Geokemisk undersökning vid Rävlidmyrgruvan, Västerbottens län : provtagning, analys och förslag till åtgärder

Sammanfattning: The mining industry, in terms of ore excavation and metal production, is and has been important to the Swedish economy. In connection with the ore excavation process, large amounts of partly sulphidic waste are produced. When the metal sulphides are exposed to air and water they are eventually oxidised, which leads to the formation of acidic water with high concentrations of metal cations and sulphate ions, so called Acid Mine Drainage (AMD). This water may reach the nearest surroundings and cause negative environmental effects. By covering the waste with soil or water, the oxidation process can be minimised. At the Rävlidmyran mine in the county of Västerbotten open pit mining took place between 1951 and 1991. When the mining activities ceased the pit was filled with water forming a pit lake. Waste rock dumps, which are partly contaminated with the sulphide mineral pyrite, and covered by till material surround the pit lake. Despite previous remediation measures, there is still a leakage of metal ions to the nearby lake Hornträsket. The lake is presently considered as an oligotrophic lake, which is contaminated with heavy metals. The main purpose of the present study was to investigate possible remediation measures for reducing heavy metal leakage, based on the results of chemical analyses. The specific purposes were: (1) to collect samples within the catchment of the Rävlidmyran mine and to predict how the metal flows take place and to investigate whether sulphate reduction is important or not (2) to collect samples in the pit lake to get a picture of its chemical and biological condition and (3) to contribute to an updating of a semiquantitative heavy metal budget for the lake Hornträsket and quantify the contribution of heavy metals from the Rävlidmyran mine. Water samples collected in the catchment were analysed for pH and electrical conductivity (EC), the total concentrations of Cu, Zn, Cd and Fe, and the concentrations of Cl-, TOC and SO42-. At several sampling points the water flow was measured with a V-notch. Minerogenic and organic sediment samples were also collected and analysed for total concentrations of Cu, Zn, Cd, Fe och As. In the pit lake, water samples were collected at 0-, 2-, 5-, 8 and 12- meter depths. These samples were analysed for pH and EC as well as the total concentrations of Cu, Zn, Cd and Fe plus the concentrations of H2PO4- and SO42-. Some leakage points were identified where the water had an exceptionally low pH and high concentrations of metals and sulphate, probably as a direct cause of pyrite oxidation. This conclusion was partly supported by an analysis of sulphur isotopes. Among the sediment samples the organic sediments had the highest concentrations of Cu, Zn and Cd, while the minerogenic sediments had the highest concentrations of Fe and As. It was concluded that the environment in the pit lake was not optimal for sulphate reducing bacteria (SRB). However, analysis of sulphur isotopes indicated that sulphate reduction might occur in microenvironments of the pit lake. One suggested measure against high acidity is to inject a slurry of reactive lime stone and sewage sludge into the waste rock dump. This should increase the pH and facilitate reducing conditions, which should keep the heavy metals in precipitated metal sulphides. Addition of sewage sludge to the pit lake should favour an increased production of algae. Decomposition of the algae may create reducing conditions that should favour the formation of precipitated metal sulphides.