Contaminated interstitial sediment is a reservoir of trace elements with exposure potential for freshwater mussels

The Powell River in Tennessee and Virginia, U.S.A. is a biodiversity hotspot for freshwater mussels, a highly imperiled faunal group in North America. Populations have been declining for several decades. Identification of the cause(s) of mussel decline is critical toward improving water quality and restoring populations in the river. We hypothesized that mussel populations may be suppressed because of exposure to toxic levels of trace elements in sediment pore water, a consequence of coal mining in the watershed. We measured concentrations of trace elements in water-column (surface) and substrate interstitial (pore) water seven times over one year at nine locations spanning 157 river kilometers. Concentrations of trace elements in filtered (0.45 μm) surface and pore water were low relative to water-quality criteria. In contrast, trace elements were higher in pore water containing interstitial fine sediment particles (< 300 μm), with all toxic elements except arsenic having potential to exceed their chronic criteria if they were in dissolved form. For example, lead and copper in some samples would be equivalent to a dissolved-ion exposure of 110- and 45-times their chronic criteria, respectively. Trace elements exhibited spatial variation but no significant (p < 0.05) longitudinal trends over the length of river studied, although concentrations were higher in river segments nearer coal-mining land use. Many trace element concentrations showed seasonal temporal variation, having statistically significant (p < 0.05) increasing trends through summer into autumn, concurrent with declining river discharge and increased accumulation of fine sediment. This temporal pattern is biologically important because it indicates peak trace-element exposure occurs when mussels are in critical life cycle phases – including reproduction, recruitment, and increased growth. Our findings underscore the need for consideration of pore-water and sediment-borne exposure as a critical pathway when assessing toxicity potential of trace elements to these sediment-dwelling and filter-feeding bivalves.