed species assessment, and included perspectives on modifying MDRs and adopting new approaches to meet taxa diversity requirements.