Simulation of anoxic lenses as exporters of reactivity in alluvial aquifer sediments

Consider the Anoxic Microsite: Acknowledging and Appreciating Spatiotemporal Redox Heterogeneity in Soils and Sediments

“…165 The export of these sulfidic colloidal species can extend the boundaries of anoxic microsites by stimulating microbial activity and accumulation of reduced S products downgradient of the "original" anoxic microsite. 166,167 Redox gradient magnitude also determines the fate of S in sediments with anoxic microsites. Sulfate reduction yields less energy to microorganisms than other electron acceptors, and therefore, SO 4 2− reduction is often limited or inhibited by other electron acceptors of higher redox potential.…”

“…Despite limitations arising from creating a simplified system (compared to nature), the knowledge gained from well-designed simulated systems is invaluable as presently it is extremely difficult if not impossible to obtain dependable measures of anoxic microsites in the field. Simulated experiments, therefore, serve as a critical link between field measurements and numerical models by elucidating key mechanisms and reaction rates in systems with anoxic microsites. , …”

“…The 3D angular pore network model was later expanded to include interactions of aggregates of different sizes and orientations and upscaled to demonstrate how aggregate size distributions alter anoxic microsite distribution and, thus, biogeochemical gas fluxes from soil profiles . Pore-scale modeling of anoxic microsites is often paired with X-ray microcomputed tomography (μCT), which can constrain values for diffusivity and water distribution. , Larger scale models, based on laboratory experiments, have accurately represented anoxic microsites and their biogeochemical influence in 2D using the principles of Darcian flow and reactive transport modeling …”

“…For example, immobilization of sulfide via reductive dissolution of Fe­(III)–(hydroxy)­oxides and Fe–sulfide formation requires high ratios of S­(II) to Fe­(II). − However, in systems with inadequate S 2– or excess Fe­(II), anoxic microsites mobilize S and associated nutrients and contaminants as colloids . The export of these sulfidic colloidal species can extend the boundaries of anoxic microsites by stimulating microbial activity and accumulation of reduced S products downgradient of the “original” anoxic microsite. , …”

Coupled Processes Involving Organic Matter and Fe Oxyhydroxides Control Geogenic Phosphorus Enrichment in Groundwater Systems: New Evidence from FT-ICR-MS and XANES

“…Various models of mobilization processes have been proposed to explain the elevated geogenic P concentrations in groundwater. − However, there is a consensus that microbe-mediated degradation/oxidation of P-containing sedimentary or dissolved organic matter (OM) and reductive dissolution and desorption from P-bearing Fe­(III) (oxyhydr)­oxides (FeOOH) play key roles in the genesis of geogenic P-rich groundwater. ,, Therefore, OM and FeOOH are the most important factors controlling geogenic P enrichment, especially in alluvial-lacustrine plains, where subsurface OM and FeOOH are abundant. , However, even considering these two most critical factors only, understanding the sources and enrichment mechanisms about geogenic P is not straightforward, particularly because of (i) the complex occurrences of various OM types with different reactivities, various FeOOH phases with different crystallinities, and various P pools associated with different hosting phases and (ii) the variable strategies for geogenic P enrichment via oxidation of OM with different reactivities coupled to reductive dissolution of FeOOH with different crystallinities. , Thus, systematic characterizations (about OM, FeOOH, and P) and detailed analysis (about variable strategies) are key to understanding the fate of P in groundwater systems.…”