Experimental study of the solute transport in the interfacial exchange zone (IEZ) of a gravel stream bed

Solute exchange between overlying water and the sediment bed of a stream have important effects on the chemical mass balance and biological activities in both the water column and the sediment. Previous studies did not consider the coupled and cumulative contributions of turbulence in the overlying water, water surface waves, and underflow to the solute transport in the Interfacial Exchange Zone (IEZ), although these processes have been studied individually. To investigate the interaction, eight carefully designed experiments of solute exchange in the pore system of a stream gravel bed were conducted. The data analysis showed that the turbulent flow over the bed, the underflow, and the wave-induced advection enhanced the solute exchange and penetration of a conservative solute into a stream gravel bed. The maximum penetration depth under the turbulent current was 15 cm in 3000s; however, the penetration depth reached 20 cm in less than 1000s when surface waves were present. A coupled hydrodynamic and solute transport model was developed to estimate vertical dispersion coefficients for solutes in a porous and permeable streambed. The enhanced dispersion coefficient that combines all three processes is a function of the near-bed coherent motion due to the turbulent current, relative dispersivity (longitudinal dispersivity/wave length), wave steepness, sediment hydraulic conductivity, and sediment porosity, and decreases exponentially with depth; however, the turbulence-enhanced dispersion without the wave effects is smaller and diminishes faster with depth compared with the wave-enhanced dispersion.