Using reactive transport modeling to link hydrologic flux and root zone geochemistry at Second Creek, a sulfate enriched wild rice stream in northeastern Minnesota

Abstract

Wild rice (Zizania palustris) is an economically, culturally, and ecologically important aquatic plant species in Minnesota. In northeastern Minnesota, iron ore and taconite mining have led to elevated surface water sulfate concentrations, which has raised concern about the potential of sulfate negatively impacting wild rice populations. Recent studies have shown that elevated sulfide concentrations in the wild rice root zone (sediment porewater) is more closely correlated with lower occurrence of wild rice than surface water sulfate. This toxic porewater sulfide can be attenuated by precipitation of iron sulfide minerals if dissolved ferrous iron is locally available. Although these geochemical reactions occur in the sediment porewater, or hyporheic zone, where groundwater and surface water mix, the effect of groundwater flux and geochemistry on geochemical processes within the hyporheic zone has not been examined in the context of wild rice and sulfate. Here, we use physical and geochemical field data collected from surface water, porewater, and groundwater to inform a reactive transport model of Second Creek, a mining impacted wild rice stream in northeastern Minnesota. The model is implemented for different hydrologic flux regimes in different locations at Second Creek to examine the geochemical response of the sediment porewater to changes in both physical and geochemical conditions. We show that porewater sulfide concentrations can are dependent on hydrologic flux direction and magnitude, as well as on concentrations of surface water sulfate, sediment organic carbon, and porewater iron. This work emphasizes the importance of constraining groundwater flux and geochemistry when examining processes at the groundwater-surface water interface.