Abstract The hydrogeochemical processes that took place during an aquifer storage and recovery (ASR) trial in a confined anoxic sandy aquifer (Herten, the Netherlands) were identified and quantified, using observation wells at 0.1, 8 and 25m distance from the ASR well. Oxic drinking water was injected in 14 ASR cycles in the period 2000–2009. The main reactions consisted of the oxidation of pyrite, sedimentary organic matter, and (adsorbed) Fe(II) and Mn(II) in all aquifer layers (A–D), whereas the dissolution of carbonates (Mg-calcite and Mn-siderite) occurred mainly in aquifer layer D. Extinction of the mobilization of SO4, Fe(II), Mn(II), As, Co, Ni, Ca and total inorganic C pointed at pyrite and calcite leaching in layer A, whereas reactions with Mn-siderite in layer D did not show a significant extinction over time. Iron(II) and Mn(II) removal during recovery was demonstrated by particle tracking and pointed at sorption to neoformed ferrihydrite. Part of the oxidants was removed by neoformed organic material in the ASR proximal zone (0 – ca. 5m) where micro-organisms grow during injection and die away when storage exceeds about 1month. Anoxic conditions during storage led to increased concentrations for a.o. Fe(II), Mn(II) and NH4 as noted for the first 50–200m3 of abstracted water during the recovery phase. With a mass balance approach the water–sediment reactions and leaching rate of the reactive solid phases were quantified. Leaching of pyrite and calcite reached completion at up to 8m distance in layer A, but not in layer D. The mass balance approach moreover showed that Mn-siderite in layer D was probably responsible for the Mn(II) exceedances of the drinking water standard (0.9μmol/L) in the recovered water. Leaching of the Mn-siderite up to 8m from the ASR well would take 1600 more pore volumes of drinking water injection (on top of the realized 460).