Acid sulfate soils (AS-soils) refer to soils or fine-grained sulfide-bearing sediments that can produce acidity through sulfide oxidation. AS-soils do not pose a risk under waterlogged conditions; however, when exposed to oxygen, they can cause significant environmental and economic impacts. AS-soils oxidation results in sulfuric acid production, acidification of water bodies, element mobilization within the soil and into aquatic systems, adverse effects on biota, infrastructure damage, and potential deleterious effects on human health. Studying AS-soils presents a significant challenge due to their widespread impacts, highlighting the need for efforts to mitigate their environmental consequences. This doctoral thesis investigated the geochemical and mineralogical characteristics of AS-soils in northern Sweden, aiming to enhance understanding of their characteristics and weathering processes and thus contribute to developing environmental management strategies.

AS-soils represent a global concern due to their extensive distribution, mainly in coastal areas that were once covered by saline or brackish water. In Sweden, AS-soils are distributed along the coastline and are derived from post-glacial sediments enriched with Fe sulfides. In the Baltic Sea, sulfide-bearing sediments are exposed to oxygen due to post-glacial isostatic uplift or groundwater table lowering. In Sweden, AS-soils are already oxidized, posing environmental risks that are further intensified in northern regions due to a greater rate of isostatic uplift. AS-soils are characterized by the accumulation of elements in a transition zone (TZ) between oxidized and reduced sediments. The oxidized zone (OZ) is distinguished by low pH values (< 4), element depletion, and the precipitation of Fe (oxy)hydroxide and Fe hydroxysulfate secondary minerals as common products of sulfide oxidation. In contrast, the reduced zone (RZ) represents the AS-soil parent material and consists of unoxidized black, fine-grained laminated sediments, characterized by a high total organic carbon content, high S content, and abundance of primary Fe sulfide minerals. Groundwater fluctuations influence soil redox conditions and pH, causing the transformation of unoxidized sediments into oxidized ones. This zone transformation induces the dissolution and reprecipitation of minerals, leading to element mobilization. 

This study analyzed AS-soils in Luleå, northern Sweden, from a geochemical and mineralogical perspective. Research on soil and groundwater was conducted in one waterlogged (SW) and one oxidized (SN) AS-soil profile located in Södra Sunderbyn, in the vicinity of the Lule River. Elemental distribution over the different zones was analyzed, identifying depletion and accumulation zones, and assessing the mobilized elements as potential contaminants in soil and water bodies. Mineralogical analyses were conducted through techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD), and microprobe, which were applied with the aim of identifying primary and secondary minerals, their composition, textures, morphologies, and distribution over the zones, and their impact on soil chemistry under weathering conditions. Furthermore, sequential extraction experiments were performed to associate the elements with each of the soil fractions and to quantify the potential elements released under oxidation. Additionally, interactions between groundwater and subsoil were evaluated, identifying sources and element mobilization pathways, acting as potential contaminants.

The results demonstrated that both AS-soil profiles have the potential to generate acidity and mobilize elements upon exposure to oxygen, posing a negative environmental impact. Different S species were identified across the soil profiles. In unoxidized samples, S mainly occurs as primary Fe sulfide, metastable Fe sulfide, and organic S. Framboidal pyrite, the most abundant sulfide mineral and the primary acidity contributor, precipitated under anoxic-euxinic conditions through microbial sulfate reduction (MSR), as evidenced by negative δ³⁴S values. In contrast, oxidized samples predominantly contain S in secondary Fe hydroxysulfate minerals, such as jarosite or schwertmannite, which display negative δ³⁴S values indicative of sulfide oxidation processes, associated with their precursor sulfide.

Incubation experiments over the profiles showed that pH decreases the most in samples with high S content, but not necessarily with high TOC content. In this research, it is demonstrated that framboidal pyrite is highly reactive and prone to rapid oxidation even with short periods of exposure to oxygen. Compositional maps obtained by microprobe analysis indicated that framboidal pyrite is a source of Cu, Mn, Mo, and Ni. These trace elements are typically distributed within the framboids, except for Mn, which surrounds the framboids, creating a Mn-rich rim. The labile and more stable organic fraction is strongly associated with Cu, Mo, and S, which are susceptible to mobilization during weathering. Despite past oxidation and element mobilization occurring in the OZ, this zone still exhibited a high percentage of elements with potential to be removed, as shown in the extraction experiments. The most soluble phases and pore water contribute with high concentrations of Cd, Cu, Mn, Mo, and S, indicating their high potential for environmental release.

Seasonal variations result in groundwater fluctuations exposing the sulfide-bearing sediments to oxygen as the water table decreases, leading to oxidation and acidification. In contrast, during high water table periods, secondary mineral dissolution and element mobilization take place. These fluctuated redox conditions were evidenced by variations in δ⁵⁶Fe groundwater values, due to dissolution and transformation of Fe phases. The SN well registered high concentrations of Al, Co, Fe, Mn, Ni, S, and Zn in May and October, during low groundwater table periods. In contrast, the SW well remained waterlogged, preventing oxidation and acidity generation. The difference between the wells is evident in δ³⁴S values, with the SW well exhibiting strong ³⁴S enrichment from active MSR, while the SN well reflects dominant sulfide oxidation and limited MSR activity. 

This study shows the environmental risks associated with sulfide oxidation in northern Sweden and demonstrates that maintaining sulfide-bearing sediments under waterlogged conditions is essential to limit oxygen exposure, reduce acid generation, maintain alkaline pH levels, and minimize the release of dissolved elements, thereby mitigating negative environmental impacts.