Comparison of parameter sensitivities between a laboratory and field scale model of uranium transport in a dual domain, distributed rate reactive system

A laboratory-derived conceptual and numerical model for U(VI) transport at the Hanford 300A site, Washington, USA, was applied to a range of field-scale scenarios of different geochemical complexity to identify the importance of individual processes in controlling the fate of U(VI), as well as to elucidate the characteristic differences between well-defined laboratory and the more complex field-scale conditions. Therefore, a rigorous sensitivity analysis was carried out for the various simulation scenarios. The underlying conceptual and numerical model, originally developed from column experiment data, includes distributed rate surface complexation kinetics of U(VI), aqueous speciation, and physical nonequilibrium transport processes. The field scenarios accounted additionally for highly transient groundwater flow and variable geochemical conditions driven by frequent water level changes of the nearby Columbia River. The results of the sensitivity analysis showed not only similarities but also important differences in parameter sensitivities between the laboratory and field-scale models. It was found that the actual degree of sorption disequilibrium, actual concentration of sorbed U(VI), and the sorption extent (i.e., theoretical concentration of sorbed U(VI) at equilibrium) are the major controls for the magnitude of the calculated parameter sensitivities. These internal model variables depended mainly on (1) the groundwater flow conditions, i.e., the relatively long phases of limited groundwater movement in the field scale (intercepted by short peak flow events) and the long sustained flow phases in the column experiment (intercepted by relatively short stop flow events), and (2) the sampling location in the field-scale model, i.e., plume fringe versus plume center. Copyright 2010 by the American Geophysical Union

Spatial and temporal patterns of pore water chemistry in the inter-tidal zone of a high energy beach

Submarine groundwater discharge (SGD) is a ubiquitous source of meteoric fresh groundwater and recirculating seawater to the coastal ocean. Due to the hidden distribution of SGD, as well as the hydraulic- and stratigraphy-driven spatial and temporal heterogeneities, one of the biggest challenges to date is the correct assessment of SGD-driven constituent fluxes. Here, we present results from a 3-dimensional seasonal sampling campaign of a shallow subterranean estuary in a high-energy, meso-tidal beach, Spiekeroog Island, Northern Germany. We determined beach topography and analyzed physico-chemical and biogeochemical parameters such as salinity, temperature, dissolved oxygen, Fe(II) and dissolved organic matter fluorescence (FDOM). Overall, the highest gradients in pore water chemistry were found in the cross-shore direction. In particular, a strong physico-chemical differentiation between the tidal high water and low water line was found and reflected relatively stable in- and exfiltrating conditions in these areas. Contrastingly, in between, the pore water compositions in the existing foreshore ridge and runnel system were very heterogeneous on a spatial and temporal scale. The reasons for this observation may be the strong morphological changes that occur throughout the entire year, which affect the exact locations and heights of the ridge and runnel structures and associated flow paths. Further, seasonal changes in temperature and inland hydraulic head, and the associated effect on microbial mediated redox reactions likely overprint these patterns. In the long-shore direction the pore water chemistry varied less than the along the cross-shore direction. Variation in long-shore direction was probably occurring due to topography changes of the ridge-runnel structure and a physical heterogeneity of the sediment, which produced non-uniform groundwater flow conditions. We conclude that on meso-tidal high energy beaches, the rapidly changing beach morphology produces zones with different approximations to steady-state conditions. Therefore, we suggest that zone-specific endmember sampling is the optimal strategy to reduce uncertainties of SGD-driven constituent fluxes

Spatial and Temporal Patterns of Pore Water Chemistry in the Inter-Tidal Zone of a High Energy Beach

Submarine groundwater discharge (SGD) is a ubiquitous source of meteoric fresh groundwater and recirculating seawater to the coastal ocean. Due to the hidden distribution of SGD, as well as the hydraulic- and stratigraphy-driven spatial and temporal heterogeneities, one of the biggest challenges to date is the correct assessment of SGD-driven constituent fluxes. Here, we present results from a 3-dimensional seasonal sampling campaign of a shallow subterranean estuary in a high-energy, meso-tidal beach, Spiekeroog Island, Northern Germany. We determined beach topography and analyzed physico-chemical and biogeochemical parameters such as salinity, temperature, dissolved oxygen, Fe(II) and dissolved organic matter fluorescence (FDOM). Overall, the highest gradients in pore water chemistry were found in the cross-shore direction. In particular, a strong physico-chemical differentiation between the tidal high water and low water line was found and reflected relatively stable in- and exfiltrating conditions in these areas. Contrastingly, in between, the pore water compositions in the existing foreshore ridge and runnel system were very heterogeneous on a spatial and temporal scale. The reasons for this observation may be the strong morphological changes that occur throughout the entire year, which affect the exact locations and heights of the ridge and runnel structures and associated flow paths. Further, seasonal changes in temperature and inland hydraulic head, and the associated effect on microbial mediated redox reactions likely overprint these patterns. In the long-shore direction the pore water chemistry varied less than the along the cross-shore direction. Variation in long-shore direction was probably occurring due to topography changes of the ridge-runnel structure and a physical heterogeneity of the sediment, which produced non-uniform groundwater flow conditions. We conclude that on meso-tidal high energy beaches, the rapidly changing beach morphology produces zones with different approximations to steady-state conditions. Therefore, we suggest that zone-specific endmember sampling is the optimal strategy to reduce uncertainties of SGD-driven constituent fluxes

Transformation of silicon in a sandy beach ecosystem: Insights from stable silicon isotopes from fresh and saline groundwaters

Dissolved silicon isotope compositions (δ30Si) have been analysed for the first time in groundwaters of beach sediments, which represent a subterranean estuary with fresh groundwater discharge from a freshwater reservoir and mixing with recirculated seawater. The fresh groundwater reservoir has high and variable dissolved silica concentrations between 136 and 736 μM, but homogeneous δ30Si of +1.0 ± 0.15‰. By contrast, the seawater is strongly depleted in dissolved silica with concentrations of 3 μM, and consequently characterised by high δ30Si of +3.0‰. The beach groundwaters are variably enriched in dissolved silica compared to seawater (23–192 μM), and concentrations increase with depth at all sampling sites. The corresponding δ30Si values are highly variable (+0.3‰ to +2.2‰) and decrease with depth at each site. All groundwater δ30Si values are lower than seawater and most values are lower than dissolved δ30Si of freshwater discharge indicating a significant amount of lithogenic silica dissolution in beach sediments. In contrast to open North Sea sediments, diatom dissolution or formation of authigenic silica in beach sediments is very low (ca. 5 μmol Si g−1). Silica discharge from the beach to the coastal ocean is estimated as approximately 210 mol Si yr−1 per meter shoreline. Considering the extent of coastline this is, at least for the study area, a significant amount of the total Si budget and amounts to ca. 1% of river and 3.5% of backbarrier tidal flat area Si input

Benthic-pelagic coupling of nutrients and dissolved organic matter composition in an intertidal sandy beach

Subterranean estuaries (STEs) are important biogeochemical land–sea interfaces, where fresh groundwater mixes with seawater in coastal aquifers. However, the sources of dissolved organic matter (DOM) and the connection of its molecular-level processing to pore water chemistry and redox conditions in these ecosystems are still not well understood. We studied the cycling of DOM in the STE of an intertidal sandy beach of the North Sea on spatial and seasonal scales. Ultrahigh-resolution mass spectrometry was used to identify thousands of DOM molecular formulae. These data were interpreted in the context of inorganic pore water chemistry, stable carbon isotope composition of solid-phase extracted (SPE) DOM and chemical tracers for bioavailable (dissolved carbohydrates, DCHOs) as well as biorefractory DOM (dissolved black carbon, DBC). Numerical modelling was used to estimate pore water residence times indicating relatively young pore water in the upper saline plume (USP, < 4 years) and decades-old groundwater in the freshwater discharge tube. The detected levels of dissolved Fe and ammonium in the USP at sediment depths exceeding 50 cm demonstrated suboxic conditions. Statistical analyses revealed complex biotic and abiotic DOM processing apart from conservative mixing of marine and terrestrial endmembers. We propose that the input of bioavailable marine and terrestrial DOM, such as DCHOs, by percolating seawater and meteoric groundwater and its degradation by microbes caused oxygen depletion favoring Fe oxide/hydroxide reduction. In the freshwater discharge tube, the presence of highly aromatic compounds, DBC, and 13C-depleted SPE-DOM indicated the intrusion of meteoric groundwater containing terrestrial DOM. The discharge of this groundwater appears to be a significant source of nutrients (e.g., ammonium) and biorefractory, e.g. combustion-derived, DOM to the adjacent water column

Hydrochemical evolution of a freshwater lens below a barrier island (Spiekeroog, Germany): The role of carbonate mineral reactions, cation exchange and redox processe

Freshwater lenses below barrier islands are a precious resource for the local water supply and important for coastal ecosystems. The aim of this study was to investigate the hydrochemical evolution of a freshwater lens, using the barrier island Spiekeroog, Germany, as an example. For this purpose, groundwater samples were obtained during several campaigns, and hydrochemical data and 13C/12C isotope ratios of dissolved inorganic carbon were linked to apparent groundwater ages. Results show that apparent groundwater ages increase with depth and range between 4 and 51 years. All groundwater samples were close to equilibrium with respect to calcite and considerably enriched in calcium and bicarbonate, suggesting calcite dissolution in the unsaturated zone of the dune sediments. The estimated average rate of decalcification was ∼13 mm/a, resulting in a decalcification depth of ∼4.6 m for the oldest sediments of an approximate age of 350 years. Moreover, 13C/12C isotope ratios of dissolved inorganic carbon indicated secondary carbonate mineral reactions within the aquifer, such as recrystallization and closed-system calcite dissolution. Cation exchange was primarily observed in older groundwater, i.e. the deeper part of the aquifer, and the calculated time of complete freshening of the aquifer is ∼600 years. Regarding redox reactants, dissolved oxygen and nitrate concentrations decreased rapidly in young groundwater, while dissolved manganese and iron were virtually absent in samples collected below the zones of oxygen and nitrate reduction. Dissolved sulfide species indicate sulfate reduction in older groundwater, and methanogenesis was detected locally. The absence of solute manganese and iron may be explained by the formation of minerals, such as iron sulfides, siderite, and rhodochrosite, that serve as possible sinks for redox-sensitive solutes. In conclusion, this study showed that hydrochemical data can be linked to groundwater ages to describe the hydrochemical evolution of a freshwater lens in a homogeneous sandy aquifer as a function of residence time