Chronic Radium-226 Bioaccumulation and Toxicity in the Aquatic Invertebrate Daphnia magna

Mining operations in Canada, including uranium mining and milling, generate by-products containing radionuclides, including radium-226 (²²⁶Ra), a long-lived, bioaccumulative calcium (Ca²⁺) analog. Despite strict discharge regulations, there is limited evidence to suggest that current thresholds for ²²⁶Ra adequately protect aquatic organisms. Furthermore, Canada lacks a federal water quality guideline for ²²⁶Ra, underscoring the need for protective limits to safeguard aquatic ecosystems. Hence, this research aimed to generate data on ²²⁶Ra toxicity to the model aquatic invertebrate Daphnia magna. For this purpose, two 21-day chronic toxicity tests with D. magna were conducted, with survival and reproduction as the endpoints, as well as a reduced water hardness experiment, a multigenerational study, and a bioaccumulation assay. These experiments demonstrated that a high activity concentration (nominal 50 Bq/L) of ²²⁶Ra can significantly impact the survival of D. magna. ²²⁶Ra was also found to bioaccumulate in D. magna with a BAF of 72.8. Since the Canadian Metal and Diamond Mining Effluent Regulations (MDMER) monthly mean effluent limit is currently set at 0.37 Bq ²²⁶Ra /L, the limit for composite samples at 0.74 Bq/L ²²⁶Ra, and the limit for grab samples at 1.11 Bq/L ²²⁶Ra, it is unlikely that toxic effects to aquatic cladocerans like D. magna from ²²⁶Ra will be observed downstream of Canadian mines and mills.