Acid sulfate soils are described as the nastiest soils on Earth and are broadly composed of metal sulfides. These minerals are mostly harmless in a reducing environment. However, when these soils are drained oxygen infiltration occurs. Oxidation initiates a geochemical process, thereby starting the generation of acidity. As the pH drops, a consortium of acidophilic microbiota begin to grow and flourish. These microbes catalyze the oxidation reactions, which further generates acidity, thus driving the pH even lower. The decreasing pH leads to the solubilization of any co-occurring metals within the system. During flushing events the built-up acidity and solubilized metals mobilize and flow out of the soils into surrounding waterways to the potentially lethal detriment of resident flora and fauna.

This dissertation firstly explores the microbial communities that inhabit acid sulfate soils throughout Sweden and around Vaasa, Finland, and secondly the analogous communities of a mine waste rock repository in northern Sweden. Results from Finland showed an increase in relative abundances of extremely acidophilic microbes correlated to the decreasing pH values that followed the oxidation front. Acidity generation was not mitigated by additions of lime. Further laboratory incubations found that higher volumes and finer material sizes of lime delayed acid generation but did not prevent the development of neutrophilic iron and sulfur oxidizing microbes. The survey of Sweden extended the distribution range of acid sulfate soils and found community differences between the northerly and southerly acid sulfate soils, which were hypothesized to be a result of regional temperature variation. Furthermore, regional differences of the field oxidized samples disappeared following laboratory incubations, further supporting temperature as a driver of regional differences. Lastly, the Swedish waste rock repository study suggested that there were tipping points associated with ongoing oxidation. Subsurface associated communities rapidly decreased following excavation and were slowly replaced by a simple acidophilic community; over time a radiation of acidophiles occurred leading to an increase in acidophile diversity.

These studies together show that metal sulfide rich environments are host to resident neutrophilic to extreme acidophilic microbial communities that play integral roles to the generation of acidity and metals leaching. The composition of those communities differ based on temperature, pH, substrate type, and oxidation age. With regard to remediation strategy development, the application of fine grained lime in combination with peat may hold potential to for short termed acidity mitigation. However cautionis required when transitioning from laboratory based approaches to field trials as the communities are dynamic and complex. 

Acid sulfate soils are some of the most problematic soils on Earth due totheir significant release and mobilisation of acidity and metals. In Sweden,these soils have been studied to a lesser extent than in some other countrieswhere they occur as frequently. The aim of this thesis was to determine thegeographical distribution and geochemical (and microbiological) featuresof acid sulfate soils in Sweden.Field work included visiting 126 sites distributed relatively evenlythroughout the coastal plains, with each site located where acid sulfatesoils were most likely to have developed. A wide range of geochemicalanalyses were carried out: analyses of pH in situ and after incubations inthe laboratory; loss on ignition; and chemical extractions using water, 1MHCl, aqua regia, and a strong 4-acid digestion followed by ICP analyses.Statistical analyses of the geochemical data included mostly nonparametrictests for univariate and bivariate methods, and compositionaldata analysis for multivariate methods. Microbiological analyses werecarried out by a co-author.A total of 47% of all the sampled sites were classified as acid sulfatesoils, either directly in the field or after laboratory incubations. Acid sulfatesoils were identified on all coastal plains, with the highest relativeoccurrence in the northeast. The southeast coastal plains had a higherrelative occurrence than expected and the lowest measured field pH, lowestincubation pH, and the highest S concentrations. A large number ofgeochemical features were identified and characterized. These includedlosses of major and trace elements from the sulfuric soil material;physicochemical differences between acid sulfate soils in the north andsouth; and high water-soluble concentrations of several elements (e.g. Cd,Co, Mn, Ni, and Zn) indicating that these elements are at a large risk ofmobilisation and leaching. Furthermore, several chemical elements wereretained in the sulfuric soil material (e.g. Fe, As, Cr, Cu, U), most likelydue to binding by secondary Fe (III) phases. These retained elements aresusceptible to future leaching, for example under remediation scenariosthat raise the groundwater table to re-introduce reducing conditions.