ARE FOSSIL AQUIFERS DOMINATED BY RADIOACTIVE WATER?

Fossil groundwaters comprise a large percentage of the global groundwater storage. However, the natural abundance of dissolved radium in these ancient groundwaters is poorly studied. Here we analyze radium along with other geochemical and isotopic data in major aquifers dominated by fossil groundwaters. Activities of Ra in the investigated aquifers greatly exceed the international standards for the maximum contaminant level (MCL) in drinking water. Our data reveal that activities exceeding the MCL are measured in anoxic, SO₄–poor groundwaters having long residence times, and Ra activities are generally lower in younger groundwaters. Long residence times are optimal for mobilization of Ra to the solution by alpha-recoil processes and the development of anoxic conditions that promote reductive dissolution of sulfate minerals (i.e., barite). The low Ra activities in SO₄–rich groundwaters imply partitioning of Ra into sulfate mineral phases including barite. The occurrence of lower levels of Ra isotopes in anoxic, low-SO₄ groundwater may indicate lower abundances of U and Th, the parent nuclides of Ra, in the aquifer solids. In some groundwater wells, Ra activities may be limited by sorption mechanisms and cation exchange reactions. However, the presence of high concentrations of total dissolved solids (TDS) counteracts the sorption capacity of Ra in other groundwater wells. We recommend that Ra assessment should be considered when managing quality of fossil groundwater resources.