Abnormal levels of co-occurring arsenic (As), fluorine (F) and iodine (I) in groundwater at Datong Basin, northern China are geochemically unique. Hydrochemical, O-18 and H-2 characteristics of groundwater were analyzed to elucidate their mobilization processes. Aqueous As, F and! ranged from 5.6 to 2680 mu g/L, 0.40 to 3.32 mg/L and 10.1 to 186 mu g/L, respectively. High As, F and I groundwater was characterized by moderately alkaline, high HCO3-, Fe(II), HS- and DOC concentrations with H3AsO3, F- and I- as the dominant species. The plots of delta O-18 values and Cl/Br ratios versus Cl- concentration demonstrate build-up of more oxidizing conditions and precipitation of carbonate minerals induced by vertical recharge and intensive evaporation facilitate As retention to Fe (hydr) oxides, but enhance F and I mobilization from host minerals. Under reducing conditions, As and I can be simultaneously released via reductive dissolution of Fe (hydr) oxides and reduction of As(V) and I(V) while F migration may be retarded due to effects of dissolution-precipitation equilibria between carbonate minerals and fluorite. With the prevalence of sulfate-reducing condition and lowering of HCO3- concentration, As and I may be sequestered by Fe(II) sulfides and F is retained to fluorite and on clay mineral surfaces. (C) 2015 Elsevier B.V. All rights reserved.