Characteristics of sulfide bearing soil materials in peat extraction areas in N-Finland

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Concern has been raised about the potential formation of acid sulfate soils and associated environmental problems related to peat extraction and, thus, peat, sediment and till characteristics of 15 well drained peat extraction fields were investigated in northern and northwestern Finland. The aim was to identify and characterize the occurrence/abundance of potential acid (hypersulfidic) and actual acid sulfate soil materials as well as metals with regards to their depositional environment (marine/non-marine), black schists and soil material properties. Sulfide-bearing marine sediments were commonly found; the highest contents and thickness of sulfide sediments were found in areas <50 m above the current sea level (a.c.s.l.), while the sulfide contents were relatively low in the mineral soil materials (mainly till) in the non-marine areas (>100 m a.c.s.l.). The highest content of sulfides in sediments was typically found just below the peat layer. The sulfides existed mostly as pyrite (up to 3.5 wt% S) but were occasionally also mixed with more reactive metastable sulfide (Fe:S ratio in the order of 1:1), which coincide with high Fe concentrations, indicating that an abundance of Fe2+ can diminish the rate of pyrite formation. Sediments contained very high amounts of potential acidity, but in-field oxidation of the sediments was very limited. Although the sulfur contents were much lower (max 0.3% S), several of the till samples still became acidic (pH < 4) upon oxidation, and contained, thus, some amounts of potential acidity due to a pore buffering capacity. Consequently, during peat extraction most of the acidity is still retained in sediments limiting environmental consequences and, thus, focus should be on appropriate management to prevent oxidation and leaching after peat extraction has been finished. Locally high Mn and Zn concentrations were found in the sediments in black schist areas, indicating that Zn and Mn have migrated with the ground water from the schists to the sediment layers. In several black schist areas, the peat layer contained high amounts of sulfides with a corresponding high potential acidity as well as elevated As, Pb and Zn. This indicates an upward transport of sulfur and some metals (As, Pb and Zn) through capillary rise and/or plant uptake and/or through lateral water transport from surrounding black schist affected soil material. Air pollution, i.e. atmospheric distribution, was most likely a source for high concentrations of Cd, Hg and Sr concentrations in the peat.