| dc.description.abstract | Sulphide-bearing gneisses of southern Norway have been observed to leach acidic waters with ecotoxic levels of metals when exposed to air and moisture following excavation for road and infrastructure projects. Traditionally, the sulphide mineral content (or % S) in these rocks has been used to estimate their acid producing potential. However, a number of other lithology related factors have been shown to influence the rate at which these minerals oxidise and the subsequent toxicity of the effluents. Through the use of column leaching experiments and detailed petrological and mineralogical analysis, the effects of mineralogy and texture, particle size and temperature on reaction rates and leaching behaviour has been investigated. Leachates resulting from gneiss material having the fastest oxidation rates, lowest pH and fastest release rates of elements such as Al, Ni, Cd and Cu were from rock samples with a well-developed and rust-coloured weathering crust. Secondary sulphate and Fe-oxide minerals, typically dominating this weathered material have proven to play a significant role in the acid-forming processes despite making up only a small percentage of the total rock mass. While overall sulphide mineral content was observed to influence ARD production, rock textural properties such as sulphide morphology and their spatial relationship to other minerals may also have influenced the observed oxidation rates. Anomalously low oxidation rates were, however, observed in one of the studied gneiss samples. This sample, characterised by a hard and crystalline texture containing significant quartz and small amounts of calcite, is believed to represent texturally controlled oxidation rates. Smaller particle size ranges (0-4mm) were expected to produce faster oxidation rates than larger ranges (0-10mm), however, this was only observed for columns containing weathered crust material. This demonstrates a relatively higher reactivity of the finer-grained secondary minerals constituting the weathered crust. Reduced water flow through most columns with the smaller grain size range (0-4mm) was responsible for lower reaction rates as mineral dissolution became transport-controlled. In the coarser material, surface control is argued to prevail. While reaction rates of rock samples were faster in warmer temperatures, a threshold temperature where Al and heavy metal element release rates increased and accelerated was observed between 4 and 10°C. Results from this study show that although playing a major role, sulphide content alone cannot dictate a rock ARD potential. In addition, the integrated weathering history, inherent rock texture, primary mineralogy, sulphide morphology and grain size all influence the calculated oxidation rates resulting from the performed column experiments. Hopefully, these findings may contribute to a revision of the current sulphur-based guidelines for the classification of ARD potential in the gneissic rocks of the Lillesand area. | eng |