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

Round Lake, Hillsborough County, Florida (U.S.A.) has been augmented with Ra-226-rich (6.2 dpm L-1) groundwater from the Floridan aquifer for more than 30 years. Water column Ra-226 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 Ra-226 activities in plants, mollusc shells and tissues, fish bones, and recent sediments. Unionid mussels bioaccumulate Ra-226 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 Ra-226 by filter-feeding pelecypods in aquatic ecosystems is an important pathway for transfer of the radio-nuclide 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 Ra-226.