Inputs and fate of contaminants in a reservoir with circumneutral water affected by acid mine drainage

Abstract

The Olivargas River drains materials from the Iberian Pyrite Belt with a large number of abandoned sulphide mining complexes that produce highly contaminated acid mine drainages (AMD). The Olivargas River is regulated by a reservoir that receives these acid leachates through its three main tributaries. In this study, the chemical composition of the waters from the reservoir and the water courses in its watershed are investigated based on samples taken throughout a hydrological year. We calculated the contaminant load for Al, Cd, Co, Cu, Fe, Mn, Ni, Pb, Zn, SO4 and other elements entering the reservoir by two methods, establishing the relations of the dissolved element concentrations with 1) the water flow and 2) the specific conductance. The most extreme conditions were found in the tributary Los Peces Creek, with a mean pH value of 2.8 and concentrations of up to 736 mg/L of Zn and 291 mg/L of Al, making it the main source of metals in the reservoir (up to 65 t/year of Zn). Given that the volume of acid mine water pouring into the reservoir is relatively low, the water pH in the reservoir remains neutral. Under these conditions, most of the mine-derived metals (especially Al and Fe) form precipitates that accumulate in the bottom sediments, while dissolved concentrations are low (0.2 mg/L of Al and <0.1 mg/L of Fe). However, some potentially toxic elements, such as Zn and Cd, remain dissolved, exceeding the limiting concentrations set by environmental legislation. These results are of interest to understand the behaviour of trace elements in other reservoirs affected by AMD and to study the effectiveness of possible remediation measures.