Hexavalent chromium in water occurs as oxoanions that are toxic for plants and animals. Furthermore, they are soluble and extremely mobile under environmental conditions. Chromium in its other common oxidative state (Cr+3) is less toxic and in general immobile. Chromium is found in nature mainly in the Cr+3 form. However, human activities and natural occurring Mn oxides that can oxidize Cr+3 to Cr+6, increase the concentration of the toxic form of Cr+6 in nature. The extended worldwide problem of Cr+6 pollution makes essential the understanding of the oxidation mechanism of Cr+3 from Mn oxides, for assessing the danger of Cr+3 oxidation as well as for developing processes to reduce its formation and presence. In this study, the natural occurrence of Cr+6 was examined in ground and surface waters of the South-West part of mountain Vermio (Kozani, Greece), an area where human activity is absent. The aim is to identify a natural background for Cr+6 concentration which can assist in legislation limit definition. Results show that in cases of waters exiting ophiolite or water permeable limestone having underlying layers of ophiolithic rocks, Cr+6 is detected. On the contrary when a thin layer of water impermeable schistolith interposes between limestone and ophiolite, then Cr+6 is not detected.