Uranium anomaly evaluation in groundwaters: a hydrogeochemical study in the Nisa region, Portugal

Abstract

The hydrogeochemistry of the Nisa region, central Portugal, an area of known U mineralization, was studied. The geology of the region comprises an S-type high-level granite intrusion with vein-type U-mineralization,its contact-metamorphic aureole developed in a schist host rock containing disseminated U-mineralization. An integrated multi-variate statistical, hydrogeochemical and thermodynamic approach was adopted for the interpretation of groundwater data from the granite and its contact aureole. The U groundwater anomalies are evaluated within the local hydrogeochemical context. One-hundred-and-fifteen water samples were collected and analyzed for pH, Ec, Eh, Na, K, Ca, Mg, Fe, Mn, Al, U, Li, Ba, Sr, Cl, HCO 3, SO 4, NO 3, PO 4, F, DOC and H 4SiO 4. Uni- and bi-variate statistical studies of these dilute waters indicate that mainly surface and land-use characteristics control the dominant chemical processes, with water-rock interaction playing a lesser role. Granite/schist compositional differences show up especially in the Mg and HCO 3 content of the waters. Multi-variate statistical analyses (Principal Component Analysis and Q-mode Cluster Analysis) were carried out separately on the granite and schist water groups. Evapotranspiration is an important process both in the schist and granite environment. The impact of fault/fracture zones, land-use, oxidation/reduction reactions and water-rock interaction is different for the waters from each bedrock type. For both rock types, the waters can be classified into genetic groups or clusters ranging from surface runoff/shallow interflow, through surface, agriculture, and infiltrated, to village-pollution type water. The distribution pattern of these groundwater types is not disrupted by the granite-schist contact, allowing for an interpretation of the U distribution in the entire study area. Thermodynamic calculations indicate that the predominant U species in solution is UO 2(HPO 4) 2 2−. Virtually all waters are undersaturated with respect to U minerals. The distribution of U in the waters is thus not influenced by precipitation of U-bearing minerals. The distribution of U in the water samples as well as that for U/Ec (in an attempt to correct for evapotranspiration) are evaluated. The U- and U/Ec-anomaly patterns are considerably biased by bedrock differences and varying genetic history. Interpretability is greatly improved if these differences are taken into account by defining U-threshold values for each genetic water cluster. All known radiometric anomalies are reflected in the hydrogeochemical U-anomaly pattern. The general hydrogeochemical features allow for a refined evaluation of the known radiometric anomalies and of the additional anomalies found in the present study. Additional hydrogeochemical anomalies from waters reflecting depth characteristics are attractive exploration targets.