Evaluation of the biotic ligand model for the development of benthic toxicity thresholds and remedial goals for metals at sediment management sites

A combined risk assessment and risk management framework is proposed for application of a multimetal biotic ligand model (mBLM) to develop bulk sediment remedial goals for metals in sediments. Sediment chemistry and toxicity data from five freshwater sediment study areas were compiled to illustrate how this framework could be applied. Based on these data, the mBLM accurately predicted toxicity to Hyalella azteca in 85.3% of the samples with respect to H. azteca 20% effect concentrations (EC20s) and in 75.1% of the samples with respect to derived final chronic values (FCVs). The sum of porewater toxic units derived using the mBLM was converted to a bulk sediment remedial goal concentration (CBSRG) using empirical partitioning relationships between metals in sediments and porewater. A median of the CBSRG values in samples with mBLM-FCV ∑ toxic units (TUs) >1 was used as a first approximation of an overall bulk sediment remedial action level (RAL) to identify sediments most in need of remedial action based on the presence of bioavailable metals in porewater in each study area. Bulk sediment RALs ultimately reflected the relative bioavailability of metals in porewater. Where metal porewater bioavailability was relatively low, RALs were higher than bulk sediment remedial goals. Conversely, where metal porewater bioavailability was relatively high, RALs were lower than bulk sediment remedial goals. There is more uncertainty where porewater metal bioavailability was not the primary driver of toxicity, as evidenced by low mBLM TU, and a high percentage of false-negative toxicity predictions. We conclude that for sediments in which porewater metal bioavailability is the primary contributor to benthic toxicity, porewater-based remedial goals derived using the mBLM can be used to derive bulk sediment remedial goals that are practical for making site remediation decisions yet still reflect metal bioavailability in porewater.