Hyporheic zones at the base of streams develop redox gradients at the groundwater-surface water interface, which makes them active regions for anaerobic and aerobic biogeochemical processes. Microbial enzymes essential for carrying out biogeochemical processes at this interface (e.g., methanogenesis, denitrification, and aerobic methanotrophy) contain trace metals (Ni, Co, Cu, and Zn) in cofactors. Low bioavailability of trace metals can inhibit microbial processes and affect carbon and nitrogen cycling in natural systems. We determined the bioavailability of trace metals in freshwater streams and sediments using the diffusive gradients in thin films (DGT) technique. DGT sediment probes were deployed at four different locations of East fork Poplar Creek (EFPC), Tennessee to obtain high-resolution depth profiles of trace metals (Cu, Co, Ni, and Zn). The total solid-phase associated trace metal concentrations at the selected sites were similar to or below geological background levels (1320 nmol g-1 zinc, 320 nmol g-1 copper, 290 nmol g-1 nickel, and 170 nmol g-1 cobalt). The depth-concentration profiles of trace metals varied across different locations. CuDGT, and CoDGT concentrations decreased at greater depths ( &gt; 6-8 cm), suggesting a change in speciation under anoxic regions. CoDGT and MnDGT exhibited closely corresponding profiles, indicating that Co has strong associations with Mn oxides/hydroxides at these locations. The increase in dissolved organic carbon concentrations at the locations substantially decreased the labile trace metal concentrations. Extended deployment times (&gt; 24 h) resulted in underestimate of the DGT-determined labile trace metal concentrations. The kinetics of transfer of trace metals from solid-phases to porewater can be interpreted by the ratio of DGT-estimated concentrations to dissolved concentrations in the sediment porewater (CDGT/Cpw). For all the trace metals, CDGT/Cpw decreased with an increase in the deployment period, indicating that the trace metals are strongly associated with solid-phases at the studied locations and are not mobilized due to local sink induced by uptake of trace metals by DGT samplers. The study indicated that uncontaminated freshwater sediments may lack bioavailable trace metal micronutrients for carrying out biogeochemical processes.

Quantifying labile trace metal concentrations and desorption kinetics in freshwater sediments using diffusion-based sampling devices

ACS Spring 2022

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