Uranium and arsenic contamination in the former Mondego Sul uranium mine area, Central Portugal

Abstract

The abandoned Mondego Sul uranium mine exploited quartz veins containing sulfides, torbernite, meta-uranocircite, saleeite, meta-saleeite and Fe-oxides and hydroxides and intersected the schist–greywacke complex. The exploitation was carried out in five open pits and produced 74,968kg U3O8 from 1987 to 1991. From July 2008 until now we observed that a lake had formed in one left open pit. Three dumps slightly covered by vegetation were located in steep slopes and streams and drainage water ran to a dam reservoir. Water, stream sediments and soils were contaminated in U and As. Uranium sulfate minerals were the most important contaminants of water, because U was positively correlated with SO42− in water. However, Fe-oxyhydroxides were the main retention minerals in soils, as U was positively correlated with Fe in soils. The Fe–Mn hydr(oxides) precipitate in streambeds retained a higher U concentration and a lower As concentration than stream sediments. Uranium and As were mainly retained in montmorillonite of stream sediments. Uranium concentration was up to 181μg/L in water, 266mg/kg in stream sediments and 377mg/kg in soils. The highest U concentration in water was reached at the beginning of spring, because the highest flow caused leaching of mineralized quartz veins, tailings and dumps and U from secondary minerals was released into water. The pH of water from the open pit was acidic and UO22+ was up to 50%, but drainage water from the largest dump was acidic to alkaline and had up to 88% UO2CO3. Arsenic concentration was up to 158μg/L in water, 211mg/kg in stream sediments and 223mg/kg in soils. The highest concentrations of most metals and As with significant As(III) in water occurred in summer due to evaporation. A restoration of the mining area is necessary to avoid a public hazard.