Evaluating the Bioavailability of Metals in Sediments from the Upper Clark Fork River Basin, Montana

A sediment quality assessment was developed to evaluate the potential bioavailability of metals from riverbed sediments. The proposed approach consisted of multiple assessment methods using bulk sediment metals concentrations, equilibrium partitioning measures, and bulk sediment and pore water toxicity tests. Metals bioavailability was evaluated using: 1) EPA’s theoretical guidelines which compare acid-volatile sulfide concentrations to simultaneously extractable metals concentrations, and compare sediment pore water metals concentrations to ambient water quality criteria; 2) correlative guidelines, which compare bulk sediment metals concentrations against sediment no-effect concentrations (NECs), and 3) sediment toxicity tests. Sediment toxicity studies were also used to derive site-specific NEC values. The study site chosen for this assessment was the metals-enriched upper Clark Fork River (UCFR) located in southwestern Montana, USA. Results of this assessment indicate only nominal risk to most aquatic organisms posed by sediment metals concentrations in depositional areas in the UCFR. The uncertainty associated with using only one of the approaches for evaluating sediment contamination should be reduced by using the combined approach outlined here