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 longerlived Ra isotopes in continuous Paleozoic carbonate aquifers of parts of Missouri, Kansas, and Oklahoma. Activities of (Ra) and (Ra) were analyzed in fresh and saline ground waters, brines, and rocks. The ̄uids have a wide salinity range (200±250,000 mg lÿ1 total dissolved solids). The (Ra) activity ranges from 0.66±7660 dpm kgÿ1 and correlates with salinity and other alkaline earth element (Ca, Sr, and Ba) concentrations. The range of (Ra:Ra) ratios in the ̄uids (0.06±1.48) is similar to that in the aquifer rocks (0.21±1.53). The relatively low mean ̄uid (Ra:Ra) ratio (0.30) re ̄ects the low Th:U ratio of the predominant carbonate aquifer rock. Radium occurs mostly (r77%) as Ra species in the ̄uids. Salinity-dependent sorption±desorption processes (with log K values from 10±10 and negatively correlated with salinity), involving Th-enriched surface coatings on aquifer ̄ow channels, can explain the rapid solid± ̄uid transfer of Ra isotopes in the system and the correlation of Ra with salinity. 7 2000 Elsevier Science Ltd. All rights reserved.