Acidity fluxes following rewetting of sulfuric material

Abstract

We selected two sites with soil materials of clay and sand and used mesocosms to study the effect of rewetting sulfuric material with sea water and fresh water. The materials behaved differently due to different water seepage rates and acid stores. The water seepage rate was low for the clay soil (0.8×10–3 m/d) compared to the sandy soil (8–15×10–3 m/d). Initial acid flux rates were higher (0.1–0.2 mol/m2/d) compared to long term rates (0.007–0.014 mol/m2/d). For the clay soil, the low seepage rate and higher stored acidity resulted in a net flux of solutes from the soil to the water column. In contrast, the higher seepage flux of the sandy material resulted in a flux of water and solutes from the water column into the soil profile, displacing acid pore water deeper into the soil profile. However there is a residual acid flux into the water column probably due to the ongoing and slow dissolution of residual acid oxidation products. In both materials, sea water mobilised more acidity. Comparing the measured flux and calculated advective and diffusive fluxes highlighted the importance of solid phase reactions such as dissolution and surface exchange reactions.