Evolution of pH, redox potential and solute concentrations in soil and drainage water at a cultivated acid sulfate soil profile

Abstract

Land drainage lowers the soil water table, which triggers enhanced oxidation of iron sulfide minerals in acid sulfate soils, causing acidification of fields and posing environmental hazards to aquatic ecosystems. The objective of this study was to develop a model for investigating the effects of oxidation of iron sulfide minerals and the cation exchange capacity (CEC) on the long-term evolution of pH, redox potential and solute concentrations in soil water and drain discharge. Two electron acceptors (oxygen and ferric iron) were included in the model. The model was implemented in the HP1 modeling platform (coupled HYDRUS-1D and PHREEQC-3) and applied to an acid sulfate soil profile of an agricultural field in Finland. The geochemical part of the model was tested against field observations. A comparison of different model versions showed that CEC had an essential role in mimicking the observed pH profile of soil. The drain discharge water quality was similar in the model versions both with and without CEC, but the buffering effect of CEC could be seen as lower iron concentrations. The oxidation of metastable iron sulfide minerals was three times faster than the oxidation of pyrite. Descriptions for the oxidation of both two minerals and for CEC were required to conduct long-term simulations that reproduce the observed geochemical state of a drained agricultural field. The HP1 modeling platform facilitated the testing of alternative chemical transformation processes of an AS soil profile in response to increased drainage efficiency after subsurface drainage installation.