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

Abstract Sierra de Gador is a karst macrosystem with a highly complex geometry, located in southeastern Spain. In this arid environment, the main economic activities, agriculture and tourism, are supported by water resources from the Sierra de Gador aquifer system. The aim of this work was to study the levels and behaviour of some of the most significant natural radionuclides in order to improve the knowledge of the hydrogeochemical processes involved in this groundwater system. For this study, 28 groundwater and 7 surface water samples were collected, and the activity concentrations of the natural U-isotopes ( 238 U, 235 U and 234 U) and 226 Ra by alpha spectrometry were determined. The activity concentration of 238 U presented a large variation from around 1.1 to 65 mBq L −1 . Elevated groundwater U concentrations were the result of oxidising conditions that likely promoted U dissolution. The PHREEQC modelling code showed that dissolved U mainly existed as uranyl carbonate complexes. The 234 U/ 238 U activity ratios were higher than unity for all samples (1.1–3.8). Additionally, these ratios were in greater disequilibrium in groundwater than surface water samples, the likely result of greater water-rock contact time. 226 Ra presented a wide range of activity concentrations, (0.8 up to about 4 × 10 2 mBq L −1 ); greatest concentrations were detected in the thermal area of Alhama. Most of the samples showed 226 Ra/ 234 U activity ratios lower than unity (median = 0.3), likely the result of the greater mobility of U than Ra in the aquifer system. The natural U-isotopes concentrations were strongly correlated with dissolution of sulphate evaporites (mainly gypsum). 226 Ra 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 234 U/ 238 U 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 226 Ra/ 234 U ratios because the Ra has a comparatively much greater mobility than U in saline conditions.