Bioaccumulation and chronic toxicity of radium-226 to the freshwater bivalve Sphaerium striatinum (Sphaeriidae)

Abstract There are limited data on radium-226 (226Ra) activity concentrations that affect freshwater organisms. The chronic toxicity of 226Ra to the native freshwater fingernail clam Sphaerium striatinum was tested under controlled laboratory conditions for 28 days. Clams were exposed to reconstituted soft water dosed with seven nominal 226Ra activity concentrations (0.1, 0.5, 1.0, 5, 10, 25 and 50 Bq L−1) and a control (0 Bq L−1). Radium-226 activity concentrations decreased substantially between static renewals, with approximately 90% of the loss attributed to adsorption to the sand substrate. Consequently, time-weighted average (TWA) concentrations were used to estimate exposure. Tissue bioaccumulation (soft and shell) was measured. Results indicated a measurable uptake of 226Ra in clam tissues, increasing with exposure concentration, suggesting potential for trophic transfer and long-term retention. Radium-226 accumulated to a greater extent in shell (278–419 L kg−1) compared to soft tissues (70–180 L kg−1). Toxicity endpoints assessed included survival, burrowing behaviour, growth, and reproduction. Survival and burrowing behaviour were not significantly affected by 226Ra exposure. In contrast, growth and reproduction showed significant concentration-dependent reductions, with TWA effect concentrations (EC10) estimated at 4.1 and 0.37 Bq L−1, respectively. These findings provide new information on the sensitivity of freshwater bivalves to 226Ra exposure and support the conclusion that chronic effects of 226Ra might arise from bioaccumulation and sediment-associated exposure pathways in addition to direct waterborne toxicity. This study contributes to the limited but expanding body of evidence needed to inform water quality guidelines and risk assessments, which are essential for designing mitigation strategies to control radionuclide releases into freshwater ecosystems from mining activities.