A review of high arsenic groundwater in Mainland and Taiwan, China: Distribution, characteristics and geochemical processes

•High As groundwaters occur in inland basins and river deltas in China.•Twenty provinces have found high As groundwaters among 34 provinces.•Aquifers are characterized by reducing conditions and low groundwater flow rates.•High As groundwater has high Fe, Mn concentrations and low NO3-, SO42- concentrations.•Both redox processes and desorption processes result in As mobilization. China is a typical high-As region, where 20 provinces have high As groundwaters among 34 provinces. These groundwaters usually occur in both arid–semiarid inland basins and river deltas. In the inland basins, mainly distributed in the northwest of China, shallow groundwaters usually have high As concentrations in alluvial lacustrine or lacustrine sediment aquifers, while high As groundwater mainly occurs in fluvial–marine sedimentary aquifers in the river deltas, which have been affected by transgression. In both the inland basins and the river deltas, high As groundwaters, mainly occurring in reducing conditions, are characterized by high Fe and Mn concentrations, high pH and HCO3- concentration, and relatively low NO3- and SO42- concentrations. Although As contents are well correlated to Fe/Mn contents in the aquifer sediments, groundwater As concentrations are generally independent of sediment As contents. Redox processes, microbe-related reduction, and desorption processes are the major geochemical processes for As enrichment in groundwaters. In reducing conditions, both reductive dissolution of Fe oxides and reductive desorption of As are believed to result in As mobilization, which would be catalyzed by indigenous microbes. Although decomposition of the low-molecular weight organic matter during microbe metabolization would also release the colloid-bound As into groundwater, the cycling of colloidal As still needs to be further investigated during redox processes. Besides, high pH and high HCO3- lead to As desorption from adsorption sites in the aquifer systems. However, the contribution of competitive desorption to high As concentrations is still unknown and remains to be discovered, relative to reductive dissolution of Fe oxides, especially in the inland basins.