Derivation of environmental quality standards for free cyanide incorporating censored data into species sensitivity distributions.

Free cyanide is considered to be the most toxic form of cyanides to aquatic life. Due to its broad range of uses and subsequent potential widespread emissions to surface water, the environmental effects of free cyanide have been extensively researched. Regulatory bodies have proposed water quality standards for free cyanide, but these are regularly debated and implementation has been inconsistent due to monitoring challenges. The aim of the present study was therefore to derive new environmental quality standards (EQS) for free cyanide according to the Water Framework Directive (WFD). Ecotoxicity data from previous derivations and an additional literature search were gathered and individually (re) evaluated on reliability. The pooled acute ecotoxicity dataset consisted of reliable results for 35 species, distributed over 8 taxonomic groups. The pooled chronic ecotoxicity dataset consisted of results for 13 species, distributed over 7 taxonomic groups. WFD criteria for deriving a species sensitivity distribution (SSD) were met, if censored data points were included. Using the R-package ETX 3.0, an SSD including censored data was constructed and acute and chronic HC values of 17 and 0.66 μg/L, respectively, were derived. Comparisons were made with alternative SSDs constructed by transforming or discarding the censored data. Applying a default assessment factor (AF) of 10 to the HC from the acute SSD resulted in a MAC-EQS of 1.7 μg CN/L for freshwater and marine water. Careful consideration was given to addressing the uncertainty around the chronic HC value for the selection of an AF of 3, resulting in an AA-EQS of 0.22 μg CN/L for freshwater and 0.044 μg CN/L for marine water by applying an additional AF of 5. It is concluded that the current environmental quality standards for free cyanide are the first to be derived according to the WFD guidance, using only reliable data and including censored values.