Abstract: In the 12 km² catchment area of Syv creek, Denmark, moderate to high concentrations of nitrate (NO₃⁻) occurred in the upper part of the oxidized zone (oxic-I), but dropped within the lower suboxic part (oxic-II), to below the detection limit in the unoxidized zone. Structural Fe²⁺ in the clay minerals made up 10 to 12% of the Fe in the oxidized zone and increased to approximately 50% in the unoxidized zone. Concurrent with changes in the distribution of structural Fe²⁺ the clay mineral constituents changed. Vermiculite was typically found in the oxidized zone whereas chlorite was found in the unoxidized zone only. A conversion of illite and chlorite into vermiculite seems to take place. A significant correlation between NO₃⁻ and the amount of reduced Fe²⁺ in the suboxic (oxic-II) zone, indicates that primary structural Fe²⁺ in the clay minerals is the reductant in a NO₃⁻ reduction process.