Roles of Tidal Cycling, Hyporheic Exchange and Bioirrigation on Metal Release From Estuary Sediments

A numerical model is developed to investigate metal release from estuarine sediments. The model includes three-dimensional (3D) large-eddy simulation of water above the sediment-water interface, 3D advective and diffusive transport within sediments by both physical and biological processes (bioturbation and bioirrigation), and biogeochemical processes within the sediment including reduction of electron acceptors, acid-base reactions, and metal sorption and precipitation. The model was applied to explore the influence of overlying water chemistry and sediment chemical, physical and biological processes on metal release from sediments. Overlying water conditions (pH, salinity, oxygen saturation) may vary diurnally due to tidal cycling and control the short-term release of metals, while over longer times (months), metals are controlled by long-term averages of sediment convective processes (groundwater upwelling, hyporheic exchange and bioirrigation). Metal release is significantly enhanced when there is bioroughness and bioirrigation due to local oxidation of surficial sediments.