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


Sierra de Gádor 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 Gádor 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 (238U, 235U and 234U) and 226Ra by alpha spectrometry were determined.

The activity concentration of 238U 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 234U/238U 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.

226Ra presented a wide range of activity concentrations, (0.8 up to about 4 × 102 mBq L−1); greatest concentrations were detected in the thermal area of Alhama. Most of the samples showed 226Ra/234U 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). 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.