U-isotopes and 226Ra as tracers of hydrogeochemical processes in carbonated karst aquifers from arid areas

The natural U-isotopes concentrations were strongly correlated with dissolution of sulphate evaporites (mainly gypsum). 226Ra had a more complex behaviour, showing a strong correlation with water salinity, which was particularly evident in locations where thermal anomalies were detected. The most saline samples showed the lowest 234U/238U activity ratios, probably due to fast uniform bulk mineral dissolution, which would minimize the impact of solubility-controlled fractionation processes. Furthermore, the high bulk dissolution rates promoted greater groundwater 226Ra/234U ratios because the Ra has a comparatively much greater mobility than U in saline conditions.