Evolution of flood levels of open pits in the Tharsis mining district and characterization of the pollutant load from acid mine drainage sources

Abstract

The Tharsis mining district is the second most important of the Iberian Pyrite Belt (IPB). The intense exploitation of sulphides carried out since the Copper Age but, above all, since the second half of 19th century to the end of the 20th century, has left large dumps and other sulphide-rich mining wastes in the area, where acid mine drainages (AMD) are generated, together with four acid pit lakes (Filón Norte, Sierra Bullones, Filón Centro y Filón Sur) A characterization of the hydrological functioning and evolution of the pit lakes (was carried out. A simple methodology based on the use of the available orthophotographs and the Digital Terrain Model (DTM), together with the water balance of the pit lakes was developed, which could be applied to other abandoned mining sites. The accumulation of large volumes (5.2 hm3) of acid and metal-rich waters in these pit lakes poses a serious environmental concern. The results showed that Sierra Bullones and Filón Norte open pits are connected underground and present the same evolution, with a water transference from Sierra Bullones to Filón Norte. The water level in both pit lakes is increasing, with an average rise of 2.8 m/yr since the beginning of flooding. However, the increase in the evaporation rate, as a result of the larger flooded area as the water level rise, would induce a hydrological equilibrium before reaching the overflow level, leading to the formation of a terminal lake. On the other hand, the water level in Filón Centro and Filón Sur pit lakes remain approximately stable. The first behaves as a flow-through or terminal lake, depending on the annual rainfall, while the second acts permanently as a flow-through lake. In addition, the influence of hydrogeochemical processes and the waste typology on the physicochemical parameters and dissolved concentrations of pollutants in the acid waters generated were studied during three different hydrological conditions (dry, wet and intermediate). Extreme leachates are produced in the area, reaching even negative pH values and concentrations of up to 2.2 g/L of As and 194 g/L of Fe. Lead was the least mobile element in dissolution probably due to the precipitation of Pb secondary minerals and/or its coprecipitation on Fe oxyhydroxysulphates. Arsenic, Cr, and V are also coprecipitated with Fe minerals. Seasonal patterns in metal contents were identified: elements coming from the host rocks, such as Al, Mn and Ni, showed their maximum concentrations in the dry period, when dilution with freshwater is lower and the water-rock interaction and evaporation is higher. On the other hand, As, Cr, Fe, Pb and V showed minimum concentrations in the dry period due to intense Fe oxyhydroxysulphate precipitation. The highest pollutant load from the Tharsis mines occurs in the wet period (139 ton/day of SO4, 44 ton/day of Fe, 6.7 ton/day of Al, etc.) while in the intermediate period, it is only a quarter, and in the dry period, it is minimal due to the low flows. Approximately half of the pollutants released reaches the Meca River basin, while the other half joins the Oraque River, although some differences occur depending on the element and the season. These discharges acidify the water of the Sancho Reservoir and will compromise the quality of the new reservoir waters, currently in construction downstream the Oraque River The concentrations and normalised patterns of rare earth elements (REEs; subdivided into light (LREEs: La to Sm), middle (MREEs: Eu to Dy) and heavy (HREEs: Ho to Lu)), their behaviour in the fluvial network, and the REEs contributions from AMD sources were also analysed. High concentrations of REEs were observed, with a mean value of 1,747 μg/L. The concentration of REEs shows a positive correlation with electrical conductivity (EC). However, the highest concentrations of REEs occurred in samples with intermediate levels of pollution and EC values. The highest correlations of middle REEs (MREEs) and heavy REEs (HREEs) occurred with elements related to hydrothermal mineralisations of Mn and Ni. The normalised patterns of the AMD showed an enrichment of MREEs over light REEs (LREEs) and HREEs in all samples. The use of REEs patterns as geochemical tracers confirmed the conservative behaviour of REEs in the fluvial network under low acid pH. The quantification of REEs released from AMD sources to water bodies revealed that the main load of REEs occurs during the wet period with 6.6 kg/day of LREEs, 1.1 kg/day of MREEs, and 0.5 kg/day of HREEs.