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 (Ra), a long-lived, bioaccumulative calcium (Ca) analog. Despite strict discharge regulations, there is limited evidence to suggest that current thresholds for Ra adequately protect aquatic organisms. Furthermore, Canada lacks a federal water quality guideline for Ra, underscoring the need for protective limits to safeguard aquatic ecosystems. Hence, this research aimed to generate data on Ra 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 Ra can significantly impact the survival of D. magna. Ra 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 Ra /L, the limit for composite samples at 0.74 Bq/L Ra, and the limit for grab samples at 1.11 Bq/L Ra, it is unlikely that toxic effects to aquatic cladocerans like D. magna from Ra will be observed downstream of Canadian mines and mills.