Evaluation of time-to-effects as a basis for quantifying the toxicity of contaminated sediments

Due to uncertainties as to appropriate procedures and dilution materials, most sediment tests are conducted only with undiluted, whole samples. Hence, it is not possible to use conventional concentration-response approaches to quantify toxicity of samples that elicit a 100% effect (e.g., mortality) at a preset test interval (typically 10 d). An alternative approach to quantifying the relative toxicity of test sediments is to determine time-to-effects. The objective of this study was to assess the utility of a time-to-effects approach for quantifying toxicity of freshwater sediments to the invertebrates Hyalella azteca and Chironomus tentans. Survival of both species and growth of C. tentans was determined using five sediments (four test samples and a control sediment) by destructively sampling replicate test chambers over the course of a "standard" 10-d assay. Studies with the control sediment and a non-toxic test sample indicated excellent recovery of test animals, even early in the test (e.g., <24 h) when individuals of both species are relatively small. Reasonable, typically monotonic, time-to-death relationships were observed for both H. azteca and C. tentans exposed to three comparatively toxic test sediments, all of which caused significant mortality by 10 d. Use of the time-to-effects approach allowed expression of toxicity of the three samples relative to one another, as well as documentation of decreases in toxicity of one of the sediments with increased storage time. These studies demonstrate the feasibility of use of time-to-effects as a basis for quantifying the relative toxicity of contaminated sediments.