A method for the toxicity assessment of wet weather events

The variable nature of wet weather flow events has made it difficult to assess the toxicity of these events with conventional toxicity testing methods. This paper proposes a test protocol that includes an event-focused toxicity test method, a corresponding test metric that more accurately represents the toxicity of brief exposures, and an event-based index that describes the acute toxicity of wet weather events. This testing protocol is designed to quantify toxic response produced by brief contaminant exposures (<24 h), as well as time-varying contaminant exposures, which are typical of wet weather events. The protocol describes the use of an event-focused toxicity test to determine a PE-LET50 (post-exposure lethal exposure time for 50% of the population) metric for individual event samples. The PE-LET50 is a new metric that represents the exposure duration to a sample necessary to produce a 50% response during a post-exposure observation period. The protocol then describes the development of an index that integrates the toxicity of event samples (PE-LET50s) into a measure of total event toxicity. This index is the event toxicity unit (ETU). The development and use of the test protocol is described and results are provided from single and multiple sequential exposures to reference toxicants and from two wet weather discharge events.