EXPERIMENTAL INVESTIGATION OF ARSENIC AND IRON CYCLING IN TIDALLY FLUCTUATING RIVERBANK AQUIFERS USING REVERSING FLOW 1D COLUMNS

Geologically sourced arsenic (As) commonly contaminates pore-waters of shallow (<50 m) alluvial aquifers across deltas in south Asia. Consumption of As from drinking water causes chronic health problems, learning disabilities, and early mortality through a host of diseases. This shallow groundwater flows towards low-lying rivers. Within riverbanks, solid-phase As accumulates on iron-oxy-hydroxides (Fe(III)OOH) within the few first few meters of riverbank sediments in the Meghna River, Bangladesh and the Red River, Vietnam. This suggests groundwater-sourced Fe(II) oxidizes, leading to precipitation of Fe(III)OOH, thereby sequestering As oxyanions in the riverbank hyporheic zone which is defined herein as a permeable natural reactive barrier (PNRB).

The objective of this study is to elucidate the physical and biogeochemical processes driving PNRB formation and long-term resistance to As release. A physical model of the riverbank mixing zone in the form of a programmable, 1-D, reversible flow direction column experiment is under development to simulate PNRB formation, to understand As adsorption/desorption under changing hydraulic conditions, and to determine how microorganisms impact PNRB formation. Specifically, it is hypothesized that: 1) under normal gaining river conditions, tidally controlled redox cycles (TCRC) generate FeOOH, immobilizing As by adsorption onto Fe-oxide surfaces; 2) under losing-river conditions, FeOOH dissolution releases dissolved As into shallow aquifers; 3) released arsenic concentrations are directly proportional to solid-phase FeOOH bound As concentrations prior to flow direction reversal.

An Arduino Due microcontroller connected to a PC controls the flow direction and magnitude with solenoid valves and peristaltic pumps. Electrical conductivity (EC), pH, dissolved oxygen (DO), and oxidation reduction potential (ORP) are measured online in the column effluent. Breakthrough curves of effluent EC, pH, DO, ORP and dissolved major ions, Fe, and As can quantify timing and direction of mass fluxes throughout a 14-day cycle of semi-diurnal tides, mimicking flow velocities in riverbanks in a field site along the Meghna River. Scanning and transmission electron microscope imagery and extractable solid-phase Fe and As will be used to test hypotheses 1-3 in detail.