Potential of controlled drainage and sub-irrigation to manipulate groundwater table for mitigating acid loadings in Finnish acid sulfate soils

Acid sulfate (AS) soils are located worldwide mainly on coastal and floodplain areas. In Europe, the largest cultivated AS soil areas are on the coasts of Baltic Sea in Finland. The oxidation of sulfidic materials in AS soils results in formation of sulfuric acid and consequent dissolution of toxic metals and deterioration of aquatic ecosystems. Their reclamation for cultivation by field drainage, dredging of main drains and poldering exposes soils for oxidation and once initiated the negative impacts may last for decades. Practical water protection measures to mitigate acid loads from cultivated AS fields are urgently needed. In this study, the impact of controlled drainage (CD) and sub-irrigation (CDI) on water quality was compared with a field that has normal subsurface drainage (ND). In CD and CDI groundwater depth was regulated by control wells and in CDI additional water was pumped into drains when groundwater dropped below the critical level. The groundwater could be kept higher in CD and in CDI than in ND and the higher groundwater slightly decreased acidity of discharge water. After dredging, the main drain groundwater level dropped deeper than earlier and consequently the acidity of discharge water increased. In CDI, the acid leaching was on average 22% less than in ND. However, ever more reduction in acid loadings is needed to reach the target of good quality of surface waters in AS soil areas till 2027, as aimed in the EU Water Framework Directive regulation.