Effects of Biogeochemical and Physical Processes on the Transformation of Trace Metals at Oxic-Anoxic Interfaces in Aquatic Systems
Chow, Stephanie Stacey
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Trace metals (e.g. Fe, Mn, Zn, Cu, Cd, Ni) are important micronutrients that have historically been regarded as toxic pollutants rather than essential components of riverine and estuarine environments. The toxicity and behavior of trace metals, in response to physical and biogeochemical processes, are determined by their individual physico-chemical properties. In this dissertation, the vertical transformation of trace metals across oxic-anoxic interfaces was investigated at two sites, a Fe-rich freshwater river with minimal sulfide and an estuary with elevated Mn and H2S concentrations. Sediment profiles obtained from the Chattahoochee River showed that dissolved arsenic, present as As(V) only, is scavenged by Fe-oxides and accumulates directly below the sediment-water interface. Depth profiles also indicate that As(V) fluxes into the overlying water during baseflow conditions as well as after storm events. The significant correlation between Fe(II) and As(V) suggest that As(V) is released from Fe-oxides during their microbial reduction. By implementing a series of sediment incubations under increasing As(V) loads, it was determined that adsorption onto Fe-oxides and microbially mediated reductive dissolution of these mineral phases drive arsenic cycling in this sediment. These incubations also reveal for the first time that arsenic, even in low concentrations, n turn, arsenic loading impacts iron cycling by stimulating anaerobic respiration of Fe-oxides and promoting recrystallization of authigenic Fe-oxides, up to a toxicity threshold up to a few micromolar in concentrations. A combination of in situ measurements with discrete water sampling was utilized to determine the effects of tidal cycling on the distribution of trace metals under changing redox conditions during two consecutive tidal cycles at Station 858 in the Chesapeake Bay. Estuarine circulation patterns driven by tidal oscillations, a defined pycnocline, and the shallow sill (~20 m) of the Chesapeake Bay promoted bottom water anoxia during the summer months that allowed dissolved sulfide and reduced manganese to accumulate below the oxycline. The distribution of barium (conservative freshwater tracer) and uranium (conservative seawater tracer) across the pycnocline over the two tidal cycles indicated that the source of dissolved species was surficial sediments. During ebb and flood tides, the shear stress from the bottom waters flowing over the sediment seems to episodically promote the advection of porewaters enriched in dissolved sulfide, manganese, uranium, barium, lead, chromium, and copper. The selective enrichment of these trace metals appears to be controlled by their reactivity with sulfide. In contrast, cobalt and nickel are retained in sediments by adsorbed or incorporated in FeS and FeS2, while arsenic co-precipitates with sulfide or iron sulfide minerals. Overall, this study demonstrates that natural aquatic systems are complex environments where the interplay between biological, chemical, and physical processes affects the distribution of trace metals over short time scales. While a great wealth of knowledge can be obtained by laboratory experiments with synthetic solutions or pure cultures of organisms, a combination of in situ measurements and incubations with real samples in necessary to characterize the processes regulating the cycling of trace metals in aquatic systems.