Abstract
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.
Highlights
Portugal has high geological and geochemical diversity and considerable potential for mineral resources as a result of the occurrence of different metallogenetic provinces (Moura and Velho, 2011)
The measurements made in situ show contributions of natural surrounding environment radioactivity because they were made in an area of natural uranium mineralization where the local background value is considerably higher
Statistical analysis of elemental concentrations of soil and stream sediment samples collected in the vicinity of the Nisa uranium ore deposit (Portugal) has identified two groups of elements: Group I, composed of U–Mb–Nb, and Group II, composed of As–Cr–V
Summary
Portugal has high geological and geochemical diversity and considerable potential for mineral resources as a result of the occurrence of different metallogenetic provinces (Moura and Velho, 2011). This diversity has generated many natural geochemical anomalies in Portugal. The magnitude of natural geochemical anomalies, as well as their local and regional effects or manifestations, must be taken into account in land-use planning, in environmental impact assessment, and in site remediation projects. Given the social relevance of geochemical contamination and the factors affecting its assessment, a study area containing a substantial uranium anomaly was selected to exemplify the need to consider the natural geochemical dispersion phenomena of metals and semi-metals in soils and stream sediments. Ontario guidelines 2011 (Table 1: Full Depth Background Site Condition Standards)
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