Temporal variation and mechanism of the geogenic arsenic concentrations in global groundwater

Abstract

Geogenic arsenic (As) in groundwater has a negative impact on public health. Recent studies have introduced the mechanisms of temporal variation in arsenic concentrations in groundwater. Short-term drastic variations in groundwater arsenic are usually associated with anthropogenic activities, such as contamination and groundwater extraction. Periodic variations in groundwater arsenic are mainly caused by seasonal precipitation/evaporation changes, periodic irrigation/pumping and periodic anthropogenic input. Hydrogeochemical processes of these temporal variations in groundwater arsenic involve the dilution/concentration of As, oxidative dissolution of arsenopyrite, competitive adsorption between As and other ions including HCO3− and PO43−, reductive dissolution of As–Fe/Mn (oxyhydr)oxide, oxidative elimination of aqueous As(III), advection between the aquifers, and the release of As from clay layers. Compared with natural factors, the temporal changes in groundwater As are more susceptible to anthropogenic behaviors; however, topography and climate still play a significant role in diverse patterns of groundwater arsenic concentrations variations. In this study, decreasing groundwater arsenic concentrations in the rainy season and increasing groundwater arsenic concentrations in the dry season were more likely to occur in oxidative aquifers in mountainous areas with fractured bedrock; in coastal areas; or in arid/semiarid inland basins with less anthropogenic interference, such as mines, ponds, farmland, etc. An inverse trend was more likely to occur in reductive aquifers in humid plain/delta areas containing paddy fields and ponds at the surface. More investigations must be conducted to determine the heterogeneity of groundwater arsenic variations at different scales and correlations with the surface environment, including crop cultivation, climate change, land use, and other factors, by using long-term monitoring data.