FORMATION OF NATURAL REACTIVE BARRIERS AT THE INTERFACE BETWEEN REDUCED AQUIFERS AND DYNAMICALLY FLUCTUATING, GAINING RIVERS

Recent studies have documented concentrated metal deposits within nearshore (<10m) sediments of riverbank aquifers. These Natural Reactive Barriers (NRBs) often form at the interface between anoxic aquifers and gaining rivers. NRBs are composed of positively charged metal oxides which can remove dissolved arsenic (As) oxyanions from groundwater discharging to the river resulting in the accumulation of solid-phase As. A reversal of groundwater flow direction, from new groundwater pumping or rising sea levels may cause the release of toxic metals into riverbank aquifers. Our objective in this study is to understand the hydrogeochemical processes behind the formation of NRBs. Models describing the formation of NRBs typically assume steady-state groundwater flow. The volume and the stability of the NRBs, however, will depend upon: 1) static properties of the aquifer like hydraulic conductivity (K) and its spatial heterogeneity; and 2) the timing and magnitudes of river level fluctuations. Aquifers with high K next to rivers with frequent and large magnitude fluctuations will form larger and more persistent NRBs. Tidal fluctuations affect rivers flowing through deltas and other lowland, coastal regions. In upper watersheds diurnal dam releases produce the same effect. On the Meghna River, semi-diurnal tides propagate up to 400 km north from the Bay of Bengal. This results in high frequency changes in hydraulic head and pore water chemistry that were detected along two transects of monitoring wells orthogonal to the Meghna River. Modeling the hydrologic and geochemical changes using a 2D numerical flow model built in COMSOL and PHREEQC, respectively, revealed two zones within the aquifer. Located close to the river was the NRB where recent mixing with oxic river water causes the precipitation of iron (Fe) and As. One hundred meters inland, mixing with older, dilute river water causes the dissolution of amorphous iron oxides liberating Fe and As. Thus, mixing between river water and anoxic aquifers first generates Fe and As and then causes their precipitation downgradient within the NRB. This is a novel conceptual model for the formation of NRBs along fresh water interfaces and may be helpful to predict the formation of them in similar settings from both geogenic and industrial sources of toxic metals.