UNDERSTANDING THE EFFECTS OF EARLY DIAGENESIS ON METAL CYCLING IN FRESHWATER LAKES USING VERTICAL CONCENTRATION PATTERNS IN POREWATER

Biogeochemical cycling at the sediment-water interface is largely driven by redox changes in sediments as a result of microbial activity. Basic principles of what these changes should be are known, but how well these principles describe metal cycling in freshwater lakes still needs study to better understand processes influencing the health of aquatic systems and environmental exposures. The overarching hypothesis for this research is that the cycling of redox sensitive elements is in dynamic equilibrium with changes in redox states in the sediments. Analyzing vertical concentration profiles of porewater and associated sediment can be used to test this hypothesis. If the hypothesis is true then 1) vertical profiles of dissolved redox sensitive species (e.g., Fe, U) should demonstrate patterns similar to what is predicted by thermodynamic constrains and 2) non redox metals influenced by redox processes should reflect patterns expected for sequestration/release (e.g., Cu sorption on iron oxyhydroxides). Vertical sediment and porewater samples were collected from 30 inland lakes with a range of land-use characteristics and benthic conditions across the Great Lakes watershed. Sediments were sectioned and porewater was extracted at the time of collection. Porewater samples were immediately filtered, acidified and both these and sediment samples were stored at 4^oC until ICPMS analysis. The results show that 1) vertical porewater concentrations of redox sensitive elements generally follow expected patterns and sequences (e.g., Fe increases with depth) and 2) evidence for sequestration/release is seen (e.g., comparing Cu and Fe porewater profiles and comparing porewater – sediment profiles). Solid phase and liquid phase chemical profiles often differ from expected due to diagenetic processes. For example, the release of P appears to influence the mobility of U through what we interpret as a precipitation mechanism. In general, the results are consistent with the hypothesis but there are confounding factors that will need to be considered. Diagenetic release of metals to the overlying water can be seen in some profiles. Understanding the importance of changes in environmental factors on this release is needed to better predict the health of aquatic systems and environmental exposures.