Pyrite in Dutch peat - influence of paleoenvironment and current land use

Peatlands in the Netherlands contain high amounts of sulfur (S). Drainage of these peatlands has led to oxidation of the peat, more recently enhanced through extended drought periods from the result of climate change. Oxidation of peat leads to the mobilization of S and elevated sulfate levels in surface waters. High sulfate concentrations are considered a water quality problem and can enhance eutrophication. In this study, the S content and S speciation in Dutch peats were investigated and their relation to paleoenvironment and current land-use.

Peat samples from eight locations in an east-west section, varying over paleoenvironment, peat type, proximity to the River Rhine and the North Sea, and current land‑use were analyzed. Sequential sulfur extraction was performed to fractionate iron-monosulfide, pyrite and organic-bound sulfur. Porewater was analyzed for sulfate, iron and nitrate concentrations to investigate their influence on the S speciation.

The analytical results could be split into 3 groups. The first group consisted of peats of marine paleoenvironment which had the highest total S content. Due to limited availability of iron (Fe), sulfur was predominantly present as organic-S and <10% of S was present as pyrite. Group 2 consisted of peat from fluvial paleoenvironment origin. In these groundwater fed, nutrient rich, minerotrophic fens, pyrite made up a larger portion of total S. In group 3, the peat was influenced by the river ‘Oude Rijn’. This river was polluted with higher sulfate concentrations, relative to rain water, and also carried clay particles rich in ferrous iron. Flooding events brought Fe to the peat-forming system resulting in pyrite formation. The influence of land-use was only visible in the top layers; high concentrations of nitrate in combination with a low pyrite content and elevated sulfate concentrations were likely caused by the input of fertilizer and subsequent denitrification and oxidation of pyrite.

Overall, paleoenvironment was found to be the predominant factor controlling S content and S speciation in Dutch peats in the Western Netherlands. Particularly the presence of pyrite was related to the presence of fen reed peats and dependent on a nearby Fe source during peat formation.