Experimental Investigation of Iron and Arsenic Fate and Transport in Fluctuating Reversing Flows Using Columns

Elucidating reactive transport dynamics of precipitating, sorbing and dissolving solutes in transient, tidally influenced riverbank systems is challenging, especially when there are dynamic flows and frequent flow direction reversals. This situation is typical of our project's context - the interface between the Meghna River in Bangladesh and the shallow (<50m) alluvial aquifers where reactive arsenic and iron are abundant. At the site, we postulate that As is sequestered by and released from iron-oxide (Fe(III)OOH) minerals which are present as a permeable, natural reactive barrier (PNRB). However, the formation of this PNRB and As adsorption has not been directly observed. Moreover, the biogeochemical mechanisms governing PNRB genesis and the extent of As sequestration in Fe-oxy-hydroxides need to be better understood. This is daunting to accomplish in the field. Lab scale physical models, on the other hand, are an excellent tool to parse out the important mechanisms.

We are building a physical model in the form of novel, programmable, reversible flow direction, 1-D column experiment. This setup will mimic flow direction reversals, corresponding hydraulic gradients, and subsequent end-member water mixing in a riverbank on the Meghna River. A series of solenoid valves and peristaltic pumps controlled by an Arduino Due are used to control the flow rate and direction from two separate reservoirs with synthetic groundwater and river water to mimic a 14-day semi-diurnal tidal cycle. Column effluent will be continually measured via Arduino compatible Atlas Scientific electrical conductivity (EC), pH, and oxidation reduction potential (ORP) probes installed into a series of fabricated flow cells, which are logged to a PC. An optical dissolved oxygen (DO) probe will also perform measurements in column effluent. By using an Arduino to automate column flow direction and magnitude changes, and to log water quality data measurements over the 14-day long experiment, human error sourced from managing the experiment can be reduced. Column effluent will also be sampled for dissolved Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl^-, Br^-, SO₄^2-, HCO₃^-, Fe, and As to quantify the timing and direction of mass fluxes and to elucidate intervals of Fe-oxy-hydroxide formation/dissolution and subsequent to As adsorption/release.