Enhancing sediment toxicity assessments: Integrating bioavailability metrics with sediment effect concentrations for...

Sediments act as both sinks and sources for contaminants, particularly heavy metals like cadmium (Cd), posing risks to aquatic ecosystems. To evaluate sediment toxicity, sediment quality guidelines (SQGs) such as Sediment Effect Concentrations (SECs) have been widely applied. However, these methods often fail to resolve toxicity in the "Uncertain" range, where contaminant concentrations approach threshold values. This limitation arises from the reliance on bulk sediment concentrations, which do not account for bioavailability. This study integrates bioavailability-focused evaluation using Interstitial Water Toxic Units (IWTU) with traditional SEC frameworks to enhance sediment toxicity assessments. Using SECs alone, the toxicity thresholds were 0.09 mg/kg for long-term ecological safety (Consensus Level 1) and 0.36 mg/kg for benthic communities (Consensus Level 2). However, predictability, defined as the ability to correctly classify sediments correctly based on bioassay results, was limited to 43 % due to ambiguity in the "grey area" between these thresholds. To address this, aqueous phase Cd concentrations, derived from a partitioning coefficient model, were normalized to the USEPA Final Chronic Value (FCV) of 0.72 μg/L to calculate IWTU values. This tiered approach integrates SECs with bioavailability-focused assessments, improving predictability to 76 % by refining classifications in the "Uncertain" category. The proposed framework effectively combines the ecological breadth of SECs with the precision of IWTU, addressing the limitations of single-method approaches. These findings underscore the importance of bioavailability-focused metrics in enhancing the reliability of sediment toxicity assessments and management practices.