Easily mobilized metals and acidity in acid sulfate soils across the Swedish coastal plains

Acid sulfate soils are found globally and have significant environmental impact as a source for metals and acidity to surrounding streams that can cause, for example, large-scale fish kills. In the face of changing climate and its effect on groundwater fluctuations, the environmental risk associated with these soils needs to be thoroughly investigated. This study examined the water-soluble concentrations of multiple elements from the oxidized, transition and reduced zones of acid sulfate soil profiles situated on the Swedish coastal plains. By comparing untreated (naturally oxidized in field) and incubated samples from these zones, we gain insight into the current and near-future mobilization and leaching of acidity and metals that occur in these soils. The results showed that concentrations of Al, Cd, Co, Mn, Ni, S and Zn mobilized from incubated samples were about an order of magnitude higher than from the untreated samples. Notably, the concentrations of mobilized Co, Mn and Ni were higher than released by 1 M HCl at the same sites, highlighting the particularly high mobility of these metals from in situ oxidation of acid sulfate soils. Conversely, Fe and Cu showed lower than expected water-soluble concentrations and were also low compared to the 1 M HCl-extractable element concentrations, likely due to rapid re-mobilization of secondary Fe minerals. Arsenic, Cr and Pb showed overall low water-soluble concentrations in both the incubated and untreated samples, consistent with these elements not being abundantly leached from acid sulfate soils. This observation was further supported by the retention of these metals in secondary Fe-mineral phases such as jarosite and schwertmannite as reported in previous studies. A strong correlation between acidity and near-total S indicated that S can serve as an indicator for the acidification risks associated with acid sulfate soil oxidation. Overall, the findings demonstrated that even a small lowering of the groundwater table can lead to significant mobilization of metals and acidity. This highlights the increased risks of environmental degradation in the face of climate change and intensified drainage operations and, thus, the need for proper management to reduce the risks.