Sources and Sinks of Arsenic in Riverbanks along the Meghna River, Bangladesh

Abstract:

The riverbanks of the Meghna River in Bangladesh are alternately scoured in the monsoon and blanketed by organic carbon, iron (Fe) and arsenic (As) rich silt as the seasonal floods subside. During the dry season, ocean tides propagate up from the Bay of Bengal, driving mixing within permeable sediments between groundwater rich in dissolved Fe and As and oxygenated river water forming iron oxides (FeOOH) which trap As. Silt layers and this well-mixed permeable zone are sources and sinks of dissolved As and Fe. Our goal is to evaluate how riverbank geologic structure impacts the timing and mode of mixing, and the timing and locations of sources and sinks of As and Fe during the early dry season by comparing three sites within 10 km of each other along the Meghna River in central Bangladesh. To achieve this monitoring wells and drive-point piezometers were installed to sample the water chemistry and measure hydraulic gradients. Site 1 is a sandy bank where river water mixes up to 3 m vertically into the bank under the influence of semi-diurnal tides (~60 cm). Dissolved As advecting towards the river is removed along with dissolved inorganic carbon (DIC) within the mixing zone. Dissolved Fe is released to pore-waters. Site 2 is a silt-capped bank with several silt layers (1-20 cm) within the aquifer to 10 m depth. Dissolved As, Fe and DIC concentrations increase towards the river with little evidence of lateral mixing with the river at the end of the flow path. Dilution, however, of the upper shallow aquifer driven by seasonal inundation of the broad floodplain during the monsoon is evident. Site 3 is a sandy bank with a buried silt layer 3 m below the surface. Here there is tidally-driven lateral mixing, like Site 1, above the silt layer, but like Site 2 As, Fe and DIC are released to pore-waters discharging to the river. The effects of annually infiltrating river water and fresh silt deposited on the floodplain overprint over processes occurring proximal to the dry season shoreline of: 1) the capture of As on FeOOH from robust tidal mixing in sandy banks; and 2) release of As, Fe and DIC from buried silt. Dams diminish the supply of fresh sediments to river banks and seasonal inundation events, and often introduces regular pulses in the river akin to tidal fluctuations. This will impact the locations and timings of sources and sinks of As in ground- and river waters.