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Dissolved organic matter (DOM) in both high As groundwater and low As groundwater were sampled from the Hetao basin and characterized by FT-ICR-MS for OM molecular characteristics. Results show that the greater number of molecular formulas was identified in DOM of high As groundwater (5125), compared to the number in DOM of low As groundwater (3603). Among them, lignins and aromatic structures were the most abundant compounds in both high As groundwater and low As groundwater, with 4434 formulas and 2944 formulas identified, respectively. The number of biodegradable compounds (aliphatic/proteins, lipids, and carbohydrates) was less in high As groundwater (197) than that in low As groundwater (279). Higher proportion of lignins and aromatic structures (86.5%) would lead to complexation between As and DOM, which enhanced As mobility. On the other hand, higher proportion of lignins and aromatic structures and lower proportion of bioavailable compounds in high As groundwater supported the hypothesis that more bioavailable compounds were consumed and resulted in the enrichment of lignins and aromatic structures and As release into groundwater.

High arsenic (As) groundwater has been found in deep aquifer of the northwestern Hetao Basin. Little is known about the source of high As deep groundwater and its role in As distribution. Based on strontium (Sr) isotopes, this study aims to investigate the provenance of high arsenic deep groundwater (DGW). 87Sr/86Sr in DGW gradually decreased from the alluvial fans to the flat plain. Results show that fissure water, plagioclase dissolution, and shallow groundwater (SGW) controlled the 87Sr/86Sr variation. Although the reductive dissolution of As-containing iron oxide controlled the As mobilization, the leaching of phyllite and meta-basalt, incongruent dissolution of plagioclase, and vertical mixing with SGW also played an important role in increasing As concentrations in DGW approximately along the flow paths.

Groundwater showed a trend from oxic-suboxic conditions in the alluvial fan to anoxic conditions in the flat plain in the Hetao basin, with increases in As, Fe2+, NH4+, alkalinity and pH, and decreases in Eh and NO3-. A 1D reactive transport model was constructed using a model flow column by PHREEQC to quantitatively interpret geochemical processes related to As mobilization in a 5000 m-long hydrogeological unit. Results indicated that groundwater As concentration was mostly controlled by Fe(III) oxide reduction and As adsorption processes. Observed Fe2+ was the combination of Fe(III) oxide reduction and mackinawite/siderite precipitation. Increasing alkalinity was the result of oxidation of organic carbon, rather than the dissolution of carbonates.

Temporal variations in water levels are crucial for understanding As behaviour in groundwater systems. Groundwater levels were recorded in irrigation wells in non-irrigation and irrigation seasons. Groundwater samples were collected yearly in irrigation wells from 2014 to 2016 and in a multilevel well from 2015 to 2016 for analysing geochemical parameters. Results showed that groundwater flow direction was reversed due to groundwater pumping. The change of groundwater flow led to the surface water, as a new groundwater recharge source, which flushed the near-surface sediments enriched in soluble components and increased groundwater TDS. The labile organic matter introduced by the surface water recharge fueled dissimilatory reduction of Fe(III) oxides and further increased groundwater As concentration.

Manganese oxides are considered as one of the effective oxides capable of oxidizing arsenite and reduce the toxicity of arsenic. Since low molecular weight organic acids (LMWOAs) commonly found in nature can act as reducing and chelating agents for manganese oxides, it is particularly important to investigate how these organic acids with different numbers of carboxyl groups like citrate and EDTA affect oxidation and adsorption of arsenic by manganese oxides. In this study, low As(V) adsorption on manganese oxide is slightly enhanced by citrate and EDTA, which results from the increase in active sites via reduction of manganese oxide by LMWOAs. However, citrate and EDTA have different effects on the oxidation of As(III). MnIII/II citrate autocatalytic cycle as a manganese-based redox system decreases As(III) oxidation rate, but EDTA does not yield autocatalysis, which slightly increases the oxidation rate of As(III). Reduction of manganese oxide by EDTA and chelation between Mn(II) and EDTA lead to exposure of more active sites. Our research highlights the different effects of low molecular weight organic acids on the reactions between arsenic and manganese oxide.

Groundwater is the dominant long-term water resource for agricultural irrigation and industrial production in the Baiyangdian basin, North China Plain. Groundwater and pore-water were investigated to evaluate chemical evolution and geochemical processes in shallow and deep aquifers. Results show that both shallow groundwater and shallow pore-water had higher TDS, Ca2+, Mg2+, SO42-, and HCO3- concentrations than deep groundwater and deep pore-water. Generally, concentrations of groundwater major ions were higher than those of pore-water in shallow aquifers, while they were slightly lower in groundwater than in pore-water from deep aquifers. Water isotopes showed the meteoric origin of groundwater and pore-water, although evaporation signature was traced in shallow groundwater. Shallow groundwater also experienced carbonate dissolution and silicate weathering. Silicate weathering and evaporite dissolution were the major hydrogeochemical processes in deep aquifers. This study indicated that deep groundwater has better water quality, but is vulnerable to contamination from shallow groundwater with high TDS and NO3- concentrations.