Soil aquifer treatment (SAT) is increasingly considered to be a favorable polishing method before the use of secondary or tertiary wastewater effluents for unrestricted irrigation or disposal to the sea. Most of the oxidation processes of organic compounds as well as of nitrogen compounds take place in the unsaturated zone of the SAT. A material balance over the unsaturated zone of a high-frequency SAT system reveals that 90% of the removal of organic matter and complete ammonium oxidative removal already occur within the unsaturated zone. This is observed, despite the short (less than 3 wk) water retention time in this zone, compared with the subsequent 6- to 36-mo retention time within the aquifer. An oxidizer balance over the unsaturated zone reveals that 42.5 ± 12 mg/L of oxygen is consumed in the unsaturated zone. This consumption level is five times higher than the oxygen saturation level under the conditions prevailing in the effluent. The implications of this excessively high oxygen demand in the vadose zone on manganese mobilization in the aquifer are addressed.