Reactive Transport of Nutrients in the Hyporheic Zone: Experiment and Simulation

The hyporheic zone (HZ) is crucial for the stream and river ecosystem for attenuation of domestic, agricultural, and industrial pollution. Flow in the hyporheic zone occurs due to bedforms (dunes and ripples), meandering, and the presence of any obstruction. Water from the stream enters this zone and either returns or infiltrates deeper depending on the relative levels of surface water and groundwater. The zone is more reactive for biogeochemical processes due to the influx of nutrients and Pollutants and the mixing of surface water and groundwater. The sediment in HZ can adsorb the nutrients and pollutants, as well as support the growth and metabolism of microorganisms.

The experiments were conducted in a recirculating hyporheic zone flume of 5 m effective length connected to a groundwater reservoir, which allowed us to simulate gaining, losing, and neutral streams by independently adjusting the surface water and groundwater levels. Three artificial sediment dunes were constructed in the shape of asymmetric triangles, 1 m in length and 0.15 m in height at 0.75 m length.

The objective of the experiments was to investigate the transport of a conservative tracer (Br^-) and nutrients (NO₃^-, PO₄^3-, NH₄⁺) within the hyporheic zone and estimate the dispersion and retardation coefficients with the help of simulation. All the experiments were conducted in duplicate. The flow was simulated using the two-dimensional steady-state classical groundwater flow equation, and the transport was simulated using the two-dimensional time-dependent advection-dispersion equation using grid sizes of 0.5 cm x 0.5 cm. The dispersivity and horizontal and vertical dispersion coefficients were estimated using the experiments with the conservative tracer. The retardation coefficients of the nutrients were computed for each of the nutrients. All the parameter estimations were carried out by minimizing the least square errors between the experimental measurements and simulation. Since the transport is time-dependent, parameters were estimated using the data from the measurements at two times and validated using the data from subsequent measurements. Uncertainties of the estimated parameters were also computed.