Hydrogeochemical impact of coarse-grained post-glacial acid sulfate soil materials

- Coarse-grained acid sulfate soil materials show a rapid drop in pH upon oxidation.
- Similar incubation pH-values as in fine-grained acid sulfate soils.
- 10-100 times lower sulfur concentrations than in fine-grained acid sulfate soils.
- Elevated acidity and metal concentrations in ground- and surface water.

Acid sulfate (a.s.) soils have long been under investigation in Finland due to their negative impact on the environment. Earlier studies have mostly focused on fine-grained (< 63 μm) a.s. soil materials, but acidification caused by coarse-grained (≥ 63 μm) post-glacial a.s. soil materials has recently gotten more attention. Using a “let the soil speak for itself” incubation approach, we investigated six soil profiles consisting of mostly coarse-grained potential a.s. soil materials in a study area in western Finland. Although the sulfur concentrations and acidities in the reduced sulfidic coarse-grained parent materials were significantly lower (10-100 times) compared to fine-grained a.s. soil materials, the pH-values were similar and well below 4.0 for most samples after a 16-week (incubation) oxidation period. The coarse-grained materials also oxidized rapidly and displayed a fast drop in pH during the incubation. This was most likely due to a poor buffering capacity caused by the low specific surface and a quartz-feldspathic mineralogy, which is likely to cause a rapid leaching of acidity. No actual a.s. soil materials (pH < 4.0) were observed in the sampled soil profiles but are likely to occur within the study area; since all, besides one, of the sampled soil profiles contained potential a.s. soil materials. Acidity and metal problems were common in a nearby groundwater area utilized for drinking water, where Al, As, Cr, Co, Fe, Mn and Ni concentrations were elevated at least 10 times over background median values. In addition, oxidation of coarse-grained a.s. soil materials appear to have caused the pH to decrease as low as 3.4 in several sand pit lakes, formed by sand mining, where median levels of SO4, Al, Mn, Ni, Se and Zn were elevated > 10 times over background concentrations. We suggest that there is a strong link between the oxidation of coarse-grained post-glacial a.s. soil materials and a high acidity and elevated metal concentrations in groundwater and sand pit lakes.