Brian D. Smerdon

Fully integrated modeling of surface-subsurface solute transport and the effect of dispersion in tracer hydrograph separation

Tracer hydrograph separation has been widely applied to identify streamflow components, often indicating that pre-event water comprises a large proportion of stream water. Previous work using numerical modeling suggests that hydrodynamic mixing in the subsurface inflates the pre-event water contribution to streamflow when derived from tracer-based hydrograph separation. This study compares the effects of hydrodynamic dispersion, both within the subsurface and at the surface-subsurface boundary, on the tracer-based pre-event water contribution to streamflow. Using a fully integrated surface-subsurface code, we simulate two hypothetical 2-D hillslopes with surface-subsurface solute exchange represented by different solute transport conceptualizations (i.e., advective and dispersive conditions). Results show that when surface-subsurface solute transport occurs via advection only, the pre-event water contribution from the tracer-based separation agrees well with the hydraulically determined value of pre-event water from the numerical model, despite dispersion occurring within the subsurface. In this case, subsurface dispersion parameters have little impact on the tracer-based separation results. However, the pre-event water contribution from the tracer-based separation is larger when dispersion at the surface-subsurface boundary is considered. This work demonstrates that dispersion within the subsurface may not always be a significant factor in apparently large pre-event water fluxes over a single rainfall event. Instead, dispersion at the surface-subsurface boundary may increase estimates of pre-event water contribution. This work also shows that solute transport in numerical models is highly sensitive to the representation of the surface-subsurface interface. Hence, models of catchment-scale solute dynamics require careful treatment and sensitivity testing of the surface-subsurface interface to avoid misinterpretation of real-world physical processes.

Helium equilibrium between pore water and quartz: a clever, but limited tool

The partitioning of Fe in sediments and soils has traditionally been studied by applying sequential leaching methods. These are based on reductive dissolution and exploit differences in dissolution rates between different reactive Fe (oxyhydr)oxide minerals. We used lab-made ferrihydrite, goethite, hematite and magnetite spiked with 58Fe and leached two-mineral mixtures with both phases abundant in excess of the methods dissolution capacity. Leaching was performed with 1) hydroxylamine-HCl and 2) Na-dithionite as the reactive agent. Following Poulton & Canfield (2005) [1], the first dissolution is designed to selectively leach the most reactive Fe-phases, ferrihydrite and lepidocrocite, whereas the second dissolution is designed to leach goethite and hematite. Magnetite would then be dissolved in a third dissolution step with oxalic acid. First results show that the hydroxylamine-HCl method for ferrihydrite dissolves only insignificant amounts of goethite and hematite. However, magnetite-Fe constitutes about 10% of the total dissolved Fe. The Na-dithionite dissolved Fe from goethite-magnetite and hematite-magnetite mixtures contain about 30% of magnetite-Fe. We applied selective sequential leaching and Fe isotope analysis to fine-grained marine sediments from a depocenter in the North Sea, which contain abundant reactive Fe (oxyhydr)oxides and show evidence for Fe sulfide formation within the upper 10 cm. Fe isotopes of the hydroxylamine-HCl leach targeting ferrihydrite shows a downcore increase of !56Fe typical for sediments undergoing microbial reductive Fe dissolution, whereas Fe isotopes of the Na-dithionite leach (goethite and hematite) and oxalic acid leach (magnetite) are identical and show no downcore variation in !56Fe. This means, that only the most reactive Fe phases participate in the Fe redox cycle in this location. The similar isotopic composition of goethite + hematite and magnetite suggests a detrital source, which is not utilized possibly due to the abundant ferrihydrite and lepidocrocite present. [1] Poulton & Canfield (2005), Chemical Geology 214, 209– 221Seasonal Methane Fluxes and Sulfate Reduction Rates in a Eutrophied Baltic Estuarine System