r(III) ions and a well resolved hyperfine structure due to manganese ions, resulting from Mn(IV) and Mn(II). The isotopic and EPR data support the hypothesis of Cr(III) being oxidized by Mn oxides which are widespread in the aquifer, possibly related to the oscillation of the phreatic level. The obtained results highlight the usefulness of chromium stable isotopes as environmental tracers and support the observations that naturally occurring Mn oxides in soils may catalize Cr oxidation from the stable Cr(III) form to the toxic Cr(VI) soluble form, yielding valuable information in planning reme