Characteristics and implication of stable carbon isotope in high arsenic groundwater systems in the northwest Hetao Basin, Inner Mongolia, China

Organic matter is believed to trigger the formation of high As groundwater, which has been widely found in China. Sixty-three groundwater samples, seven multilevel-well groundwater samples and four surface water samples were collected to trace and delineate groundwater organic matter by using stable carbon isotopes of dissolved organic carbon, dissolved inorganic carbon, and sedimentary organic carbon in this study. Low delta C-13(DIC) mainly occurred in high As groundwater in the flat plain area. High As concentrations corresponding to high delta C-13(DOC) values indicated that As was released during microbial degradation of DOC coupled with reduction of Fe oxides. Along the flow path, delta(13)(DIC), ORP values and SO4/Cl ratios showed decreasing trends with the increases in As and dissolved Fe concentrations, illustrating a gradual increase in anaerobic microbial activities that caused reductive dissolution of Fe oxides and As enrichment. Results of groundwater samples from a multi-level well showed that small Delta delta (defined as the difference between delta C-13(DOC) and delta(13)(DIC)) indicated that more sufficient DOC degradation contributed to groundwater DIC concentrations and facilitated groundwater As mobilization.