Examination and Modeling of Seiche-Driven Redox of Sulfur and Mobilization of Sediment Pore Fluids

Redox chemistry plays a significant role in nutrient cycling in aquatic sediment. An acceleration in cycling can occur in locations where periodic inundation allows oxygen penetration deeper into reduced sediments, disturbing the porewater profile and allowing for significant redox reactions. Cyclical hydrological phenomena perturbing redox interfaces in many different contexts have been well studied, however nutrient cycling in anoxic sediments of freshwater estuaries in response to seiche-induced activity has not been examined. The objective of this research is to study the effect of oxygen intrusion on nutrient cycling and mobilization in porewater of anoxic sediments subjected to periodic wetting and drying driven by a seiche. A tank simulating an 8-hour, 8 cm seiche cycle and containing a sediment wedge was constructed in the lab with sulfur-rich mud and coarse sand and divided into three different zones to compare: a dry zone, splash zone exposed to periodic wetting and drying, and saturated zone. Sulfate concentration in surface and porewater was measured in each of the zones and used as an indication of the impact of oxygenation on sulfur cycling and movement. A description of groundwater flow in response to seiche fluctuations based on the Dupuit assumption yielded a one-dimensional nonlinear diffusion equation to describe groundwater mechanics. The results of this model were compared to tracer measurements in the laboratory to constrain inundation frequency and the rate at which oxidation products were flushed from pore fluids. Observations indicate elevated sulfate in the upper portion of the splash zone, presumably since it experiences longer atmospheric interaction; and mobilization of sulfate into the saturated zone and surface water. Collectively, spatial patterns in observational and modeling results indicate that porewater concentrations may be dependent on both proximity to the surface-water interface and inundation frequency. These findings suggest that freshwater estuarine sediments affected by seiche activity can experience faster rates of nutrient cycling and porewater flushing, potentially contributing to the release of nutrients and metals to surface waters.