Round Lake, Hillsborough County, Florida (U.S.A.) has been augmented with R
a-226-rich (6.2 dpm L-1) groundwater from the Floridan aquifer for more tha
n 30 years. Water column Ra-226 activity in Round Lake is 3.4 dpm L-1, abou
t one third of the U.S. drinking water standard value. Radionuclide measure
ments in biota and sediment cores from the lake indicate high Ra-226 activi
ties in plants, mollusc shells and tissues, fish bones, and recent sediment
s. 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 residentia
l and industrial use. Biological accumulation of Ra-226 by filter-feeding p
elecypods in aquatic ecosystems is an important pathway for transfer of the
radio-nuclide to higher trophic levels of both aquatic and terrestrial foo
d webs. Radionuclide analysis of soft tissues from aquatic bivalves provide
s a rapid means of identifying waterbodies that receive substantial inputs
of dissolved Ra-226.