Understanding Mechanism of Iron Cycling Coupled to Natural Organic Matter in a Wetland Environment Under Dynamic Hydrogeological Conditions

Iron redox cycling has been studied under dynamic hydrogeological conditions in a wetland stream system in Hunnicutt Creek (Clemson, SC) through a time period of six months with an focus on understanding the geochemical controls on iron flux from shallow groundwater into the stream. An emphasis was placed on detailed characterization of pH and EH gradients as well as natural organic matter (NOM) and major ions. NOM has a major impact on iron and other trace element speciation, mobility, transportation, and bioavailability in the environment. In a gaining stream, transport of ferrous iron from shallow groundwaters into the stream can lead to the formation of iron-bearing flocs which have the potential to enhance contaminant transport. In this work, iron speciation was determined to test multiple working hypotheses describing iron flux from sediments to the stream: soluble free Fe(II), soluble Fe(II)-OM, soluble Fe(III)-OM and Fe(III)-OM facilitating flocs forming. Measurements of physical and chemical properties of pore waters, such as EH, pH and total organic carbon (TOC), concentrations of iron and other trace elements, as well as piezometer measurements of water level fluctuations were taken along a select transect in Hunnicutt Creek. Seasonal and spatial changes of parameters were observed. Redox potential (EH) largely influenced the concentrations of iron and some trace elements, while TOC and pH showed less impact on the system chemistry and iron appeared to be the primary redox buffer in shallow groundwater which then reacted with dissolved oxygen in surface waters. Piezometers measurements of water levels across the transect show a constant higher water level on the bank than in the stream, providing evidence of the gaining stream condition. Future work to match fluctuations in stream water level (and thus, dissolved oxygen availability) with changes of pore water chemistry is required