Environmental Fate and Biological Effects of Radium-226 in Freshwater Environments: Evidence from Fingernail Clams (Sphaeriidae)

Abstract

Radium-226 (²²⁶Ra) is a naturally occurring radionuclide that is typically present at low concentrations in freshwater environments but can be elevated by industrial activities that disturb or concentrate naturally occurring radioactive materials, including mining, oil and gas extraction, coal production, and wastewater treatment. Despite concerns regarding its mobility, bioaccumulation potential, and radiotoxicity, the ecological effects of ²²⁶Ra remain poorly understood. This knowledge gap has contributed to the absence of a Canadian freshwater quality guideline for the protection of aquatic life. This thesis investigated the occurrence, environmental fate, bioaccumulation and toxicity of 226Ra to a sensitive freshwater species of fingernail clams (Sphaerium striatinum), with a particular focus on the influence of calcium on its behaviour and toxicity. As alkaline earth metals with similar physicochemical properties, calcium and radium are expected to exhibit comparable environmental and biological behaviours, making calcium a potentially important factor influencing 226Ra fate, bioavailability, bioaccumulation, and toxicity.

Analysis of Canadian mining effluent data demonstrated that ²²⁶Ra is released from multiple mining sectors and that uranium mines are not uniquely responsible for elevated discharges. Final treated effluent concentrations ranged from 0.2 to 7800 mBq L-1, with average concentrations in several sectors exceeding the regional background upper tolerance limit of 21.7 mBq L-1. To address ecotoxicological data gaps, laboratory methods were developed for the freshwater fingernail clam Sphaerium striatinum, a sensitive bivalve species. Experimental results demonstrated that 226Ra bioaccumulates in both soft tissues and shells, with preferential partitioning to shells. Chronic exposure reduced growth (EC10 = 4.1 Bq L-1) and reproduction (EC10 = 0.37 Bq L-1) at environmentally relevant concentrations, while survival was comparatively less sensitive. Sorption experiments further showed that the environmental fate of 226Ra is strongly controlled by mineralogy and water chemistry, with adsorption to reactive clay minerals rapidly reducing dissolved concentrations. Increasing calcium concentrations reduced sorption and increased the persistence of dissolved 226Ra, whereas organic matter had comparatively little influence.

Combined bioaccumulation and toxicity studies demonstrated that calcium exerts competing effects on ²²⁶Ra behaviour. Although calcium can reduce biological uptake through competitive interactions at biological membranes, it also increases ²²⁶Ra dissolved exposure by limiting sorption to mineral phases. Consequently, calcium did not consistently mitigate toxicity and, for some endpoints, resulted in increased organism sensitivity. Dose modelling further indicated that waterborne exposure was the dominant pathway under the conditions tested. Overall, this research demonstrates that the environmental risk of ²²⁶Ra is governed by coupled geochemical and biological processes and that water chemistry can substantially influence both exposure and toxicity. These findings provide critical information for environmental risk assessments and support the future development of science-based freshwater quality guidelines and management strategies for ²²⁶Ra releases.