Groundwater is one of the major sources of freshwater in Oman. Therefore, it is important to identify mechanisms which regulate the quality of groundwater which is used for either drinking or agriculture irrigation. Hexavalent chromium is a toxic metal which could be released to the groundwater from anthropogenic or geologic source. The concentration of total chromium in drinking water in world health organization should not exceed 50 ug/L. The major mechanism for mobilization of Cr(VI) is through oxidation of Cr(III) which is released from ultramafic rocks. It is well established that oxidation of Cr(III) to Cr(VI) in the presence of manganese oxides. The aim of this study is the investigation of Cr(VI) presence in groundwater within ophiolite in two areas (the Nakhal chromite mining area and in Yanqul copper mine), in sedimentary basin which is the alluvial fan in Barka area and the control sample was taken from Hadash. The ground water as well as solid samples (rock, soil and sediment) from four areas were analyzed. The results showed that there are many groundwater samples have total Cr exceeding the permissible limit in both summer and winter. The Hadash control samples have large variability total Cr. Barka4 groundwater sample have the highest concentration of Cr(VI) in both summer and winter while Cr(VI) in Nakhal and Yanqul was moderate. According to X-ray fluorescence spectrometry (XRF) results all the solid samples were rich in Cr and the lowest concentration was seen in Hadash soil. X-ray diffraction (XRD) showed that all the solid samples contain serpentine minerals (Lizardite) and clinochlore which are capable to host Cr. The leaching test indicated that various forms of Cr in the sediments and soils which are related with chromite, serpentine (clinochlore) but also absorbed in the form of Cr(III) and Cr(VI). The main mechanism for releasing Cr into groundwater in this study was dissolution of serpentine minerals which released Cr(III) and then oxidized by manganese oxides to Cr(VI). In case of Barka water samples, sulfate ionic exchange can play a role in concentration of Cr(VI) in ground water. Finally, according to our results of this study the main source of Cr(VI) in groundwater is geogenic source from oxidation of Cr(III) which come from weathering products of mafic and ultramafic rocks to Cr(VI) by manganese oxides.