| dc.description.abstract | The coupled processes of biogeochemical and hydrological processes in hyporheic zones (HZ) commonly determine the fate of contaminants in both groundwater and surface water. Many parts of the HZ experience both transient and bidirectional flow owing to hydraulic gradients created by the pairing of rapidly changing river stage with relatively steady groundwater tables. The specific contributions of end-member chemistries of the river and groundwater, and the periodicity of the river stage fluctuation, on the fate of a groundwater or river water contaminant is difficult to tease out empirically owing to laborious field conditions. Recently developed bidirectional flow column experiments offer a solution. No published studies, however, have ramped flow up and down smoothly as happens in natural riverbanks under the influence of daily ocean tides and dam releases, as well as sporadic or seasonal floods. Therefore, transient flow, bidirectional column experiments are necessary to accurate simulate how paired river water stage fluctuations and river and groundwater chemistry impact determine the fate of a contaminant within a watershed.
The objective of this project was to construct a programmable transient and reversing flow column experiment apparatus (R1D) that can mimic mixing in a sediment column between two reservoirs of disparate chemistries under a flow regime analogous to a semi-diurnal tidal sequence. To validate this device, once constructed, breakthrough curves (BTC) of a conservative NaCl tracer were compared for an experiment under reversing flow conditions using constant flux conditions and variable (sine wave) flux conditions. Calculated Peclet numbers indicated that mechanical dispersion (dispersivity) dominated over molecular diffusion. The dispersivity of the BTC were fitted to a 1-D numerical model in order to discern whether fitted dispersivity was dependent on the flow direction. Dispersivity did not change with flow direction which demonstrates homogeneous packing of the sand column. The average linear velocities fitted to the BTC were (0.025-0.091 m/d). This apparatus is well suited to investigate a number of systems of kinetically limited and equilibrium biogeochemical reactions within permeable riverbanks. One such system is the permeable natural barriers (PNRBs) along rivers flowing through deltas with anoxic shallow aquifers with high concentrations of arsenic (As). These PNRBs capture dissolve iron (Fe) as Fe-oxides and arsenic (As) bound to their surfaces from groundwater discharging to the Meghna River, however, the initial formation and fate of these deposits under seasonal fluctuations in river stage is unknown. | |
