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Structural hierarchy is a fundamental characteristic of natural porous media. Yet it provokes one of the grand challenges for the modeling of fluid flow and transport since pore‐scale structures and continuum‐scale domains often coincide independent of the observation scale. Common approaches to represent structural hierarchy build, for example, on a multidomain continuum for transport or on the coupling of the Stokes equations with Darcy's law for fluid flow. These approaches, however, are computationally expensive or introduce empirical parameters that are difficult to derive with independent observations. We present an efficient model for fluid flow based on Darcy's law and the law of Hagen‐Poiseuille that is parameterized based on the explicit pore space morphology obtained, for example, by X‐ray μ‐CT and inherently permits the coupling of pore‐scale and continuum‐scale domain. We used the resulting flow field to predict the transport of solutes via particle tracking across the different domains. Compared to experimental breakthrough data from laboratory‐scale columns with hierarchically structured porosity built from solid glass beads and microporous glass pellets, an excellent agreement was achieved without any calibration. Furthermore, we present different test scenarios to compare the flow fields resulting from the Stokes‐Brinkman equations and our approach to comprehensively illustrate its advantages and limitations. In this way, we could show a striking efficiency and accuracy of our approach that qualifies as general alternative for the modeling of fluid flow and transport in hierarchical porous media, for example, fractured rock or karstic aquifers. A model for the simulation of pore‐scale and continuum‐scale flow in hierarchically structured porous media is developedExplicit pore space morphology obtained by image analysis of X‐ray micro‐CT images is used for parameterizationPredictions of solute breakthrough obtained by particle tracking perfectly match observations

The identification of transport parameters by inverse modeling often suffers from equifinality or parameter correlation when models are fitted to measurements of the solute breakthrough in column outflow experiments. This parameter uncertainty can be approached by performing multiple experiments with different sets of boundary conditions, each provoking observations that are uniquely attributable to the respective transport processes. A promising approach to further increase the information potential of the experimental outcome is the closed‐flow column design. It is characterized by the recirculation of the column effluent into the solution supply vessel that feeds the inflow, which results in a damped sinusoidal oscillation in the breakthrough curve. In order to reveal the potential application of closed‐flow experiments, we present a comprehensive sensitivity analysis using common models for adsorption and degradation. We show that the sensitivity of inverse parameter determination with respect to the apparent dispersion can be controlled by the experimenter. For optimal settings, a decrease in parameter uncertainty as compared to classical experiments by an order of magnitude is achieved. In addition, we show a reduced equifinality between rate‐limited interactions and apparent dispersion. Furthermore, we illustrate the expected breakthrough curve for equilibrium and nonequilibrium adsorption, the latter showing strong similarities to the behavior found for completely mixed batch reactor experiments. Finally, breakthrough data from a reactive tracer experiment is evaluated using the proposed framework with excellent agreement of model and experimental results. Dispersion is fitted with a tenfold decrease in uncertainty in closed‐flow experimentsEquifinality between adsorption kinetics and dispersion is decreasedClosed‐flow mode breakthrough curves resemble equilibration curves of completely mixed batch reactor experiments

Transport studies that employ column experiments in closed-flow mode complement classical approaches by providing new characteristic features observed in the solute breakthrough and equilibrium between liquid and solid phase. Specific to the closed-flow mode is the recirculation of the effluent to the inflow via a mixing vessel. Depending on the ratio of volumes of mixing vessel and water-filled pore space, a damped oscillating solute concentration emerges in the effluent and mixing vessel. The oscillation characteristics, e.g., frequency, amplitude, and damping, allow for the investigation of solute transport in a similar fashion as known for classical open-flow column experiments. However, the closed loop conserves substances released during transport within the system. In this way, solute and porous medium can equilibrate with respect to physicochemical conditions. With this paper, the features emerging in the breakthrough curves of saturated column experiments run in closed-flow mode and methods of evaluation are illustrated under experimental boundary conditions forcing the appearance of oscillations. We demonstrate that the effective pore water volume and the pumping rate can be determined from a conservative tracer breakthrough curve uniquely. In this way, external preconditioning of the material, e.g., drying, can be avoided. A reactive breakthrough experiment revealed a significant increase in the pore water pH value as a consequence of the closed loop. These results highlight the specific impact of the closed mass balance. Furthermore, the basis for the modeling of closed-flow experiments is given by the derivation of constitutive equations and numerical implementation, validated with the presented experiments. A complementary approach of conducting column experiments is presentedConcentration oscillations emerge in closed-flow column experimentsA numerical model was developed and successfully applied to observations

Soil texture is one of the main factors controlling soil organic carbon (SOC) storage. Accurate soil‐texture analysis is costly and time‐consuming. Therefore, the clay content is frequently not determined within the scope of regional and plot‐scale studies with high sample numbers. Yet it is well known that the clay content strongly affects soil water content. The objective of our study was to evaluate if the clay content can be estimated by a simple and fast measure like the water content of air‐dried soil. The soil samples used for this study originated from four different European regions (Hainich‐Dün, Germany; Schwäbische Alb, Germany; Hesse, France; Bugac, Hungary) and were collected from topsoils and subsoils in forests, grasslands, and croplands. Clay content, water content of air‐dried soil, and SOC content were measured. Clay content was determined either by the Pipette method or by the Sedigraph method. The water content of air‐dried soil samples ranged from 2.8 g kg–1to 63.3 g kg–1and the corresponding clay contents from 60.0 g kg–1to 815.7 g kg–1. A significant linear relationship was found between clay content and water content. The scaled mean absolute error (SMAE) of the clay estimation from the water content of air‐dried soil was 20% for the dataset using the Pipette method and 28% for the Sedigraph method. The estimation of the clay content was more accurate in fine‐textured than in coarse‐textured soils. In this study, organic‐C content played a subordinate role next to the clay content in explaining the variance of the water content. The water retention of coarse‐textured soils was more sensitive to the amount of organic C than that of fine‐textured soils. The results indicate that in our study the water content of air‐dried soil samples was a good quantitative proxy of clay contents, especially useful for fine‐textured soils.

It is not clear from the literature how the spatial distribution of topsoil and vegetation properties is affected by grazing cessation. Thus, the objective of this study was to elucidate if longterm grazing exclosure increases spatial heterogeneity of topsoil and vegetation properties in a steppe ecosystem in NE China. Variograms and crossvariograms were calculated for bulk density, organic carbon OC, total N, and total S concentration, 13C, pH, Ah horizon thickness, vegetation cover, and aboveground biomass. Five sites with different grazing intensities ungrazed since 1979, ungrazed since 1999, winter grazing, continuously grazed, heavily grazed were sampled with two different grid sizes, allowing the exploration of scale effects. Small grids 15 m spacing, 5 m nested sampling exhibited a different spatial structure compared to large grids 50 m spacing, 10 m nested sampling. Spatial distribution in small grids changed with grazing intensity. Generally, heterogeneity of topsoil properties increased with decreasing grazing intensity from a homogeneous to a patchy distribution. We attribute this to vegetation recoverysuccession and deposition of windblown material in ungrazed areas. The plot ungrazed since 1999 showed different spatial dependencies than continuously and heavily grazed plots, but has not yet reached the high variability of the plot which was ungrazed since 1979. Large grid sampling did not detect smallscale variability or grazing impacts, but showed spatial dependencies that were attributed to topography or soil erosiondeposition. Low OC concentration and low Ah thickness were associated with hilltop and shoulder positions, resulting in lower OC stocks at these topographic units.

Kurzfassung Kolloide sind Bestandteile aller aquatischen Systeme. Das Verhalten der Kolloide in der Umwelt, deren Mobilisierung, Transport und Abscheidung und somit auch die Relevanz sind noch nicht vollständig verstanden. Dies liegt zum Teil an der Tatsache, dass die Kolloidanalytik extrem aufwändig ist. Sie gehört nicht zur Standardanalytik eines Wasserlabors. Keine Technik ist bisher zufriedenstellend in der Lage, Kolloide in ihrem gesamten Größenbereich auch bei geringsten Konzentrationen gleichzeitig zu quantifizieren und zu identifizieren. In der Regel ist eine Kombination von verschiedenen Techniken notwendig, um ausreichend Informationen über das Verhalten der Kolloide zu erhalten. Für die Bestimmung der Oberflächeneigenschaften stehen kaum Methoden zur Verfügung. Eine weitere, nicht zu unterschätzende Schwierigkeit ist die Probennahme und Probenaufbereitung. Für die Untersuchung von Kolloiden sind spezielle Probennahmetechniken und Protokolle notwendig, da ansonsten die ungestörten, natürlichen Bedingungen durch (Probennahme-)Artefakte maskiert werden. Dieser Artikel soll in Form einer Übersichtsarbeit Hinweise zur Probennahme geben und die verschiedenen Möglichkeiten der Kolloidanalytik aufzeigen.

Kurzfassung Kolloide sind Bestandteile aller aquatischen Systeme. Sie umfassen Feststoffe, deren Größe in zumindest einer Dimension zwischen 1 und 1.000 nm liegt. Sie können anorganischer (z. B. Tonteilchen, Karbonate oder Silikate) oder organischer (z. B. Ruß oder höhermolekulare organische Verbindungen wie Huminstoffe) Natur sein. Auch Bakterien, Viren, Sporen und Algen in diesem Größenbereich können zu den Kolloiden gerechnet werden (häufig als “Bio-Kolloide“ bezeichnet). Kolloide können den Transport von (Schad-)stoffen im Untergrund und in Oberflächengewässern beeinflussen, insbesondere von Spurenelementen und hydrophoben organischen Verbindungen, oder zu unerwünschten Effekten wie Porenraumreduktion („Clogging“) führen. Für die Hygiene des Trinkwassers ist das Verhalten von Bakterien, Viren und Sporen (den „Bio-Kolloiden“) von besonderer Bedeutung. Bisher ist das Verhalten von Kolloiden nur unzureichend verstanden. Dieser Artikel soll einen Beitrag zu dem Verständnis des physikalischen Verhaltens der Kolloide, deren Vorkommen in der aquatischen Umwelt sowie Relevanz in Form einer Übersichtsarbeit leisten.

Depositions originating from a central Slovak Al smelter may increase metal solubility in adjacent soils because they contain F (mainly HF). The reason for fluoro-mobilization of metals may be the formation of soluble fluoro-metal complexes or the mobilization of organic matter and subsequent formation of organo-metal complexes. The objectives of our work were (1) to assess the extent of metal mobilization by fluoride in a Slovak Lithic Eutrochrept affected by the emissions of an Al smelter and (2) to model the dissolved metal species with the help of a chemical equilibrium model (MINEQL+). The O (Moder), A, and B horizons were equilibrated with solutions at F concentrations of 0, 0.9, 2.7, and 9.0 mmol l^—1. In the extracts, the concentrations of Al, Ca, Cd, Cr, Cu, Fe, K, Mg, Mn, Ni, Pb, Zn, dissolved organic carbon (DOC), free and complexed F, and the pH and electrical conductivity (EC) were determined. The heavy metal concentrations in the O horizon (Cd: 0.99, Cr: 18.0, Cu: 44, Ni: 26, Pb: 110, and Zn: 84 mg kg^—1) were 2.5 to 9 times larger than those in the A and B horizons. The concentrations of H2O-soluble F decreased from the O (261 mg kg^—1) to the A (103 mg kg^—1) and B horizon (92 mg kg^—1). In batch experiments increasing addition of F increased the equilibrium concentrations of Al, Cr, Cu, Fe, Ni, Pb, and DOC in all samples, of Cd in the A, and of K in the B horizon. At the same time the concentrations of complexed F and pH increased whereas EC decreased. Chemical equilibrium modelling indicated that the mobilizing effect of F resulted from the formation of fluoro-Al complexes and organo-complexes of all other metals.

Fluoride in soil solution may cause substantially increased leaching of dissolved organic carbon (DOC), Al, and heavy metals from soils. Our objective was to examine whether increased metal solubility is attributable to formation of fluoro‐ or organo‐complexes. The A horizon of a Slovak soil contaminated with heavy metals and F by Al smelter emissions was used to conduct two column experiments under saturated and steady‐state water flow conditions. The columns were leached with 5 mMNa2SO4, 1 mMNaCl, and 9 mMNaF solutions at pH 5.5 and an electrical conductivity of 1000 µS cm−1. Samples were collected in 70‐min intervals for approximately 7 d. Two experiments were performed. In the first experiment (single F pulse), a fluoride (F) pulse was applied for 36 pore volumes (PVs), while in the second experiment (dual F pulse), two consecutive F pulses (each for 2.8 PVs) were applied. For both experiments, Cl breakthrough curves (BTCs) revealed a convection‐dominated transport regime. The breakthrough of Cd, Cr, Fe, Ni, and Pb paralleled DOC breakthrough in both experiments indicating metal leaching as organo‐complex. In the dual F pulse experiment, peak separation of the DOC and the complexed F (Fbound) breakthrough was achieved. The highest Fboundconcentration occurred 3.8 PVs later than that of DOC. The Al BTC paralleled that of Fbboundbut showed tailing with a turning point simultaneous to the highest DOC concentration. Thus, the increase in Al solubility was mainly caused by fluoro‐complexation and only to a lesser extent by organo‐complexation.

Kurzfassung: Die Berücksichtigung der räumlichen Heterogenität von Schadstoff-Quellen und -Senken ist eine wesentliche Voraussetzung für eine verläßliche Abschätzung der Sickerwasserfrachten von polyzyklischen aromatischen Kohlenwasserstoffen (PAK) in belasteten Böden. Ziel dieser Arbeit ist die Kennzeichnung der PAK-Belastung an einem ehemaligen Gaswerkstandort (Testfeld Süd) unter Berücksichtigung der standörtlichen Heterogenität. Im Rahmen einer Probenahme wurden tiefendifferenziert und räumlich aufgelöst die hydraulischen und physikochemischen Bodeneigenschaften sowie die PAK-Belastungssituation aufgenommen. Kennzeichnend für die Belastungssituation der ungesättigten Zone sind konzentrations- und tiefenunabhängige PAK-Muster, die sich deutlich vom Muster im Grundwasserleiter und von Böden unter Versiegelung unterscheiden. Die PAK-Summenkonzentrationen weisen – wie auch die Bodeneigenschaften – eine hohe Schwankungsbreite auf. Durch stochastische Simulation auf Basis der Theorie Markovscher Ketten erfolgte die Ableitung typischer Bodenprofile. Über die dadurch ermittelten Wahrscheinlichkeiten der Profilausprägungen ist eine Gewichtung lokaler PAK-Sickerwasserkonzentrationen hinsichtlich ihres Beitrags zum flächigen Austrag möglich.

Groundwater environments provide habitats for diverse microbial communities, and although Archaea usually represent a minor fraction of communities, they are involved in key biogeochemical cycles. We analysed the archaeal diversity within a mixed carbonate-rock/siliciclastic-rock aquifer system, vertically from surface soils to subsurface groundwater including aquifer and aquitard rocks. Archaeal diversity was also characterized along a monitoring well transect that spanned surface land uses from forest/woodland to grassland and cropland. Sequencing of 16S rRNA genes showed that only a few surface soil-inhabiting Archaea were present in the groundwater suggesting a restricted input from the surface. Dominant groups in the groundwater belonged to the marine group I (MG-I) Thaumarchaeota and the Woesearchaeota. Most of the groups detected in the aquitard and aquifer rock samples belonged to either cultured or predicted lithoautotrophs (e.g., Thaumarchaeota or Hadesarchaea). Furthermore, to target autotrophs, a series of 13CO2 stable isotope-probing experiments were conducted using filter pieces obtained after filtration of 10,000 L of groundwater to concentrate cells. These incubations identified the SAGMCG Thaumarchaeota and Bathyarchaeota as groundwater autotrophs. Overall, the results suggest that the majority of Archaea on rocks are fixing CO2, while archaeal autotrophy seems to be limited in the groundwater.