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  • 523  (27)
  • 550.724  (18)
  • 2020-2022  (45)
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  • 2020-2022  (45)
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  • 2021  (45)
  • 1
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    Carbon release and transformation from coastal peat deposits controlled by submarine groundwater discharge: a column experiment study (2021)
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    Publication Date: 2021-12-06
    Description: Abstract Although the majority of coastal sediments consist of sandy material, in some areas marine ingression caused the submergence of terrestrial carbon-rich peat soils. This affects the coastal carbon balance, as peat represents a potential carbon source. We performed a column experiment to better understand the coupled flow and biogeochemical processes governing carbon transformations in submerged peat under coastal fresh groundwater (GW) discharge and brackish water intrusion. The columns contained naturally layered sediments with and without peat (organic carbon content in peat 39 ± 14 wt%), alternately supplied with oxygen-rich brackish water from above and oxygen-poor, low-saline GW from below. The low-saline GW discharge through the peat significantly increased the release and ascent of dissolved organic carbon (DOC) from the peat (δ13CDOC − 26.9‰ to − 27.7‰), which was accompanied by the production of dissolved inorganic carbon (DIC) and emission of carbon dioxide (CO2), implying DOC mineralization. Oxygen respiration, sulfate (SO42−) reduction, and methane (CH4) formation were differently pronounced in the sediments and were accompanied with higher microbial abundances in peat compared to sand with SO42−-reducing bacteria clearly dominating methanogens. With decreasing salinity and SO42− concentrations, CH4 emission rates increased from 16.5 to 77.3 μmol m−2 d−1 during a 14-day, low-saline GW discharge phase. In contrast, oxygenated brackish water intrusion resulted in lower DOC and DIC pore water concentrations and significantly lower CH4 and CO2 emissions. Our study illustrates the strong dependence of carbon cycling in shallow coastal areas with submerged peat deposits on the flow and mixing dynamics within the subterranean estuary.
    Keywords: 550.724 ; coastal peatlands ; coastal peatlands ; biogeochemical processes ; carbon release ; column experiments
    Language: English
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  • 2
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    Efficiency and Accuracy of Micro-Macro Models for Mineral Dissolution (2021)
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    Publication Date: 2021-12-03
    Description: Micro-macro models for dissolution processes are derived from detailed pore-scale models applying upscaling techniques. They consist of flow and transport equations at the scale of the porous medium (macroscale). Both include averaged time- and space-dependent coefficient functions (permeability, porosity, reactive surface, and effective diffusion). These are in turn explicitly computed from the time- and space-dependent geometry of unit cells and by means of auxiliary cell problems defined therein (microscale). The explicit geometric structure is characterized by a level set. For its evolution, information from the transport equations solutions is taken into account (micro-macro scales). A numerical scheme is introduced, which is capable of evaluating such complex settings. For the level-set equation a second-order scheme is applied, which enables us to accurately determine the dynamic reactive surface. Local mesh refinement methods are applied to evaluate Stokes type cell problems using P2/P1 elements and a Uzawa type linear solver. Applications of our permeability solver to scenarios involving static and evolving geometries are presented. Furthermore, macroscopic flow and transport equations are solved applying mixed finite elements. Finally, adaptive strategies to overcome the computational burden are discussed. We apply our approach to the dissolution of an array of dolomite grains in the micro-macro context and validate our numerical scheme.
    Keywords: 551.49 ; 550.724 ; porous media ; reactive flow ; dissolution processes ; modeling
    Language: English
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  • 3
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    Laboratory and field investigations on freeze and gravity core sampling and assessment of coring disturbances with implications on gas bubble characterization† (2021)
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    Publication Date: 2021-10-14
    Description: The quantification of greenhouse gas emissions from aquatic ecosystems requires knowledge about the spatial and temporal dynamics of free gas in sediments. Freezing the sediment in situ offers a promising method for obtaining gas-bearing sediment samples, unaffected by changes in hydrostatic pressure and sample temperature during core withdrawal and subsequent analysis. This article presents a novel freeze coring technique to preserve the in situ stratigraphy and gas bubble characteristics. Nondestructive X-ray computed tomography (CT) scans were used to identify and characterize coring disturbances of gravity and freeze cores associated with gassy sediment, as well as the effect of the freezing process on the gas bubble characteristics. Real-time X-ray CT scans were conducted to visualize the progression of the freezing process. Additional experiments were conducted to determine the freezing rate to assess the probability of sediment particle/bubble migration, and gas bubble nucleation at the phase transition of pore water to ice. The performance of the freeze coring technique was evaluated under field conditions in Olsberg and Urft Reservoir (Germany). The results demonstrate the capability of the freeze coring technique for the preservation of gas-bearing sediments and the analysis of gas bubble distribution pattern in both reservoirs. Nevertheless, the obtained cores showed that nearly all gravity and freeze cores show some degree of coring disturbances.
    Keywords: 550.724 ; gas-bearing sediments ; freeze coring technique
    Language: English
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  • 4
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    Ice Particle Properties Inferred From Aggregation Modelling (2021)
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    Publication Date: 2021-10-14
    Description: We generated a large number 105,000 of aggregates composed of various monomer types and sizes using an aggregation model. Combined with hydrodynamic theory, we derived ice particle properties such as mass, projected area, and terminal velocity as a function of monomer number and size. This particle ensemble allows us to study the relation of particle properties with a high level of detail which is often not provided by in situ measurements. The ice particle properties change rather smoothly with monomer number. We find very little differences in all particle properties between monomers and aggregates at sizes below 1 mm which is in contrast to many microphysics schemes. The impact of the monomer type on the particle properties decreases with increasing monomer number. Whether, for example, the terminal velocity of an aggregate is larger or smaller than an equal-size monomer depends mostly on the monomer type. We fitted commonly used power laws as well as Atlas-type relations, which represent the saturation of the terminal velocity at large sizes (terminal velocity asymptotically approaching a limiting value) to the data set and tested the impact of incorporating different levels of complexity with idealized simulations using a 1D Lagrangian super particle model. These simulations indicate that it is sufficient to represent the monomer number dependency of ice particle properties with only two categories (monomers and aggregates). The incorporation of the saturation velocity at larger sizes is found to be important to avoid an overestimation of self-aggregation of larger snowflakes.
    Keywords: 551.5 ; 550.724 ; agreggation modeling ; cloud microphysics ; ice particle properties ; Lagrangian modeling ; terminal velocity
    Language: English
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  • 5
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    Comment on “A New One-Equation Model of Fluid Drag for Irregularly Shaped Particles Valid Over a Wide Range of Reynolds Number” by F. Dioguardi et al. (2021)
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    Publication Date: 2021-10-14
    Description: With this comment we want to clarify a number of aspects of the paper recently published by Dioguardi, Mele, and Dellino “A New One-Equation Model of Fluid Drag for Irregularly Shaped Particles Valid Over a Wide Range of Reynolds Number” (hereafter referred to as DMD2018). In particular, we show that contrary to the conclusions of DMD2018, the model of Bagheri and Bonadonna (2016, https://doi.org/10.1016/j.powtec.2016.06.015), hereafter referred to as BB2016, is the best model in predicting the drag and terminal velocity of particles measured by DMD2018, as demonstrated here by comparison of estimation errors. The discrepancy is mainly due to a production error (misplaced parentheses) introduced in BB2016 during the publication process and partly due to the incorrect methodology used by DMD2018 to calculate particle terminal velocity. Here we present the correct sets of equations and methodology to show that typo-free model of BB2016 outperforms all existing drag models including the new model suggested by DMD2018.
    Keywords: 550.724 ; drag coefficient ; terminal velocity ; particle shape ; non-spherical ; irregular
    Language: English
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  • 6
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    Lithic artifact assemblage transport and microwear modification in a fluvial setting: A radio frequency identification tag experiment (2021)
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    Publication Date: 2021-10-14
    Description: River processes are widely assumed to have impacted the integrity of lithic assemblages when artifacts are found in fluvial sediments, but the specifics of these influences remain largely unknown. We conducted a real-world experiment to determine how the initial stages of fluvial entrainment affected lithic artifact assemblages. We inserted replica artifacts with radio frequency identification tags into a gravel-bedded river in Wales (UK) for seven months and related their transport distances to their morphology and the recorded streamflow. In addition, nine artifacts were recovered at the end of the experiment and analyzed for microwear traces. In sum, our results show that in a gravel-bedded river with a mean discharge of 5.1 m3/s, artifact length and width were the main variables influencing artifact transport distances. The experiment also resulted in characteristic microwear traces developing on the artifacts over distances of 485 m or less. These results emphasize the multifaceted nature of alluvial site formation processes in a repeatable experiment and highlight new ways to identify the transport of replica Paleolithic material.
    Keywords: 550.724 ; 551.35 ; experimental archaeology ; fluvial dynamics ; Paleolithic ; RFID ; taphonomy ; use-wear
    Language: English
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  • 7
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    The Impact of Wettability and Surface Roughness on Fluid Displacement and Capillary Trapping in 2-D and 3-D Porous Media: 2. Combined Effect of Wettability, Surface Roughness, and Pore Space Structure on Trapping Efficiency in Sand Packs and Micromodels (2021)
    Details
    Publication Date: 2021-10-13
    Description: A comprehensive understanding of the combined effects of surface roughness and wettability on the dynamics of the trapping process is lacking. This can be primarily attributed to the contradictory experimental and numerical results regarding the impact of wettability on the capillary trapping efficiency. The discrepancy is presumably caused by the surface roughness of the inner pore-solid interface. Herein, we present a comparative μ-CT study of the static fluid-fluid pattern in porous media with smooth (glass beads) and rough surfaces (natural sands). For the first time, a global optimization method was applied to map the characteristic geometrical and morphological properties of natural sands to 2-D micromodels that exhibit different degrees of surface roughness. A realistic wetting model that describes the apparent contact angle of the rough surface as a function surface morphology and the intrinsic contact angle was also proposed. The dynamics of the trapping processes were studied via visualization micromodel experiments. Our results revealed that sand and glass beads displayed opposite trends in terms of the contact angle dependence between 5° and 115°. Sand depicted a nonmonotonous functional contact angle dependency, that is, a transition from maximal trapping to no trapping, followed by an increase to medium trapping. In contrast, glass beads showed a sharp transition from no trapping to maximal trapping. Since both porous media exhibit similar morphological properties (similar Minkowski functions: porosity, surface density, mean curvature density, Euler number density), we deduce that this difference in behavior is caused by the difference in surface roughness that allows complete wetting and hence precursor thick-film flow for natural sands. Experimental results on micromodels verified this hypothesis.
    Keywords: 550.724 ; impact of wettability on trapping efficiency ; impact of surface roughness and pore space structure on trapping efficiency ; porous media with rough surface ; natural sand and glass ceramic micromodels ; wettability-controlled crossover from snap-off to bypass trapping ; spontaneous precursor thick-film flow (Wenzel's argument)
    Language: English
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  • 8
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    Leaching of hexavalent chromium from young chromite ore processing residue (2021)
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    Publication Date: 2021-10-13
    Description: Chromite ore processing residue (COPR) is a waste derived from the chromate extraction from roasted ores and is deposited in some countries in landfills. The objective of this study was to investigate the leaching characteristics of hexavalent Cr [Cr(VI)] from two COPR samples obtained from unlined landfills in the Kanpur area of northern India. Column experiments were conducted under water-saturated conditions to simulate Cr release from the wastes caused by tropical heavy-rain events. Leached Cr(VI) decreased from 1,800 to 300 mg L−1 (Rania site) and 1,200 to 163 mg L−1 (Chhiwali site) during exchange of 12 pore volumes, which approximately corresponds to 2 yr of monsoon precipitation. Flow interruptions for 10, 100, and 1,000 h had little effect on Cr(VI) concentrations in the leachate, suggesting that Cr(VI) leaching was not limited by slow release kinetics. Calcium aluminum chromium oxide hydrates (CAC), and highly soluble phases such as Na2CrO4 may play a role in controlling Cr(VI) concentration in the leachates. The amount of Cr(VI) leached from the columns accounted for 16% of the total Cr(VI) present in both COPR samples. A decrease in the solid-phase Cr(VI)/Crtotal ratio along the column was identified by X-ray absorption near edge structure (XANES) spectroscopy. Consistently, the smallest Cr(VI)/Crtotal ratios were found in the lower column section closest to the inflow. Our results suggest that Cr(VI) leaching from the unlined COPR landfills will continue for centuries, highlighting the urgent need to remediate these dumpsites.
    Keywords: 577.275 ; 550.724 ; Northern India ; chromite ore processing residue ; leaching ; experiments
    Language: English
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  • 9
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    Transport and Retention of Sulfidized Silver Nanoparticles in Porous Media: The Role of Air-Water Interfaces, Flow Velocity, and Natural Organic Matter (2021)
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    Publication Date: 2021-10-13
    Description: The sulfidation and aging of silver nanoparticles (Ag-NPs) with natural organic matter (NOM) are major transformation processes along their pathway in wastewater treatment plants and surface waters. Although soils appear to be a sink for disposed Ag-NPs, the impact of variable saturation on the transport and retention behavior in porous media is still not fully understood. We studied the behavior of sulfidized silver nanoparticles (S-Ag-NPs, 1 mg L−1) in saturated and unsaturated sand columns regarding the effects of (i) the presence of NOM (5 mg L−1) in the aquatic phase on retention, transport, and remobilization of S-Ag-NPs and (ii) the distribution and quantity of air-water and solid-water interfaces for different flow velocities determined via X-ray microtomography (X-ray μCT). Unsaturated transport experiments were conducted under controlled conditions with unit gradients in water potential and constant water content along the flow direction for each applied flux. It was shown that (i) NOM in S-Ag-NP dispersion highly increased the NP-mobility; (ii) differences between saturated and unsaturated transport were increasing with decreasing flux and, consequently, decreasing water contents; (iii) both, solid-water and air-water interfaces were involved in retention of S-Ag-NPs aged by NOM. Using numerical model simulations and X-ray μCT of flow experiments, the breakthrough of Ag-NP could be explained by a disproportional increase in air-water interfaces and an increasing attachment efficiency with decreasing water content and flow velocity.
    Keywords: 577.275 ; 550.724 ; silver nanoparticles ; X-ray tomography ; phase distribution ; air-water interface ; natural organic matter ; transport
    Language: English
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  • 10
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    The Impact of Wettability and Surface Roughness on Fluid Displacement and Capillary Trapping in 2-D and 3-D Porous Media: 1. Wettability-Controlled Phase Transition of Trapping Efficiency in Glass Beads Packs (2021)
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    Publication Date: 2021-10-13
    Description: Fluid invasion, displacement of one fluid by another in porous media, is important in a large number of industrial and natural processes. Of special interest is the trapping of gas and oil clusters. We study the impact of wettability on fluid pattern formation and capillary trapping in three-dimensional glass beads packs (dmean = 1 mm) during fluid invasion at capillary numbers of 10−7 using μ-CT. The invading fluid was water, and the defending fluid was air. The contact angle of the glass beads was altered between 5° and 115° using Piranha cleaning and silanization. We analyzed the front morphology of the invading fluid, the residual gas saturation, the fluid occupation frequency of pores, and the morphology and statistics of the trapped gas clusters. We found a sharp transition (crossover) at a critical contact angle θc = 96°. Below θc the morphology of the displacement front was flat and compact caused by the strong smoothing effect of cooperative filling. Above θc the morphology of the displacement front was fractal and ramified caused by single bursts (Haines jumps). Across this dynamical phase transition the trapping efficiency changes from no trapping to maximal trapping. For θ 〉 θc the experimental results show that invasion percolation governs the fluid displacement. Strong indicators are the universal scaling behavior of the size distribution of large clusters (relative data error εdata 〈 1%) and their linear surface-volume relationship (R2 = 0.99).
    Keywords: 550.724 ; percolation transition ; sharp transition (crossover) at a critical contact angle θc ; below θc, the displacement front was flat and compact ; above θc, the displacement front was fractal and ramified ; trapping efficiency changes from no trapping to maximal trapping ; trapped gas clusters show universal scaling law
    Language: English
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