Aquifer samples from the precipitation zone of an in situ iron and manganese removal plant that was operated for 10 years were analyzed for iron and manganese minerals. Measurements were performed by various chemical extraction techniques (5 M HCl, 0.0081 M Ti(III)-EDTA, 0.114 M ascorbic acid), X-ray diffraction and Mossbauer spectroscopy. Chemical extractions showed that iron was precipitated as ferric oxides, whereas manganese was not oxidized but deposited as Mn(II) probably within carbonates. The ferric oxides in particular accumulate preferentially in the smaller grain-size fractions. This tendency was observed to a lesser extent for manganese. X-ray diffraction and Mossbauer spectroscopy showed that the ferric oxides were mainly crystalline (goethite, 50% to 100% of the iron). Ferrihydrite was found as well, but only as a minor fraction (less than or equal to 12%). Pure manganese minerals were not found by X-ray diffraction. The precipitated amounts of iron (5 to 27 mu mol/g Fe as ferric oxide) and manganese (1 to 4 mu mol/g Mn) during 10 years operation of the treatment plant agree with values that were estimated from operational parameters (9 to 31 mu mol/g Fe and 3 to 6 mu mol/g Mn). Considering the small amounts of precipitated iron and manganese, no long-term risks of clogging of the aquifer are expected.