Abiotic Mixing‐Dependent Reaction in a Laboratory Simulated Hyporheic Zone

Abstract

AbstractGroundwater (GW) contaminants upwelling toward surface water (SW) can attenuate in the hyporheic zone, with dissolved oxygen (DO) frequently controlling the attenuation. In a laboratory mesocosm, we induced downwelling of SW into the sediments to create a hyporheic flow cell (HFC). We added DO to downwelling SW and sodium sulfite (Na2SO3) to anoxic upwelling GW to induce an abiotic mixing‐dependent reaction along the mixing zone between the HFC and upwelling GW. Using planar optodes and SO4 measurements, we observed movement of the DO mixing zone (oxic front position), extent of DO mixing (mixing zone thickness), and location of MD reaction (SO4 peak concentration). Oxic front position and mixing zone thickness were stable during nonreactive control experiments, indicating that dispersion of DO across the mixing zone had come into equilibrium with supply of DO to the mixing zone. By contrast, mixing zone thickness shrank over time during the reaction experiments, as MD reaction consumed DO in the mixing zone. The decrease in mixing zone thickness for the reaction experiments indicates steeper DO gradients and greater dispersion (transport) limitation, quantified by Damköhler numbers farther above unity. Maximum SO4 concentrations always occurred further from the center of the HFC (i.e., more toward surrounding upwelling GW) than did the oxic front. In most riverbeds, transport and mixing dynamics are thus superimposed upon existing hydraulic dynamics, with implications for monitoring and attenuation of contaminants.