ABSTRACT: Nitrate pollution of shallow groundwater in Flanders is widespread. The assessment of its distribution is made difficult however because of the redox zoning in groundwater. Indeed, nitrates are stable only in the oxic groundwater zone, in the presence of oxygen. After oxygen has been (largely) consumed, mainly by oxidation of organic matter and sulphides, the redox potential has dropped to a level allowing for nitrate reduction. This nitrate-reducing zone is found at a certain depth, depending on the flow regime and the reduction capacity of the aquifer matrix. After nitrates have been reduced, at still larger depth and lower redox potential, Mn-and Fe-oxides become unstable and reduced Mn2+ and Fe2+ appear in the solution. At still lower redox potential, sulphates are reduced. It is evident that nitrate monitoring wells should have their screens in the oxic zone, in order to allow conclusions as to whether or not nitrate pollution of groundwater occurs. They should not have their screens below the nitrate-reducing zone, except to confirm the redox zone, and possibly to monitor the shift of redox zones towards larger depths as a function of time. The reduction capacity of aquifers will indeed be depleted as oxygen, nitrates and Fe3± are reduced, resulting in a shift of redox zones towards depth. Especially in lowly reactive aquifers to which highly nitrateloaded infiltration water is recharged, this may lead to a noticeable shift over the years.