Characteristics of boron isotopes and their indicative significance in groundwater arsenic mobilization from an alluvial basin

Groundwater with high arsenic (As) concentration is widely distributed all over the world and seriously threatens human health. Due to the similar chemical properties, boron (B) would be used to understand the formation mechanism of high As groundwater. Thirty groundwater samples were collected from alluvial fan, transition area, and flat plain generally along the flow path in the northwestern Hetao Basin, China. Groundwater As concentration generally showed an increasing trend along the path. The δ¹¹B values ranged from 3.36 ‰ to 26.19 ‰, and exhibited a decreasing trend (from alluvial fan to transition area; Stage I) followed by an increasing trend (from transition area to flat plain; Stage II). Boron release from incongruent dissolution of silicate minerals and B adsorption onto secondary clay minerals resulted in high δ¹¹B values in groundwater with low As concentrations from the alluvial fan, where As was adsorbed and immobilized on Fe(III) oxides or clay minerals. During Stage I, B concentrations increased slightly and the δ¹¹B values decreased, as the result of B desorption. A negative correlation between As concentrations and δ¹¹B values illustrated that desorption was an important process of As enrichment. During Stage II, degradation of organic matter and reductive dissolution of Fe(III) oxides increased concentrations of B and As. However, the decreasing trend of B/Cl and the increasing trend of δ¹¹B showed that co-precipitation of B and carbonates removed B from groundwater. The positive correlation between As and SI_{calcite+dolomite} supported that secondary Ca precipitation decreased As adsorption by directly isolating As from Fe(III) oxides, which promoted As enrichment. This study provides insights into hydrogeochemical processes associated with As and B enrichment in groundwater.