Evaluation of groundwater quality based on radiological and hydrochemical data from two uraniferous regions of Western Iberia: Nisa (Portugal) and Ciudad Rodrigo (Spain)

The regions of Nisa (Portugal) and Ciudad Rodrigo (Spain) are characterized by a high radiation background due to the presence of uranium-enriched rocks. To evaluate the water quality of the shallow aquifers and the relationship between radiological and non-radiological variables, a set of 41 samples was collected (mainly from springs) and analyzed for the radioisotopes ²³⁸U, ²³⁴U, ²²⁶Ra and ²²²Rn as well as for some major and minor cations. The results obtained for the cationic composition in both areas show some differences in terms of geological bedrock, with higher mineralization and hardness in the Spanish samples (Ca–Mg facies); the Nisa samples are included in the Na–K facies. The indicative effective dose was calculated for several age groups. On average, the effective dose is higher in Nisa due to the activity in ²²⁶Ra, and 25 % of the total dataset puts ²²²Rn above the recommended acceptable maximum. Despite the low-scale ancient uranium exploitation works in the Nisa area in comparison to those carried out on the Spanish side, the higher risk calculated in the Portuguese region reinforces the importance of taking into account the natural background when the goal is to control the exposure of the population to ionizing radiation.