Characterization of local geochemical anomalies in soils and stream sediments for improving land-use planning: A case study from Nisa, Portugal

One of the largest and most important near-surface deposits of uranium in Portugal is found in the region of Nisa. Unexploited resources in the Nisa deposit are estimated at about 32 million tonnes of ore. To better understand the processes of natural dispersion of chemicals, such as hydrogravitational erosion and reactive transport, samples of soils and of stream sediments were collected in the vicinity of the Nisa deposit and analysed for chemical signatures of metals (uranium, chromium, molybdenum, niobium, vanadium, and zinc) and semi-metals (arsenic). Stream sediment samples were taken from sites along a 470-m-long stretch of a river bed near the mineralization, with soil samples being collected from riverbanks. Univariate, bivariate, and multivariate statistical analysis as well as GIS mapping tools were used to identify and interpret variations in geochemical concentrations of the different elements and the relationships between them. The analyses allowed local background (reference) values to be established with respect to the geochemical anomaly caused by the mineral deposit. These background values were compared with legislation standards established for the remediation of soils and sediments. Recommendations to be considered in land-use planning with respect to the occurrence of natural geochemical anomalies are presented.