Background/Objectives. Mercury is a constituent of potential concern in sediments, specifically in its more bioavailable, organic form of methyl mercury. Methyl mercury production is microbially mediated, and the zone of the most active net methylation is often found at the onset of sulfate reduction in the sediment column.
Methylation is, in part, a function of the amount of bioavailable inorganic mercury, which depends on mercury binding with sulfide and other ligands. As such, characterizing redox processes within sediment sites improves the understanding of mercury fate and transport and remedy efficacy. The Conceptual Site Model (CSM) of mercury speciation, fate, and transport has been developed through multiple phases and lines of evidence at the Berry’s Creek Study Area (BCSA) in Bergen County, NJ. However, given the complexity of the tidally-influenced marsh, this model can be strengthened through the direct, empirical measurement of in situ redox conditions and variation throughout the Site.
Approach/Activities. Voltammetry has emerged as a viable technique for characterizing redox profiles in situ. Voltammetric microelectrodes use the application of a voltage scan in order to identify and quantify concentrations of dissolved chemical species including oxygen, manganese, iron, and sulfide. The simultaneous evaluation of multiple analytes at a fine resolution minimizes sediment disturbances in the areas studied. Voltammetry was deployed in both marsh and waterway settings at the BCSA in tandem with a multi-media (sediment and surface water) investigation in order to evaluate temporal and spatial patterns of redox development. The redox profile collection spanned throughout the Site and was evaluated against high-resolution bulk sediment concentration profiles, tidal stage data, elevation, and Site locations. In addition, profiles were collected within the Pilot Study thin cover and control plots of the Feasibility Study in order to evaluate the effects of thin-layer placements at the Site.
Results/Lessons Learned. The use of voltammetry highlighted several patterns throughout the BCSA. Primarily, the contrast between redox development in mudflats and marshes was well-established. A consistent, shallow onset of reducing conditions was observed in mudflats, whereas reduction onset was deeper and more spatially variable in marshes. The variability in marshes can be attributed to a complex interplay of factors including less frequent inundation, fluctuating sediment saturation, secondary porosity and oxygen translocation from Phragmites roots, system-wide gradients in redox conditions, and sediment composition. Additionally, the study was able to evaluate thin-layer placements that are utilized in marshes and mudflats to support evaluation of Site-specific remedial performance. The in situ measurements demonstrated that thin-layer placements applied on mudflats increased the depth to the onset of anoxic conditions, and thus the likely depth of peak net mercury methylation. Similar, though weaker, patterns were observed within the marsh thin-layer placements. Finally, the voltammetry study confirmed the understanding of the relationship between mercury methylation and redox, as peak methyl mercury concentrations were observed to fall within the expected redox zone. Overall, the voltammetry study of Site-wide redox development was successful in informing both the baseline CSM and remedy evaluation.