Laboratory experiments were used to study the geochemical controls on sediment reactivity and buffering processes during the injection of oxygen (O 2) saturated waters into a deep anoxic heterogeneous siliclastic sedimentary aquifer. Detailed geochemical characterisation and sediment incubation experiments identified pyrite (20-100%), sedimentary organic matter (SOM; 3-56%), siderite (3-28%) and Fe(II)-aluminosilicates (8-55%) as the main O 2 reductants. Trace-levels of carbonate acted as a pH buffer, while a lower boundary pH of 3 indicated acid buffering by K-feldspar dissolution. The processes identified were used to formulate a kinetic reaction modelling framework that was able to reproduce (i) the observed, transient O 2 consumption and CO 2 production (ii) the major ion composition at completion, and (iii) the observed trace metal releases in each of the treatments. The experiments showed that the approach was useful for identifying and quantifying key geochemical reactions that affect the water quality evolution in artificially recharged anoxic aquifers.