Impact of peatland forestry on runoff water quality in areas with sulphide-bearing sediments; how to prevent acid surges

Abstract

Several rivers around the Baltic Sea coast have poor water quality status due to occasional low pH and high metal concentrations. One potential cause of low pH is intensive drainage of shallow peat soils (<80cm thick peat layer) overlying sulphide-rich sediments. This results in oxidation of iron sulphides to sulphuric acid, lowering water pH dramatically, and increases the concentrations of soluble toxic metals (e.g. Al, Cd, Ni, Zn). This study examined soil properties and hydrology in order to investigate the leaching of acidity derived from peatland forestry and devise potential ways to prevent acid leaching after drain maintenance work. Groundwater (GW) level, runoff amount and water quality were recorded at regular intervals for three small forested catchments (drained area 8.5–13ha) during two growing seasons, 2010 and 2011. The soil was sampled and analysed for pH, electrical conductivity (EC) and sulphur. Undisturbed soil columns consisting of mineral subsoil were taken to the laboratory and exposed to lowered water levels in a controlled drainage experiment. The DRAINMOD model was used to simulate the impact of different drainage configurations. The simulation results suggest that the oxidation risk of sulphidic materials in subsoil during normal hydrological years is small in drained peatland forestry areas with a thick peat layer. The risk of oxidation of sulphidic materials increases during dry summers with a clear decline in GW level into the sulphidic subsoil. According to laboratory data, this can considerably lower the pH (down to 2.5) and increase metal concentrations (e.g. maximum Al, Cu and Zn concentrations of 1010, 257 and 186μgL−1, respectively) in runoff waters. However, with effective management methods these effects can be avoided. Changes in drainage practices (avoiding deep drainage) and controlled drainage may in some cases reduce the GW level drawdown and reduce the risk of sulphide oxidation.