Evaluation of long-term effects of excavated sulfide-bearing rock : - A case study in western Sweden

Sulfide-bearing rock that is excavated in conjunction with construction projects can cause environmental and construction engineering problems. Sulfidic minerals oxidize when they are exposed to an oxidant and water, which can make them generate acid rock drainage (ARD) with high concentrations of metals and sulphate. Predicting the overall oxidation process and drainage properties is, however, difficult since these depend on various factors. In this master thesis the current state of a 20-year-old ARD case is studied and compared to early site conditions to improve the knowledge of how different sulfide-bearing rocks may behave over time. The study investigates the drainage water and its impact on a local watercourse, but also chemical and mineralogical properties of oxidized rock from the site. The concentrations of Al, Cd, Co, Cr, Cu, Fe, Mn, Ni, S, and Zn in the drainage and watercourse has been studied and compared to data from when the sulfide-bearing rock was excavated and deposited. The chemical composition and occurrence of sulfides in different rock types from the area has been examined through chemical analysis, SEM-EDS techniques, and sequential extractions. XRD data has been collected but was not interpreted within this project. The results from the water chemistry investigations showed that the concentrations of Cd, Co, Cr, Cu, Ni, and Zn had decreased since 2004, while the concentrations of Al, Fe, and Mn had increased. These elements mainly originated from a drainage pipe below ground level and later precipitated in the watercourse; only nickel and sulphate concentrations remained elevated 800 meters downstream from the studied rock storage. It was also shown that other sources in the area contaminated the watercourse. Masses of the previously deposited sulfide-bearing rock were moved approximately 10 years ago, which increase the uncertainty of conclusions drawn from the drainage. However, the elevated electrical conductivity and metal content of the drainage are signs of sulfide oxidation. The high concentration of aluminium likely indicates that the pH is buffered by silicates in the rock storage, and the concentrations of iron and manganese could also originate from silicates, or from sulfide oxidation. Pyrite was found abundantly in clusters in metasedimentary rock. Mainly pyrite, but also sphalerite and pyrrhotite was found in less oxidized, mica rich metafelsic rock. Both pyrite and sphalerite appeared in conjunction with mica, whereof pyrite as crack fillings. In more oxidized samples of mica rich rock, close to no sulfides were observed. The substantial difference in this rock is likely because the sulfides in conjunction with mica are particularly exposed to oxidation since fractures tend to appear along mica, and the sulfides located there have large specific surface areas. The presence of sulfides in the mica poor rock grouping of this study proved to be very heterogenous. In future studies there is therefore a need to further divide this group and study the sulfide-bearing metasedimentary rock from this site separately. In addition, the mineralogical source of the manganese in the water should be investigated in further studies