Oxidation of iron sulfides in subsoils of cultivated boreal acid sulfate soil fields – based on soil redox potential and pH measurements

Acid sulfate soil leachates deteriorate the aquatic ecosystems of their recipient waters around the world. In Finland, AS soils are located mainly on the coast of the Baltic Sea, where rivers and estuaries suffer from acid leachates and waters do not meet with the criteria of good water quality set by the EU. Field drainage of cultivated AS soils is attributable to leaching of acidity, but regardless of various mitigation measures, the acidity of discharge water in these areas has not decreased significantly. In order to better understand the pathways involved in the formation of acidity, the redox status of 56 Finnish AS soil fields was examined using redox potential and pH data measured down to 2 m. The findings indicated that the oxidation of soils has occurred at depths below the drainage pipes, with the median being at a depth of 1.6 m. In fields cultivated for a long time, soil texture had a stronger effect on the depth of the redox interface than the drainage method; open ditch drainage and subsurface drainage; oxidation being faster in sandy and silty soils than in clayey soils. The isostatic land uplift also seems to affect the depth of the redox interface in the long run. Most of the studied fields had been cultivated for at least 30 years prior to the study. However, the pH values of the soils were still very low, probably due to actual and retained acidity. The prevention of oxidation of sulfidic materials in subsoils is important, but measures for neutralizing the acidity are needed. Without them it seems that the leaching of acidity will continue and may decrease only slowly. However, severe droughts during summers and the reclamation of unripe AS soils for any purpose will increase the leaching of acidity.