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5F - Probabilistic approaches (MOP/195) A new technique for estimating thresholds of toxicity in ecological risk assessment. Hanson, Mark1, Solomon, Keith1, 1 Centre for Toxicology, University of Guelph, Guelph, Ontario, Canada ABSTRACT- The use and utility of the no observed effect concentration (NOEC) in ecological risk assessment is a contentious issue. A new method has been developed to estimate the threshold of toxicity, or a true NOEC, for aquatic plants. The method involves determining the effective concentration (ECx) of a number of endpoints from one species. These ECx values are plotted on a log-probability scale. The x-intercept, or a low-centile, of the distribution can be interpreted as the threshold of toxicity for that plant at that response level. This threshold is the concentration at which no effects should be observed for any endpoint above that response level. It is based on the assumptions that multiple effect measures from a single species will be log-normally distributed and that the distribution contains all possible endpoints for that species. The thresholds and the distributions can then be used as a substitute for the NOEC or ECx in risk assessment techniques, such as hazard quotients and probabilistic ecological risk assessment. This new method of estimating toxicity thresholds is more realistic than the use of arbitrary uncertainty factors, is more conservative than current probabilistic risk assessment methods, allows for simple comparison between species and exposure duration to a toxicant, and maybe useful for assessing mixture toxicity. This technique was applied to laboratory and field toxicity data with aquatic plants to assess potential risks from haloacetic acids. Using this new risk assessment method, we conclude that HAAs do not appear to pose a risk to aquatic macrophytes at current environmental concentrations under field or laboratory conditions. Key words: toxicity thresholds, probabilistic ecological risk assessment, hazard quotients, NOEC |
