Evidence from whole-sediment, porewater, and elutriate testing in toxicity assessment of contaminated sediments

Bioassays with microorganisms are widely used as tools in estimating the potential risk of contaminated sediments. Although whole-sediment testing seems the most realistic approach to evaluate the bioavailability of contaminants in sediments, porewaters and aqueous extracts are frequently used. The object of this study was to compare whole sediment, porewater, and elutriate exposure of contaminated sediment simultaneously with two bacterial bioassays. Artificial sediment was spiked with five chemicals of different chemical and physical properties as single substances and as a mixture of all substances. Depending on the quality of the toxicants the test organisms were affected via aqueous phase (elutriate or porewater) or via solid phase (whole sediment). Sediment quality criteria based on the distribution of chemicals between sediment and aqueous phase and assessment of the toxicity of a sediment with water quality criteria were not confirmed by this study. The DHA solid-phase assay illustrated that threshold concentrations based on the NOEC values of single chemicals are not always sufficient to assess the risk of polluted sediments for organisms. It is suggested that for the evaluation of the potential hazard of a sediment contaminated with an unknown mixture of chemicals all exposure routes of contaminants should be examined by elutriate, porewater, and whole-sediment testing.