AN EXPERIMENTAL INVESTIGATION OF THE EFFECTS OF SEICHE ACTIVITY ON SULFUR-CONTAINING SEDIMENT AND SEDIMENT POREWATER

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. Perturbation of redox interfaces and porewater mobilization via tidal pumping has been well studied, however nutrient cycling in anoxic sediments of freshwater estuaries in response to seiche-induced activity have not been examined. The objective of this research is to study the mobilization of porewater and effect of oxygen intrusion on nutrient cycling in anoxic sediments subjected to periodic wetting and drying driven by a seiche in a laboratory setting. A tank simulating an 8-hour, 8 cm seiche cycle and containing a sediment wedge was constructed with sulfur-rich mud and coarse sand and divided into three vertical zones to compare: a dry zone, splash zone exposed to periodic wetting and drying, and saturated zone. Sulfate concentrations in surface and porewater were measured in each of the zones and used as an indication of movement and the impacts of oxygenation on sulfur cycling. A model 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. Data reveals elevated sulfate in the upper portion of the splash zone, presumably since it experiences longer atmospheric interaction; and mobilization of sulfate into surface water and areas further from the sediment-water interface. 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.