BIOGEOCHEMISTRY OF ARSENIC CYCLING IN A TIDALLY INFLUENCED RIVER-BANK AQUIFER IN BANGLADESH

Groundwater arsenic (As) contamination is a common problem in southeast Asia yet few studies have investigated the fate of As in groundwater discharging to rivers. River water movement into shallow aquifers, driven by short-term tidal fluctuations and seasonal changes in low-lying deltas, may contribute in determining the timing and location of As accumulation and mobilization in riverbank sediments. The river water level in the Meghna River, Bangladesh varies due to tidal fluctuations, which causes the groundwater flow direction in the immediate vicinity of the river (<10 m) to change regularly. This study evaluates the role of aquifer heterogeneity on the cycling of As within two riverbank aquifers with contrasting sedimentology along the Meghna River in Bangladesh. East-west oriented monitoring well transects were installed perpendicular to the river and sampled in January 2016 for geochemical constituents. The geochemical conditions were modeled to determine zones of similar reactions between the aqueous phase and sediments. During January, the Meghna River near the two study sites experiences gaining conditions; specific conductance and major element data revealed that surface water infiltration due to tidal fluctuations caused groundwater dilution up to 50 m from the river’s edge and down to 15 m depth. We searched for a natural reactive barrier, which are known to form along the Meghna River as reducing groundwater, rich in dissolved Fe and As passes through the oxidized hyporheic zone. Bacterial strains were found up to 50 m inland at the southern site, that are capable of catalyzing the reductive dissolution of As attached to Fe oxides, resulting in As mobilization. These results suggest that substantial concentrations of As are produced within the dilute mixing zone as groundwater discharges to the river.

Previous studies have revealed important geochemical processes involved in As cycling in southeast Asia, however few studies have focused on the fate of As in groundwater discharging to rivers. Our results indicate that there are distinct water mixing zones depending on vertical depth and horizontal distance from the river and these impact when and where As and Fe accumulates and is released into the riverbank aquifer. Silt/clay layers at various depths within a shallow aquifer greatly limit this process.