Guerrero José Luis, Vallejos Ángela, Cerón Juan Carlos, Sánchez-Martos Francisco, Pulido-Bosch Antonio, Bolívar Juan Pedro
Department of Applied Physics, University of Huelva, Marine International Campus of Excellence (CEIMAR), 21071 Huelva, Spain.
Water Resources and Environmental Geology Research Group (RNM-189), Marine International Campus of Excellence (CEIMAR) University of Almería, 04120, Almería, Spain.
J Environ Radioact. 2016 Jul;158-159:9-20. doi: 10.1016/j.jenvrad.2016.03.015. Epub 2016 Apr 6.
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.
加多尔山脉是一个岩溶宏系统，其几何形状高度复杂，位于西班牙东南部。在这个干旱环境中，主要经济活动——农业和旅游业——依赖于加多尔山脉含水层系统的水资源。这项工作的目的是研究一些最重要的天然放射性核素的水平和行为，以增进对该地下水系统中水文地球化学过程的了解。为进行这项研究，采集了28个地下水样本和7个地表水样本，并通过α能谱法测定了天然铀同位素（（238）U、（235）U和（234）U）以及（226）Ra的活度浓度。（238）U的活度浓度变化很大，从约1.1到65 mBq L⁻¹不等。地下水中铀浓度升高是氧化条件导致的，这种条件可能促进了铀的溶解。PHREEQC模拟代码表明，溶解态铀主要以碳酸铀酰络合物的形式存在。所有样本的（234）U/（238）U活度比均高于1（1.1 - 3.8）。此外，这些比值在地下水中的不平衡程度比地表水样本更大，这可能是水 - 岩接触时间更长的结果。（226）Ra的活度浓度范围很广，从0.8到约4×10² mBq L⁻¹；在阿尔哈马的热区检测到最高浓度。大多数样本的（226）Ra/（234）U活度比低于1（中位数 = 0.3），这可能是因为在含水层系统中铀的迁移性比镭更强。天然铀同位素浓度与硫酸盐蒸发岩（主要是石膏）的溶解密切相关。（226）Ra的行为更为复杂，与水的盐度有很强的相关性，在检测到热异常的地点尤为明显。盐度最高的样本显示出最低的（234）U/（238）U活度比，这可能是由于快速的均匀整体矿物溶解，从而使溶解度控制的分馏过程的影响最小化。此外，高整体溶解速率导致地下水中（226）Ra/（234）U比值更高，因为在含盐条件下镭的迁移性比铀大得多。
