Pore Water Velocity Research Articles

Non-pumping reactive wells filled with mixing nano and micro zero-valent iron for nitrate removal from groundwater: Vertical, horizontal, and slanted wells

Non-pumping reactive wells (NPRWs) filled by zero-valent iron (ZVI) can be utilized for the remediation of groundwater contamination of deep aquifers. The efficiency of NPRWs mainly depends on the hydraulic contact time (HCT) of the pollutant with the reactive materials, the extent of the well capture zone (Wcz), and the relative hydraulic conductivity of aquifer and reactive material (Kr). We investigated nitrate removal from groundwater using NPRWs filled by ZVI (in nano and micro scales) and examined the effect of NPRWs orientations (i.e. vertical, slanted, and horizontal) on HCT and Wcz. The dependence of HCT on Wcz for different Kr values was derived theoretically for a homogeneous and isotropic aquifer, and verified using particle tracking simulations performed using the semi-analytical particle tracking and pathlines model (PMPATH). Nine batch experiments were then performed to investigate the impact of mixed nano-ZVI, NZVI (0 to 2 g l−1) and micro-ZVI, MZVI (0 to 4 g l−1) on the nitrate removal rate (with initial NO3−=132 mg l−1). The NPRWs system was tested in a bench-scale sand medium (60 cm length × 40 cm width × 25 cm height) for three orientations of NPRWs (vertical, horizontal, and slanted with inclination angle of 45°). A mixture of nano/micro ZVI, was used, applying constant conditions of pore water velocity (0.024 mm s−1) and initial nitrate concentration (128 mg l−1) for five pore volumes. The results of the batch tests showed that mixing nano and micro Fe0 outperforms these individual materials in nitrate removal rates. The final products of nitrate degradation in both batch and bench-scale experiments were NO2−, NH4+, and N2(gas). The results of sand-box experiments indicated that the slanted NPRWs have a higher nitrate reduction rate (57%) in comparison with vertical (38%) and horizontal (41%) configurations. The results also demonstrated that three factors have pivotal roles in expected HCT and Wcz, namely the contrast between the hydraulic conductivity of aquifer and reactive materials within the wells, the mass of Fe0 in the NPRWs, and the orientation of NPRWs adopted. A trade-off between these factors should be considered to increase the efficiency of remediation using the NPRWs system.

Modification, characterization and investigations of key factors controlling the transport of modified nano zero-valent iron (nZVI) in porous media

ABSTRACTEnhancement of nano zero-valent iron (nZVI) stability and transport in the subsurface environment is important for in situ degradation of contaminants. Various biodegradable dispersants (poly (acrylic acid) (PAA), Tween 20 and Reetha Extracts) have been tested to evaluate their effectiveness in this regard. Application of dispersants during the synthesis of nZVI have positively affected the reduction in particle size. The transport capacity in terms of fraction elution at different pore water velocities and collector grain size (filter media) was analyzed using correlation equation for the filtration model by Rajagopalan and Tien (RT model). At a surfactant concentration of 5% for PAA, Tween 20 and Reetha (Sapindus trifoliata) extracts, the lowest particle size and the highest zeta potential achieved are 8.67 nm and −55.29 mV, 75.24 nm and −62.68 mV, 61.6 nm and −37.82 mV, respectively. The trend of colloidal stability by The Derjaguin—Landau—Verwey—Overbeek (DLVO) Theory model for PAA and Reetha applied concentration was 3% > 4% > 5% > 2% > 1% > 0%. For Tween 20, modified nZVI particle shows a higher repulsive force with increasing Tween 20 concentration. Results indicated that some mechanisms such as aggregation, ripening and surface modification with different carrier pore water velocities had a considerable impact on nZVI retention in porous media. The results indicate that natural surfactant like Reetha extracts exhibits an alternative potential capacity for nZVI modification in comparison with synthetic surfactants (PAA and Tween 20).

Modelling solute transport in homogeneous and heterogeneous porous media using spatial fractional advection-dispersion equation

This paper compared the abilities of advection-dispersion equation (ADE) and spatial fractional advection-dispersion equation (sFADE) to describe the migration of a non-reactive contaminant in homogeneous and heterogeneous soils. To this end, laboratory tests were conducted in a sandbox sizing 2.5 × 0.1 × 0.6 m (length × width × height). After performing a parametric sensitivity analysis, parameters of sFADE and ADE were individually estimated using the inverse problem method at each distance. The dependency of estimated parameters on distance was examined. The estimated parameters at 30 cm were used to predict breakthrough curves (BTCs) at subsequent distances. The results of sensitivity analysis indicated that average pore-water velocity and dispersion coefficient were, respectively, the most and least sensitive parameters in both mathematical models. The values of fractional differentiation orders (α) for sFADE were smaller than 2 in both soils. The scale-dependency of the dispersion coefficients of ADE and sFADE was observed in both soils. However, the application of sFADE to describe solute transport reduced the scale effect on the dispersion coefficient, especially in the heterogeneous soil. For the homogeneous soil, the predicting results of ADE and sFADE were nearly similar, while for the heterogeneous soil, the predicting results of sFADE were more satisfactory in comparison with those of ADE, especially when the transport distance increased. Compared to ADE, the sFADE simulated somewhat better the tailing parts of BTCs and showed the earlier arrival of tracer. Overall, the solute transport, especially in the heterogeneous soil, was non-Fickian and the sFADE somewhat better described non-Fickian transport.

Comparison of microbial removal between slow dead-end versus tangential sand filtration.

Both River-Bank Filtration and dead-end sand-filtration are becoming increasingly applied in rural areas to improve quality of fecally contaminated water. To evaluate the capacity of both treatments to remove E. coli, fecal streptococci, somatic and K13 phages, we have studied their concentrations in diluted wastewater after short-distance tangential sand-filtration and after dead-end sand-filtration.Bacteria were almost undetectable in both systems after sixty cm depth and at a pore water velocity of 1 m/d. Both phages underwent removal of 2.5 logs by tangential filtration, whereas dead-end filtration removed 5.1 logs and 3.9 logs of K13 and somatic phages, respectively. After discounting removal by the schmutzdecke, observed only in the dead-end filtration, both systems removed phages similarly.We conclude that short-distance river bank filtration alone does not meet WHO requirements for drinking water. However, the reduction of suspended solids renders the filtered water amenable to further treatment steps.

Evaluating Hydraulic Properties of Biochar‐Amended Soil Aggregates by High‐Performance Pore‐Scale Simulations

Core Ideas SR‐mCT was used to characterize soil pore structures. Graphics processing unit–enhanced LBM was used to simulate the water flow. Effects of biochar amendment on soil hydraulic properties were evaluated. Biochar amendment can change the soil pore architecture and improve soil physical and hydraulic properties. In this study, two clayey soils were selected and treated with or without biochar amendment (four treatments). Synchrotron‐based X‐ray micro‐computed tomography (SR‐mCT) was used to characterize pore structures of the selected aggregates from the treatments. Based on the high‐resolution soil pore structure information, the three‐dimensional lattice Boltzmann (LB) method was used to simulate water flow in pores. The high‐resolution soil pore structures were also used to define the internal boundary conditions in the LB method that was accelerated by graphics processing unit parallel computing. Results showed that biochar amendment increased soil permeability by at least one order of magnitude, reduced tortuosity by 20 to 30%, and resulted in more uniformly distributed pore water velocities, which was attributed to the change in soil pore geometry. This study is the first physically based modeling effort to assess the effect of biochar amendment on hydraulic properties of soil aggregates. The SR‐mCT method and modeling approach developed in the research play an important role in understanding the mechanisms of water and nutrient transport in soil at the pore scale.

Linking pore network characteristics extracted from CT images to the transport of solute and colloid tracers in soils under different tillage managements

The understanding of relations between quantitative information of soil structure from X-ray computed tomography (CT) and soil functions is an important topic in agronomy and soil science. The influence of tillage on macroporosity (i.e., pores measured by CT >240 μm in all directions) could be manifested in their effects on solute and colloid transport properties. Tillage will also have crucial importance on preferential flow; i.e., a direct flow through root and earthworm channels. Increasing knowledge of the relationships between soil tillage, structure, and transport contributes to a deeper insight into the key factors of soil management influencing productivity, environmental quality and crop health. The aim of this work is the identification of relationships between soil management of the pore network and the influence of the characteristics of the paths identified by CT on the transport of solute and colloidal tracers. In this work, we used CT to characterize the macropore network ( >0.24 mm) of sixteen columns (100 height × 84 diameter, mm) of adjacent plots under different soil management as follows: conventional management with shallow tillage after sowing (4 samples), conventional management with no tillage after sowing (4 samples), and organic vegetables (8 samples). The soil samples were installed in columns under a dripper, and the transport behavior was examined during breakthrough of Br− and 1-μm latex microspheres in samples near saturation. Transport of Br− and latex microspheres was modeled using a two-region physical non-equilibrium model (dual porosity). Preferential flow was higher under organic management, although the pore water velocities were, in general, lower. The preferential flow of Br− was correlated with the total volume of macropores extracted from each tomography, and the local increase in the Hounsfield value (i.e., CT matrix density, CTMatrix) surrounding the macropores. The denser lining, produced by the earthworms in the inner walls of the pores, was inversely correlated with the kinetic exchange coefficient between mobile and immobile zones of the dual-porosity model. The macropore roughness indicated by the CT-macropore surface area was correlated with the solute dispersion coefficient and with the solute travel time. Finally, we found that the overall CTMatrix density is inversely related to the preferential flow. The importance of this work lies in the improvement of the accuracy of predictions related to flow and transport through soils, especially those processes that include particles traveling through the soil.

Transport and fate of viruses in sediment and stormwater from a Managed Aquifer Recharge site

Enteric viruses are one of the major concerns in water reclamation and reuse at Managed Aquifer Recharge (MAR) sites. In this study, the transport and fate of bacteriophages MS2, PRD1, and ΦX174 were studied in sediment and stormwater (SW) collected from a MAR site in Parafield, Australia. Column experiments were conducted using SW, stormwater in equilibrium with the aquifer sediment (EQ-SW), and two pore-water velocities (1 and 5 m day−1) to encompass expected behavior at the MAR site. The aquifer sediment removed >92.3% of these viruses under all of the considered MAR conditions. However, much greater virus removal (4.6 logs) occurred at the lower pore-water velocity and in EQ-SW that had a higher ionic strength and Ca2+ concentration. Virus removal was greatest for MS2, followed by PRD1, and then ΦX174 for a given physicochemical condition. The vast majority of the attached viruses were irreversibly attached or inactivated on the solid phase, and injection of Milli-Q water or beef extract at pH = 10 only mobilized a small fraction of attached viruses (<0.64%). Virus breakthrough curves (BTCs) were successfully simulated using an advective–dispersive model that accounted for rates of attachment (katt), detachment (kdet), irreversible attachment or solid phase inactivation (μs), and blocking. Existing MAR guidelines only consider the removal of viruses via liquid phase inactivation (μl). However, our results indicated that katt > μs > kdet > μl, and katt was several orders of magnitude greater than μl. Therefore, current microbial risk assessment methods in the MAR guideline may be overly conservative in some instances. Interestingly, virus BTCs exhibited blocking behavior and the calculated solid surface area that contributed to the attachment was very small. Additional research is therefore warranted to study the potential influence of blocking on virus transport and potential implications for MAR guidelines.

Kinetics of inorganic nitrogen turnover in a sandy seepage face on a subterranean estuary

Subterranean estuary seepage faces are recognized as important reactive interfaces that regulate solute transport to coastal ecosystems via Submarine Groundwater Discharge (SGD). Here we describe benthic processes and rates driving the biogeochemical regulation of SGD-borne inorganic N loading into the Ria Formosa lagoon (Iberian peninsula) through a sandy seepage face. Maximum potential NO3− reduction rates, obtained by kinetic modeling of advection-controlled flow-through reactor experiments, ranged from 2.33 ± 1.06 to 14.17 ± 0.22 nmol cm−3 “bulk” sediment (bs) h−1. Maximum potential nitrification ranged from 0 to 7.5 ± 1.3 nmol cm−3 bs h−1 while potential ammonium assimilation was valued at 2.0 ± 0.3 nmol cm−3 bs h−1. These NO3− reduction rates are in good agreement with previous estimates obtained by diagenetic modeling of in-situ porewater NO3− vertical profiles at the same location. Potential NO3− reduction rates were very sensitive to temperature (Q10 = 3.5 ± 0.2). Porewater velocity seems to control net NO3− reduction rates, probably by determining solute distribution but also its supply to the microbial community by shaping the diffusive boundary layer around sediment particles. Nevertheless, NO3− reduction rates seem ultimately limited by organic matter availability at high velocities. Half-saturation constants of NO3− for NO3− reduction were low, suggesting that the NO3− reducing microbial community had high affinity for NO3−. In addition, our experiments provide evidence for the occurrence of alternative NO3− reduction pathways, including Dissimilatory Nitrate Reduction to Ammonium (DNRA) and apparent aerobic NO3− reduction within the shallow subsurface sediment layer (2–12 cm depth).

Nonlinear seepage–erosion coupled water inrush model for completely weathered granite

ABSTRACTWater inrush from completely weathered granite has presented a significant challenge for tunnel construction, suggesting the urgency and importance of revealing the water inrush mechanism. In this paper, a seepage erosion model is proposed to describe the water inrush. Assuming that completely weathered granite consists of solid grain phase, fluid phase, and fluidized grain phase, the three-phase interaction was constrained by mass balance equations, and the erosion of fluidized grains was described by a modified porosity evolution equation. The fluid flow is governed by a coupled Darcy–Brinkman/Navier–Stokes equation, which responds to the variation of the flow pattern in the evolution process. Then, the validity of the model has been proved, and the superiority has been studied by comparing with the previous models. The comparison results showed that the flow pattern has a significant impact on pore pressure, water velocity, and mutation time, and the proposed model can more accurately predict the development of velocity. Furthermore, this model was used to simulate the development of water inrush and achieved good results in predicting the direction, channel size, and whole evolution process of water inrush. The research findings from the paper can benefit tunnel engineering in the case of water inrush disasters.

Submarine groundwater discharge within a landslide scar at the French Mediterranean coast

Submarine groundwater discharge (SGD), the flow of fresh and saline groundwater from the seabed into the coastal ocean, has been intensively investigated in the recent years. This research has usually been restricted to shallow water and intertidal areas, whereas knowledge about groundwater seepage in deeper water is mainly limited to point sources from karstic aquifers. In this study we observed submarine groundwater seepage and a subterranean estuary in sediments at water depths of 20–44 m located within the Ligurian Margin, western Mediterranean Sea. Here, a catastrophic submarine landslide occurred near the Nice airport (French Ligurian coast) in the fall of 1979 after a period of heavy rainfall. During two research cruises, gravity cores were recovered in and around the area of the landslide scar. Pore water samples collected from sediment cores indicated sediments containing freshwater within the landslide scar. Pore water profiles of selected ions, such as chloride, ammonium, manganese, sulfate and barium were used to assess transport and reaction processes within the sediment. A 1-dimensional transport model indicates in most cores upward pore water velocities of 2.3–8.8 cm yr−1. This study shows that submarine groundwater seepage along the French Mediterranean coastline can occur at water depths reaching 44 m.

Solute Transport Properties of Fen Peat Differing in Organic Matter Content.

There is a limited understanding of solute transport properties of degraded peat soils as compared to mineral substrates. A lower organic matter (OM) content is often the result of peat degradation and mineralization following artificial drainage. In this study, we aimed at deducing changes in solute transport properties of peat soils differing in OM content. Miscible displacement experiments were conducted on 70 undisturbed soil columns with OM contents ranging from 11 to 86% w/w under saturated steady-state conditions using tritium and bromide as conservative tracers. Measured breakthrough curves (BTCs) were subjected to model analysis using three different approaches: single-porosity model (SPM), mobile-immobile model (MIM), and two-flow region model (TFRM). The results indicated that (i) nonequilibrium solute transport processes are common in peat soils; (ii) the TFRM improved predictions of BTCs with heavy tailing or two peaks; (iii) applied tracers, tritium and bromide, were retarded in peat soils with higher OM content; and (iv) pronounced preferential flow mainly occurred in peat soils with lower OM content. This type of strong preferential flow had a small ratio of measured to fitted pore water velocity and a greater ratio of velocities (/ > 3.0) in the fast and slow transport region as obtained from the TFRM. We conclude that shallow groundwater resources are more likely to become polluted in drained and degraded fen peats that are used for agricultural purposes.

Advective solute transport through porous media along streamlines

Transforming the standard solute transport models into one-dimensional ones along streamlines by excluding from them diffusive and dispersive components and direct including in simulation procedures the mechanical dispersion, the predominate factor defining solute transport in day-to-day applications, permit reducing these models to ordinary differential equations of the first order, making simulations less cumbersome and more flexible and permitting analytical solutions under about any practical conditions. The models of solute transport along flux tubes and streamlines are discussed in numerous publications. Most of those models, if not all, are one-dimensional copies of the mass conservation law and do not take in consideration streamline specificity: the absence of sinks and source within flux tubes to which the streamlines belong. This makes the obtained equations either inadequate or unnecessary oversimplified by assuming the pore water velocity constant along the streamlines. The development of the solute transport equations along streamlines taking in account the above mentioned specificity is presented in Appendix 1. Appendix 2 contains development of analytical solutions to the corresponding models for spatially varying actual pore water velocities, arbitrary initial and boundary conditions, linear and non-linear (Freundlich and Langmuir isotherms) interaction of solutes with the surroundings. Calculating concentration at given points and instants, the concentrations for the involved streamlines must be summarized accordingly to mechanical dispersion. The method of weighted summarizing is suggested. The approach is illustrated by field examples.

Covariation in patterns of turbulence‐driven hyporheic flow and denitrification enhances reach‐scale nitrogen removal

AbstractCoinjections of conservative tracers and nutrients are commonly used to assess travel time distributions and nutrient removal in streams. However, in‐stream tracer data often lack information on long‐term hyporheic storage, and removal rate coefficients are often assumed to be uniform despite plentiful evidence that microbially mediated transformations, such as denitrification, exhibit strong spatial variability in the hyporheic zone. We used process‐based particle‐tracking simulations to explore the coupled effects of spatial patterns in hyporheic flow and denitrification on reach‐scale nitrogen removal. We simulated whole‐stream nitrogen dynamics with exponential, layered, and uniform profiles of hyporheic denitrification. We also simulated nitrogen dynamics in Little Rabbit Creek, an agricultural headwater stream in the Kalamazoo River Basin (Michigan, USA) where vertical profiles of hyporheic denitrification were measured in situ. Covariation between pore water velocity and mixing causes rapid exchange in the near‐surface bioactive region and substantially prolonged exchange in the deeper hyporheic. Patterns of hyporheic denitrification covary with patterns of hyporheic flow. This covariation directly controls tailing of in‐stream breakthrough curves and hence reach‐scale nutrient removal. Enhanced denitrification near the sediment‐water interface strongly tempers breakthrough curve tails at time scales associated with flushing of the near‐surface region, while more spatially uniform denitrification causes weaker tempering over a wider range of hyporheic exchange time scales. At the reach scale, overall nitrogen removal increases with heterogeneity of hyporheic denitrification, indicating that covariation between flow and denitrification—particularly the rapid flushing of highly bioactive regions near the sediment‐water interface—controls whole‐stream transformation rates.

Unraveling the complexities of the velocity dependency of E. coli retention and release parameters in saturated porous media

Escherichia coli transport and release experiments were conducted to investigate the pore-water velocity (v) dependency of the sticking efficiency (α), the fraction of the solid surface area that contributed to retention (Sf), the percentage of injected cells that were irreversibly retained (Mirr), and cell release under different (10–300mM) ionic strength (IS) conditions. Values of α, Sf, and Mirr increased with increasing IS and decreasing v, but the dependency on v was greatest at intermediate IS (30 and 50mM). Following the retention phase, successive increases in v up to 100 or 150mday−1 and flow interruption of 24h produced negligible amounts of cell release. However, excavation of the sand from the columns in excess electrolyte solution resulted in the release of >80% of the retained bacteria. These observations were explained by: (i) extended interaction energy calculations on a heterogeneous sand collector; (ii) an increase in adhesive strength with the residence time; and (iii) torque balance consideration on rough surfaces. In particular, α, Sf, and Mirr increased with IS due to lower energy barriers and stronger primary minima. The values of α, Sf, and Mirr also increased with decreasing v because the adhesive strength increased with the residence time (e.g., an increased probability to diffuse over the energy barrier) and lower hydrodynamic forces diminished cell removal. The controlling influence of lever arms at microscopic roughness locations and grain-grain contacts were used to explain negligible cell removal with large increases in v and large amounts of cell recovery following sand excavation. Results reveal the underlying causes (interaction energy, torque balance, and residence time) of the velocity dependency of E. coli retention and release parameters (ksw, α, and Sf) that are not accounted for in colloid filtration theory.

Implication of surface modified NZVI particle retention in the porous media: Assessment with the help of 1-D transport model

Retention of surface-modified nanoscale zero-valent iron (NZVI) particles in the porous media near the point of injection has been reported in the recent studies. Retention of excess particles in porous media can alter the media properties. The main objectives of this study are, therefore, to evaluate the effect of particle retention on the porous media properties and its implication on further NZVI particle transport under different flow conditions. To achieve the objectives, a one-dimensional transport model is developed by considering particle deposition, detachment, and straining mechanisms along with the effect of changes in porosity resulting from retention of NZVI particles. Two different flow conditions are considered for simulations. The first is a constant Darcy’s flow rate condition, which assumes a change in porosity, causes a change in pore water velocity and the second, is a constant head condition, which assumes the change in porosity, influence the permeability and hydraulic conductivity (thus Darcy’s flow rate). Overall a rapid decrease in porosity was observed as a result of high particle retention near the injection points resulting in a spatial distribution of deposition rate coefficient. In the case of constant head condition, the spatial distribution of Darcy’s velocities is predicted due to variation in porosity and hydraulic conductivity. The simulation results are compared with the data reported from the field studies; which suggests straining is likely to happen in the real field condition.

Parameterization and prediction of nanoparticle transport in porous media: A reanalysis using artificial neural network

AbstractThe continuing rapid expansion of industrial and consumer processes based on nanoparticles (NP) necessitates a robust model for delineating their fate and transport in groundwater. An ability to reliably specify the full parameter set for prediction of NP transport using continuum models is crucial. In this paper we report the reanalysis of a data set of 493 published column experiment outcomes together with their continuum modeling results. Experimental properties were parameterized into 20 factors which are commonly available. They were then used to predict five key continuum model parameters as well as the effluent concentration via artificial neural network (ANN)‐based correlations. The Partial Derivatives (PaD) technique and Monte Carlo method were used for the analysis of sensitivities and model‐produced uncertainties, respectively. The outcomes shed light on several controversial relationships between the parameters, e.g., it was revealed that the trend of with average pore water velocity was positive. The resulting correlations, despite being developed based on a “black‐box” technique (ANN), were able to explain the effects of theoretical parameters such as critical deposition concentration (CDC), even though these parameters were not explicitly considered in the model. Porous media heterogeneity was considered as a parameter for the first time and showed sensitivities higher than those of dispersivity. The model performance was validated well against subsets of the experimental data and was compared with current models. The robustness of the correlation matrices was not completely satisfactory, since they failed to predict the experimental breakthrough curves (BTCs) at extreme values of ionic strengths.

Cotransport of human adenoviruses with clay colloids and TiO2 nanoparticles in saturated porous media: Effect of flow velocity

This study focuses on the effects of two clay colloids (kaolinite, KGa-1b and montmorillonite, STx-1b) and titanium dioxide (TiO2) nanoparticles (NPs) on human adenovirus transport and retention in water saturated porous media at three different pore water velocities (0.38, 0.74, and 1.21cm/min). Transport and cotransport experiments were performed in 30-cm long laboratory columns packed with clean glass beads with 2mm diameter. The experimental results suggested that the presence of KGa-1b, STx-1b and TiO2 NPs increased human adenovirus inactivation and attachment onto the solid matrix, due to the additional attachment sites available. Retention by the packed column was found to be highest (up to 99%) in the presence of TiO2 NPs at the highest pore water velocity, and lowest in the presence of KGa-1b. The experimental results suggested that adenoviruses would undergo substantial aggregation or heteroaggregation during cotransport. However, no distinct relationships between mass recoveries and water velocity could be established from the experimental cotransport data. Note that for the cotransport experiments, collision efficiency values were shown to be higher for the higher flow rate examined in this study.

Upscaling of nanoparticle transport in porous media under unfavorable conditions: Pore scale to Darcy scale

Transport and deposition of nanoparticles in porous media is a multi-scale problem governed by several pore-scale processes, and hence, it is critical to link the processes at pore scale to the Darcy-scale behavior. In this study, using pore network modeling, we develop correlation equations for deposition rate coefficients for nanoparticle transport under unfavorable conditions at the Darcy scale based on pore-scale mechanisms. The upscaling tool is a multi-directional pore-network model consisting of an interconnected network of pores with variable connectivities. Correlation equations describing the pore-averaged deposition rate coefficients under unfavorable conditions in a cylindrical pore, developed in our earlier studies, are employed for each pore element. Pore-network simulations are performed for a wide range of parameter values to obtain the breakthrough curves of nanoparticle concentration. The latter is fitted with macroscopic 1-D advection-dispersion equation with a two-site linear reversible deposition accounting for both equilibrium and kinetic sorption. This leads to the estimation of three Darcy-scale deposition coefficients: distribution coefficient, kinetic rate constant, and the fraction of equilibrium sites. The correlation equations for the Darcy-scale deposition coefficients, under unfavorable conditions, are provided as a function of measurable Darcy-scale parameters, including: porosity, mean pore throat radius, mean pore water velocity, nanoparticle radius, ionic strength, dielectric constant, viscosity, temperature, and surface potentials of the particle and grain surfaces. The correlation equations are found to be consistent with the available experimental results, and in qualitative agreement with Colloid Filtration Theory for all parameters, except for the mean pore water velocity and nanoparticle radius.

Pore water velocity and ionic strength effects on DOC release from peat-sand mixtures: Results from laboratory and field experiments

Organic soils are the most important source of dissolved organic carbon (DOC) in surface water. To date, most studies have focused on natural and re-wetted peatlands, but in Central Europe a large proportion of organic soils are drained and under agricultural use. Furthermore, measures such as deep ploughing or sand addition have been conducted to improve trafficability and have resulted in topsoil horizons consisting of a peat-sand mixture. Very little is known about DOC losses from such soils. Moreover, peat soils frequently feature both mobile zones, characterised by active water and solute transport, and immobile zones, which exchange solutes with the mobile zone by diffusion. Surprisingly, however, the effects of this dual porosity on DOC transport have not yet been explored. This study investigated the physicochemical controls on DOC concentrations in a peat-sand mixture by means of a saturated column experiment with undisturbed columns. The soil came from a former bog in northern Germany where peat layers remaining after peat extraction were mixed with the underlying mineral soil by ploughing. Three pumping rates and two levels of electrical conductivity (EC) were applied. The transport properties of the soil were obtained by analysing breakthrough curves of potassium bromide using the transport model STANMOD, which is based on the two-region non-equilibrium concept. The results of the column study were compared to DOC concentrations measured bi-weekly for two years at the field site from where the columns were taken. Despite a similar texture and soil organic carbon (SOC) content, the fraction of the mobile zone in the columns varied between 51% and 100% of total porosity. Thus even heavily degraded organic soils mixed with sand still showed a dual porosity comparable to degraded peat soils. Percolating the columns with the high EC solution caused low pH values, probably due to ion exchange and cation bridging. The combination of high EC and low pH greatly decreased DOC concentrations at the outlet of the columns. DOC concentrations decreased and fluxes increased as the pumping rates increased. Taking pore water velocity in the mobile zone into account could help to explain the differences between the columns. Overall, transport of DOC did not seem to be limited by production of DOC, but by rate-limited exchange processes. In contrast to the column experiment, field concentrations of DOC were much higher and were not related to pH, but increased with higher electrical conductivity. These higher concentrations could be explained by low pore water velocities and the slightly higher SOC content in the field. This first experiment on DOC transport in peat-sand mixtures taking the dual-porosity nature of organic soils into account clearly demonstrated the importance of pore water velocity and thus the residence time for DOC concentrations. While hydrochemical conditions are frequently addressed in laboratory studies, there is a need for improved understanding of their interaction with hydrology and soil-physical properties, especially when attempting to interpret DOC data on different spatial and temporal scales.

Response of pavement and stratified ground due to vehicle loads considering rise of water table

ABSTRACTA two-dimensional model has been developed to investigate the dynamic response of pavement–embankment–subsoil system considering the rise of water table. The ground consists of pavement, embankment, topsoil, saturated soil under the natural water table and semi-infinite substratum. Above the natural water table, the material is assumed to be dry and viscoelastic. Below the natural water table, the soil is saturated and modelled as poroviscoelasitc material. The semi-infinite substratum is assumed to be water-resistant and taken as a viscoelastic half-space. When the water table rises, the submerged part is then saturated and taken as poroviscoelastic medium. Biot’s theory and Fourier transform are employed to describe the problem, and some improvements are made based on traditional stiffness matrix method to give semi-analytical solution to the problem. The solution has been verified using a simplified problem reported by existing research. The influences of load characteristics and rise of water table are investigated. Resonance load frequency exists, at which the maximum surface vertical displacement of the pavement reaches its largest value. The rise of water table decreases the resonance frequency and generally increases the maximum surface vertical displacement of the pavement. The pore water pressure and pore water velocity are also substantially affected by the rise of water table. In general, the rise of water table has an amplifying effect on the vibration of the pavement system and increases the damage risk, which needs to be considered in the design and maintenance of highways.