title: U-isotopes and (226)Ra as tracers of hydrogeochemical processes in carbonated karst aquifers from arid areas.
abstract: 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 ((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.
authors.affiliation: Department of Applied Physics, University of Huelva, Marine International Campus of Excellence (CEIMAR), 21071 Huelva, Spain. Electronic address: joseluis.guerrero@alu.uhu.es.
authors.forename: José Luis
authors.initials: JL
authors.lastname: Guerrero
authors.affiliation: Water Resources and Environmental Geology Research Group (RNM-189), Marine International Campus of Excellence (CEIMAR) University of Almería, 04120, Almería, Spain. Electronic address: avallejo@ual.es.
authors.forename: Ángela
authors.initials: Á
authors.lastname: Vallejos
authors.affiliation: Centro de Investigación para la Ingeniería en Minería Sostenible, Escuela Técnica Superior de Ingeniería, Universidad de Huelva. Ctra. Palos de la Frontera. s/n. 21819, Palos de la Frontera, Huelva, Spain. Electronic address: ceron@uhu.es.
authors.forename: Juan Carlos
authors.initials: JC
authors.lastname: Cerón
authors.affiliation: Water Resources and Environmental Geology Research Group (RNM-189), Marine International Campus of Excellence (CEIMAR) University of Almería, 04120, Almería, Spain. Electronic address: fmartos@ual.es.
authors.forename: Francisco
authors.initials: F
authors.lastname: Sánchez-Martos
authors.affiliation: Water Resources and Environmental Geology Research Group (RNM-189), Marine International Campus of Excellence (CEIMAR) University of Almería, 04120, Almería, Spain. Electronic address: apulido@ual.es.
authors.forename: Antonio
authors.initials: A
authors.lastname: Pulido-Bosch
authors.affiliation: Department of Applied Physics, University of Huelva, Marine International Campus of Excellence (CEIMAR), 21071 Huelva, Spain. Electronic address: bolivar@uhu.es.
authors.forename: Juan Pedro
authors.initials: JP
authors.lastname: Bolívar
mesh_terms: D002254:Carbonates; D060587:Groundwater / Q000032:analysis*; D011834:Radiation Monitoring; D011883:Radium / Q000032:analysis*; D054712:Salinity; D013030:Spain; D014501:Uranium / Q000032:analysis*; D014875:Water Pollutants, Radioactive / Q000032:analysis*
chemical_list: D002254:Carbonates; D014875:Water Pollutants, Radioactive; D014501:Uranium; D011883:Radium
keywords: (226)Ra; Arid areas; Karst aquifer; Radioactive disequilibrium; Sierra de Gádor; Uranium isotopes