Effects of water-table changes following rainfall events on arsenic fate and transport in groundwater–surface water mixing zones

Abstract

This study investigated the effects of groundwater–surface water (GW–SW) interactions on the fate and transport of arsenic (As) following rainfall events and subsequent water-table changes in GW–SW mixing zones, comprising the riparian and hyporheic zones, near an abandoned gold mine. During the dry and wet periods, stream conditions changed from flow-through to gaining, respectively. Water-table changes caused by rainfall events controlled flow paths between riparian zones and the stream, affecting spatiotemporal variation in the redox and pH conditions of the aquatic environment. Subsequently, the fate and transport of As in GW–SW mixing zones was responsive to variations in redox and pH conditions. Through the oxidative dissolution of As-bearing sulfide minerals and the reductive dissolution of iron (Fe) oxides with adsorbed As, As was released into the groundwater in the riparian zones and transported to the stream and streambed along the baseflow discharge. However, As was also immobilized in the sediment through adsorption onto Fe-oxides and coprecipitation with calcium (Ca) and zinc (Zn), suggesting that the sediment acts as a sink-and-source of As in aquatic environments. Therefore, water-table changes and GW–SW interactions could play an important role in the fate and transport of As in aquatic environments, specifically groundwater–riparian–streambed–stream systems. The findings of this study will provide scientific insights into the mechanisms of As in aquatic environments, aiding in improved decision–making to ensure safe and sustainable water management in response to future climate change.