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

The purpose of this study was to elucidate the processes controlling the di stribution and behavior of the longer-lived Ra isotopes in continuous Paleo zoic carbonate aquifers of parts of Missouri, Kansas, and Oklahoma. Activit ies of (Ra-228) and (Ra-226) were analyzed in fresh and saline ground water s, brines, and rocks. The fluids have a wide salinity range (200-250,000 mg l(-1) total dissolved solids). The (Ra-226) 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 (Ra-228:Ra-226) 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 (Ra-228:Ra-226) ratio (0.30) reflects the low Th :U ratio of the predominant carbonate aquifer rock. Radium occurs mostly (g reater than or equal to 77%) as Ra2+ species in the fluids. Salinity-depend ent sorption-desorption processes (with log K values from 10(0)-10(4) and n egatively 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. (C) 2000 E lsevier Science Ltd. All rights reserved.