Predicting metal toxicity in sediments: a critique of current approaches

The ability to predict metal toxicity in sediments based on measurements of simple chemical parameters is not possible using currently available sediment-quality guidelines (SQGs). Past evaluations of available SQGs for metals indicated little difference in their predictive abilities; however, the scientific understanding of cause-effect relationships is progressing rapidly. Today, it is clear that they can be protective of benthic ecosystem health, but single-value SQGs will be ineffective for predicting the toxicity of metals in sediments. Recent exposure-effects models and the sediment biotic ligand model both indicate that a better approach would be to have SQG concentrations, or ranges, that are applied to different sediment types. This review indicates that significant improvements in laboratory and field-based measurements, better recording of parameters that influence metal toxicity in sediments, as well as quantification of the metal exposure routes and the relative contribution of dissolved and particulate sources to toxic effects are needed to improve the power of predictive models and the overall effectiveness of SQGs for metals. Simply exposing benthic organisms to contaminated sediments and reporting effects concentrations or thresholds based on particulate metal concentrations will provide little information to aid future SQG development. For all tests, careful measurement and reporting of concentrations of particulate metal-binding phases (e.g., sulfide, organic carbon, and iron phases), metal partitioning between porewater and sediments, and porewater pH are considered as minimum data requirements. When using metal-spiked sediments, much better efforts are required to achieve sediment properties that resemble those of naturally contaminated sediments. Our current understanding of metal toxicity indicates that considerably greater information requirements will be needed to predict sublethal and chronic effects of metals, because the toxic, metabolically available concentration of metals within an organism will fluctuate over time. Based on the review of exposure and effects models, along with improved measurement of metal exposure-related parameters, the measurement of the short-term uptake rate of metals into organisms is likely to improve future models.