IDENTIFICATION OF HYDRO-BIOGEOCHEMICAL PROCESSES CONTROLLING SEASONAL VARIATIONS IN ARSENIC CONCENTRATIONS AT JIANGHAN PLAIN, CHINA

Consumption of groundwater containing elevated arsenic impacts about 60 thousand people at Jianghan Plain, China. Dramatic seasonal variations of arsenic concentrations were observed within a single year (20 -1200 μg/L). It is crucial to understand the mechanisms of arsenic transform and transport in aquifer and develop method to control the arsenic in groundwater.

To identify hydro-biogeochemical processes controlling the seasonal variations in arsenic concentration, a 2D reactive transport model is developed to descript the major processes at the site. The reaction network includes the following processes: 1) reductive dissolution of iron, 2) reductive of arsenate, 3) oxidation of arsenite, 4) arsenic surface complexation reactions.

Modeling results confirmed the seasonal changes in arsenic concentrations caused by surface-groundwater interaction. In wet seasons, surface water recharges groundwater and brings oxidizing agent to the anoxic aquifer. Oxidizing condition reduces arsenic release and surface water recharge dilutes the arsenic concentration in aquifer, causing a transient drop in arsenic concentrations. However, simulation results indicate that the affected region is limited to about a hundred meters nearby the river, so it cannot be accounted for the general seasonal variations of arsenic concentrations at wells far from rivers. Instead, vertical flow caused by groundwater extraction is identified as the main factor controlling the arsenic release and transport processes. A strong downward hydraulic gradient formed by pumping, will induce oxidizing water from the upper aquitard recharging the lower aquifer and cause a sudden drop in arsenic drop. Besides, as the redox condition changes by oxidant, arsenate becomes dominant in aquifer at low arsenic concentration period. When vertical recharge ceases, aquifer recovers to be anoxic after oxidant exhaust and rapid arsenic release promote the arsenic concentration in groundwater which is predominated by arsenite again. The reactive transport model provides an effective tool to analyze arsenic cycling mechanisms at Jianghan Plain and other similar alluvial basins.