Dispersion dynamics of reactive solutes in tidal wetlands under pulsatile wind conditions

Tidal wetlands serve as critical interfaces between terrestrial and aquatic ecosystems, playing a significant role in nutrient cycling and pollutant attenuation. This study investigates the dynamics of reactive solute dispersion in tidal wetlands, specifically examining the influence of reversible and irreversible reactions under pulsatile wind conditions. By employing Mei's multi-scale homogenization technique [1], we aim to elucidate how unsteady wind patterns affect the transport and reaction mechanisms of solutes in these unique environments. The impacts of reaction parameters, such as the Damköhler number (Da) and irreversible reaction rate, along with wind parameters and vegetation factors, on solute mixing have been analyzed. It has been found that an increase in the wind oscillation period (τ_w) corresponds to a decrease in the frequency of wind oscillations, which no longer effectively resist flow pulsation but instead enhance it when the wind aligns with the flow direction, regardless of its strength. When the wind opposes the primary flow, an interesting trend in the dispersion coefficient has been observed for varying wind oscillation periods and amplitudes. The dispersion coefficient first decreases with increasing wind strength, reaches a minimum, and then begins to increase. Vegetation-induced drag is more pronounced when the wind opposes the flow. This significantly reduces dispersion compared to wind flowing in the same direction as the primary flow. Results also indicate that slow phase exchange kinetics (Da<<1)  lead to higher dispersion coefficients compared to fast kinetics (Da>>1)  in both wind directions. Introducing an irreversible reaction rate causes absorption at the wetland bed, lowering the solute concentration near the channel bed. This creates a depletion effect, where solutes are continuously removed from the fluid phase, leading to an overall reduction in fluid-phase concentration throughout the wetland system.