English: Conceptual diagram of weathering in the subsurface driven by acid production in an overlying soil. Some of the acid generated in soil may be neutralized by mineral weathering reactions in the soil and vadose zone (i.e., unsaturated zone between soils and the water table). The acid that is not neutralized there may react with the aquifer matrix. Two example weathering reactions are shown, one in which potassium feldspar (KAlSi3O8) reacts with acid to produce kaolinite (Al2Si2O5(OH)4) and aqueous potassium and silica, and the other in which calcium carbonate (CaCO3) reacts with acid to produce aqueous calcium and bicarbonate. Solutes generated by these reactions can be transported by groundwater flow to nearby surface water bodies or into the deep subsurface.