EFFECTS OF SURFACE WATER AND GROUNDWATER INTERACTION ON ARSENIC CYCLING IN THE DATONG BASIN, CHINA: RESULTS FROM ONE-YEAR MONITORING

To elucidate the mechanism of arsenic (As) transport, a field monitoring site was established in Datong Basin, northern China. The site is adjacent to Sanggan River, which is ephemeral with peak flows and flooding of the surrounding areas occurring in March to April and October every year. A total of 20 monitoring boreholes were installed in 2010. The groundwater was sampled to measure the composition and concentration varieties of chemical species monthly from November, 2010 to November, 2011. The results show that when the flooding of the Sanggan river reaches the groundwater monitoring site in March, the As concentration decline slightly, followed by an tenfold rise, and eventually decrease again. Once the flooding recedes in May, the concentration increase gradually.

On the basis of the As concentration data and previous hydrochemical and sediment analysis, we hypothesize the potential mechanisms for observed As concentration variations. The river water recharges the groundwater during peak flows and flooding. Additional oxygen (O₂) is carried into the aquifers and it reacts rapidly with existing Fe²⁺ in the groundwater to produce Fe³⁺. The weakly alkaline condition promotes formation of Fe(OH)₃. Consequently, As concentrations decrease as As is incorporated into newly formed Fe(OH)₃ via surface-adsorption and/or co-precipitation. In addition, O₂ also reacts with As-bearing Fe(II) sulfides, which allows release of As into the groundwater and leads to sharp rises of As concentration. This reaction also generates Fe²⁺ and more Fe(OH)₃ and As surface-adsorption on and/or co-precipitation with Fe(OH)₃ occurs. Consequently, As concentration declines.

When the flooding recedes, the additional O₂ inputs diminish. The reducing environments prevail in aquifers. Possible microbial reduction dissolution of newly formed Fe(OH)₃ dominates As mobilization in the groundwater and releases Fe²⁺ and As. Therefore, gradual increases in As concentration were observed. Additionally, newly formed Fe²⁺ reacts with S^2- and Fe(II) sulfides form. Fe(II) sulfides can immobilize As via surface-adsorption and/or co-precipitation, which decreases As concentration in the groundwater. When surface water recharges groundwater again, the biogeochemistry processes repeat as discussed above.