Effects of irrigation-induced water table fluctuation on arsenic mobilization in the unsaturated zone of the Datong Basin, northern China

Abstract

High level of arsenic-containing groundwater has been used for irrigation purposes for several decades in Asia, leading to fluctuating water table and redox conditions in the unsaturated zone, thereby potentially affecting the mobilization of arsenic in the unsaturated and saturated zones. A field plot experiment was conducted in the arsenic – affected area of the Datong Basin, China to determine the effects of irrigation return flow on the hydrogeochemical behavior of arsenic and iron in the unsaturated zone. High-arsenic groundwater was extracted from a shallow aquifer far from the irrigation site and used as irrigation water. Soil water/shallow groundwater and soil/sediment samples were collected at different depths from the field site during the experiment. Water samples were analyzed to determine hydrochemical properties and arsenic concentrations, and soil/sediment samples were analyzed to determine total iron and arsenic. Infiltration of irrigation water into the unsaturated zone led to fluctuations in the water table, varying redox conditions, and the redistribution of arsenic and iron in the near-surface soil/sediment. Soil/sediment bulk geochemical analysis results indicate obvious increases of arsenic and iron in topsoil after irrigation. Infiltration of irrigation water carrying organic matter, sulfate, and nitrate into the subsurface affects the (bio)geochemistry of the unsaturated and saturated zones, and ultimately the behavior of arsenic and iron. Geochemical modeling results suggest desorption and leaching processes are responsible for the temporal changes of arsenic concentrations in both pore water, groundwater and sediment during irrigation. The results of this study indicate that flood irrigation using arsenic-contaminated groundwater should be controlled and gradually replaced by drip irrigation, sprinkler irrigation or irrigation using non-contaminated water resources to mitigate arsenic accumulation in the unsaturated zone and shallow groundwater.