Biological accumulation of 226Ra in a groundwater‐augmented Florida lake

Round Lake, Hillsborough County, Florida (U.S.A.) has been augmented with 226Ra‐rich (6.2 dpm L−1) groundwater from the Floridan aquifer for more than 30 years. Water column 226Ra activity in Round Lake is 3.4 dpm L−1, about one third of the U.S. drinking water standard value. Radionuclide measurements in biota and sediment cores from the lake indicate high 226Ra activities in plants, mollusc shells and tissues, fish bones, and recent sediments. Unionid mussels bioaccumulate 226Ra and display very high activities in soft tissues (219.9–454.8 dpm g−1 dry). Throughout Florida, groundwaters are increasingly pumped for agricultural irrigation, as well as residential and industrial use. Biological accumulation of 226Ra by filter‐feeding pelecypods in aquatic ecosystems is an important pathway for transfer of the radionuclide to higher trophic levels of both aquatic and terrestrial food webs. Radionuclide analysis of soft tissues from aquatic bivalves provides a rapid means of identifying waterbodies that receive substantial inputs of dissolved 226Ra.