Assistant Professor
Dept. Geology & Geophysics, Texas A&M University

Title: “Formation of Natural Reactive Barriers at the interface between reduced aquifers and tidally fluctuating, gaining rivers”

Abstract: 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 water level fluctuations will form larger and possibly more persistent NRBs. Frequent water level fluctuations are driven by ocean tides on many rivers flowing through deltas and other lowland, coastal regions. On the Meghna River, semi-diurnal tides propagate up to 400 km north from the Bay of Bengal. The resulting high frequency changes in flow direction and pore water chemistry were measured 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 riverbank aquifer. Located within 10 m of the river was the NRB where recent mixing with oxic river water causes the precipitation of iron (Fe) and As. Up to 100 m inland, however, 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 both generates Fe and As and causes their downgradient accumulation within the NRB. This 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. As well as their stability in light of changing ocean, rivers and groundwater levels.