The distribution characteristics and geological control factors of shallow high-arsenic groundwater in the Hetao Plain, Inner Mongolia, from the perspective of Late Pleistocene–Holocene depositional environments

The alluvial–lacustrine strata that were formed by the evolution of rivers and lakes in the Hetao Plain during the Late Quaternary have an important influence on the formation and distribution of shallow high-arsenic groundwater. This study analyzed the distribution characteristics and depositional environments of shallow high-arsenic groundwater in study area using 1179 groundwater samples and more than 1100 pieces of drilling data. The indicator kriging statistics and the study results of the Quaternary lithofacies paleogeography show that the study area can be divided into three high-arsenic probabilistic distribution areas, namely, the Houtao Plain (HTP), the Yellow River Channel Belt (YRCB), and the Eastern Hubao Plain (EHBP). The depositional environment of the HTP was shaped by the alluviation of the Yellow River during the Late Pleistocene–Holocene. The YRCB is still affected by the alluviation of the Yellow River presently, and the EHBP was almost unaffected by the Yellow River. The high-arsenic groundwater in the EHBP is mostly distributed in the relatively continuous alluvial–lacustrine strata and has a typical hydrochemical type of HCO3, with the highest Meq(HCO3−/SO42−) and the highest reduction degree of SO42−. By contrast, the high-arsenic groundwater in the alluvial–lacustrine environments of the HTP and the YRCB accounts for only 14.77% and 20.13%, respectively, and has only less than 40% of HCO3 dominant type water. The high-arsenic groundwater in these two areas is generally located in the alluvial or alternating fluvial–lacustrine strata. However, the two areas exist more than three alluvial–lacustrine layers with a thickness of over 2 m each, which play a critical role in the formation of high-arsenic groundwater. Moreover, affected by alluvial aquifers in the same system, the high-arsenic groundwater in both the HTP and the YRCB is not intensively distributed and does not represent a typical HCO3 dominant type. The S2− produced by the massive reduction of SO42− might co-precipitate with Fe and As, which may explain why the EHBP has lower arsenic concentration than the HTP and the YRCB, both of which have a lower reduction degree of SO42−.