Potential human health impact of groundwater in non-exploited uranium ores: The case of Horta da Vilariça (NE Portugal)

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The anomalous concentration of chemical elements in rocks can have consequences on the human health population even in the absence of anthropogenic activities. Uranium contamination of groundwater is being increasingly recognized as a health threat to rural residents relying on home wells for their drinking water, not only in communities with a legacy of mining, but also where there's naturally occurring uranium. In the Horta da Vilariça region (Torre de Moncorvo, Northern Portugal), a few uranium (U) ores were recognized in the metamorphic aureole of a hercynian granite; the host rocks are metasedimentary schists and greywacks of the pre-ordovician “Complexo Xisto-Grauváquico”. To assess water-rock interaction 11 groundwater wells were selected and sampled, for major and trace elements as well as for radioisotopes 222Rn, 226Ra, 234U, 238U analyses. Analytical results point to Na-Ca-bicarbonate groundwater with a mean electric conductivity of 477μS·cm−1 and pH between 5.4 and 6.5. The concentration of dissolved arsenic (As) reaches 15μgL−1, mostly in the oxidized form of As(V), and is above the parametric value for drinking water (10μgL−1) in 3 wells used for drinking and/or irrigation. The radioisotope activity (U, 226Ra and 222Rn) is also very high in the most of collected water samples. About 65% of the samples are above the WHO recommended limit for drinking water with respect to U, with a median of 129mg·L−1 and a maximum of 3483mg·L−1. About 30% of the samples are above the EU (2013) recommended limit for drinking water for 222Rn (1000Bq·L−1), with a median of 400Bq·L−1 and a measured maximum of 9784Bq·L−1. The human health impact was evaluated only for the water ingestion vector through calculation of the annual effective dose. As the health impact depends of the body mass the dose was calculate for different age groups of the population. The results indicate that, in 60% of the samples, the effective dose exceeds the limits suggested by existing guidelines. The use of groundwater for drinking and crop irrigation in the studied region should be subject to previous assessment for U, Ra, Rn and As. In the activity concentrations found in the present work they are clearly a potential health risk for the population.