Radium geochemistry of ground waters in Paleozoic carbonate aquifers, midcontinent, USA

The purpose of this study was to elucidate the processes controlling the distribution and behavior of the longer-lived Ra isotopes in continuous Paleozoic carbonate aquifers of parts of Missouri, Kansas, and Oklahoma. Activities of (228Ra) and (226Ra) were analyzed in fresh and saline ground waters, brines, and rocks. The fluids have a wide salinity range (200–250,000 mg l−1 total dissolved solids). The (226Ra) activity ranges from 0.66–7660 dpm kg−1 and correlates with salinity and other alkaline earth element (Ca, Sr, and Ba) concentrations. The range of (228Ra:226Ra) ratios in the fluids (0.06–1.48) is similar to that in the aquifer rocks (0.21–1.53). The relatively low mean fluid (228Ra:226Ra) ratio (0.30) reflects the low Th:U ratio of the predominant carbonate aquifer rock. Radium occurs mostly (≥77%) as Ra2+ species in the fluids. Salinity-dependent sorption–desorption processes (with log K values from 100–104 and negatively correlated with salinity), involving Th-enriched surface coatings on aquifer flow channels, can explain the rapid solid–fluid transfer of Ra isotopes in the system and the correlation of Ra with salinity.