Presence Of Preferential Flow Paths Research Articles

Enhancing Mixing During Groundwater Remediation via Engineered Injection‐Extraction: The Issue of Connectivity

AbstractIn the context of in situ groundwater remediation, mixing is vital for a successful outcome. A slow mixing rate between the contaminated groundwater and the injected treatment solution can severely weaken the effective degradation rate. Engineered Injection‐Extraction (EIE) has been proposed as a means to accelerate dilution within the porous medium. However, existing studies on the subject have not considered the potential impact of connectivity and preferential flow‐paths. Neglecting connectivity can lead to an overestimation of EIE's capabilities, since the fluid may in reality be carried mainly through a few high‐permeability channels, thus hampering mixing and reaction. Due to the fact that channeling can be found in many actual sites, in this work we aim to evaluate EIE methods in both poorly connected (represented as Multigaussian fields) and well‐connected fields (represented as non‐Multigaussians). The approach is to identify, for each given medium, a stirring protocol—defined by a specific combination of rotation angle and rotation rate—which maximizes mixing. To that end, metrics are proposed in order to (a) quantify both the mixing and the containment of the treatment solution within a given remediation volume, and (b) characterize the particle trajectories to explicitly evaluate if preferential paths are broken. The results obtained from these metrics are quite similar for both types of fields, proving that the enhancing of mixing by means of EIE is effective regardless of the presence of preferential flow paths. This study demonstrates that EIE via rotating dipoles diminishes the remediation outcome uncertainty induced by medium heterogeneity.

Transport of plastic particles in natural porous media under freeze-thaw treatment: Effects of porous media property.

Freeze-thaw (FT) cycles would alter physical and chemical properties of soil and thus influence the transport of plastic particles (one type of emerging contaminant with great concerns). This study was designed to investigate the effects of FT treatment on the mobility of plastic particles (nanoplastics as representative) in columns packed with natural soils (i.e. loamy sand and sandy soil, quartz sand employed as comparison). We found that FT treatment of different types of porous media would induce different transport behaviors of plastic particles. Specifically, FT treatment of quartz sand did not affect plastic particles mobility. While FT treatment of loamy sand and sandy soil increased plastic particles transport. The increased pore sizes and disintegration of small soil particles from soils (the detached soil would serve as mobile vehicle for the transport of plastic particle) led to the facilitated mobility of plastic particles in two types of soils after FT treatment. The presence of preferential flow paths induced by FT treatment also drove to the enhanced mobility of plastic particles in sandy soil with FT treatment. This study clearly showed that the mobility of model plastic particles in two types of natural soils was greatly enhanced by FT treatment.

Geochemical analysis of formation water as a tool for better understanding of water flooding

In oil production, a reservoir flooding system is often used. This is necessary to compensate for oil production in the reservoir and maintain reservoir pressure. But the water cutting of wells is constantly increasing, that makes the exploitation less profitable, in the future this leads to the shutdown of wells. The source of water supply to the well is not always clear, and in the presence of flooded below and overlying reservoirs from which production was previously carried out, the possible number of water supply routes increases. The first aim of this work is to identify channels with high permeability by using tracer test, the second aim is to characterize geochemical state of the reservoir as a whole, and finally to choose the optimal sidetrack point using reservoir modelling.The reservoir represents terrigenous deposits as an independent Bobrikovian horizon and the incision channel in the carbonate massif. The permeability of these zones is expected to be different. Therefore, in order to achieve the aims, the following studies were conducted:1) Tracer test. A tracer blindly follows the fluid phase in which it is injected. Tracer tests are most often used to obtain information on the direction and velocity of the flow of brines, hydraulic conductivity, effective porosity, transport parameters (e.g., dispersivity values), and the presence of preferential flow paths. For this, we used a spectrofluorimeter.2) Geochemical studies of the composition of all wells on the site. A unified Oil Water Contact (OWC) for carbonate and terrigenous deposits indicates the possibility of their hydrodynamic communication. To confirm this hypothesis, areal geochemical studies of the isotopic composition of reservoir waters were carried out;3) Using the previous methods, we conducted a Field Development Analysis (FDA) and modeled the position of the horizontal wellbore in special software. One well showed better results compared to the other, as it is located in a zone with large oil reserves and the length of the horizontal wellbore was 1.3 times longer.As a result of geochemical studies, three zones were identified based on the isotopic composition of water: the southwest - the waters of the Bobrikovian horizon, the central part - the transition zone, and the northeast - the Tournaisian waters. Zones with current reserves have been determined based on 3D modeling. For further involvement of these volumes of reservoir oil in the development, the sidetracking is proposed.

Impact of fate properties, groundwater fluctuations and the presence of worm burrows on pesticide leaching assessments through golf areas

The risk of pesticide leaching from recreational areas such as golf course turfs is not distinguished in a regulative framework within the EU where the focus is on agricultural soils. But with increasing popularity of golf, and thus, increasing number of golf courses leading to potentially increasing use of pesticides, understanding the processes determining pesticide leaching are critical to ensure optimal quality of both groundwater and golf turf. This study input the measured variation in fate properties of tebuconazole (TBZ) and MCPA as pure active ingredients and commercial products in simulations with realistic hydrological conceptualizations to investigate their implication in leaching assessments. Scenarios with (i) fluctuating and fixed groundwater levels and (ii) preferential flows including fluctuating and fixed groundwater levels were evaluated. The results showed that mobile MCPA leached in higher concentrations by a factor of 1.3 with fluctuating groundwater levels than with fixed groundwater levels. When preferential flow paths were incorporated in the models, the leaching was substantial for MCPA regardless of its formulation as active ingredient or commercial product, while in multiple simulations without preferential pathways there was no leaching of MCPA. Compared to MCPA leaching without preferential flow paths, the leaching concentrations increased up to a factor of 13.9 when preferential flows were included. With preferential flow paths, the increase in leaching concentration from fixed groundwater levels to fluctuating groundwater levels was up to a factor of 2.3 depending on the formulation of MCPA. This study demonstrated that it is imperative to assess fate parameters in the topsoil of golf courses and consider realistic groundwater BC (boundary condition) and the presence of preferential flow paths to obtain representative pesticide leaching risk assessments.

Improving the computational efficiency of first arrival time uncertainty estimation using a connectivity-based ranking Monte Carlo method

The first arrival time of a solute plume migrating from its source to an environmentally sensitive target is one of the key quantities of interest when assessing the risks of groundwater contamination. First arrival times are correlated with the hydraulic connectivity properties of spatially heterogeneous porous formations. Hydraulic connectivity leads to the presence of preferential flow paths which in turn control the transport dynamics of the leading edge of the solute plume and therefore first arrival times. In applications, these arrival times are subject to uncertainty given the lack of a detailed site characterization. The Monte Carlo method is commonly adopted to estimate the uncertainty of solute arrival times, however it leads to a computational burden. In this work, we build upon the existing knowledge regarding the correlation between connectivity and first arrival times to propose an innovative connectivity-based ranking Monte Carlo approach to quantify the uncertainty of first arrival times. The proposed method is tailored to predict first arrival times and allows to alleviate the computational costs when compared to the traditional Monte Carlo method. Our method consists of ranking the randomly generated spatially heterogeneous hydraulic conductivity fields according to their connectivity. The connectivity metric adopted is based on the concept of the minimum hydraulic resistance and can be obtained at a very low computational cost through the use of graph theory. We illustrate the methodology by analyzing the convergence rate of the first arrival time means and standard deviations. We compare the convergence rate of the first arrival times statistics obtained through the proposed methodology with those computed through the traditional Monte Carlo method. Overall, our results indicate the that the proposed methodology ensures a faster convergence of the considered quantities, thus reducing the time required for their estimation and the associated computational burden.

Estimation of groundwater recharge in savannah aquifers along a precipitation gradient using chloride mass balance method and environmental isotopes, Namibia

The quantification of groundwater resources is essential especially in water scarce countries like Namibia. The chloride mass balance (CMB) method and isotopic composition were used in determining groundwater recharge along a precipitation gradient at three sites, namely: Tsumeb (600 mm/a precipitation); Waterberg (450 mm/a precipitation) and Kuzikus/Ebenhaezer (240 mm/a precipitation). Groundwater and rainwater were collected from year 2016–2017. Rainwater was collected monthly while groundwater was collected before, during and after rainy seasons. Rainwater isotopic values for δ18O and δ2H range from −10.70 to 6.10‰ and from −72.7 to 42.1‰ respectively. Groundwater isotopic values for δ18O range from −9.84 to −5.35‰ for Tsumeb; from −10.85 to −8.60‰ for Waterberg and from −8.24 to −1.56‰ for Kuzikus/Ebenhaezer, while that for δ2H range from −65.6 to −46.7‰ for Tsumeb; −69.4 to −61.2‰ for Waterberg and −54.2 to −22.7‰ for Kuzikus/Ebenhaezer. Rainwater scatters along the GMWL. Rainwater collected in January, February and March are more depleted in heavy isotopes than those in November, December, April and May. Waterberg groundwater plots on the GMWL which indicates absence of evaporation. Tsumeb groundwater plots on/close to the GMWL with an exception of groundwater from the karst Lake Otjikoto which is showing evaporation. Groundwater from Kuzikus/Ebenhaezer shows an evaporation effect, probably evaporation occurs during infiltration since it is observed in all sampling seasons. All groundwater from three sites plot in the same area with rainwater depleted in stable isotopic values, which could indicates that recharge only take place during January, February and March. CMB method revealed that Waterberg has the highest recharge rate ranging between 39.1 mm/a and 51.1 mm/a (8.7% – 11.4% of annual precipitation), Tsumeb with rates ranging from 21.1 mm/a to 48.5 mm/a (3.5% – 8.1% of annual precipitation), and lastly Kuzikus/Ebenhaezer from 3.2 mm/a to 17.5 mm/a (1.4% – 7.3% of annual precipitation). High recharge rates in Waterberg could be related to fast infiltration and absence of evaporation as indicated by the isotopic ratios. Differences in recharge rates cannot only be attributed to the precipitation gradient but also to the evaporation rates and the presence of preferential flow paths. Recharge rates estimated for these three sites can be used in managing the savannah aquifers especially at Kuzikus/Ebenhaezer where evaporation effect is observed that one can consider rain harvesting.

High-resolution characterization of nanoparticle transport in heterogeneous porous media via low-field nuclear magnetic resonance

Understanding the transport of nanoparticles (NPs), including nanoscale zero-valent iron (nZVI) particles, through water-saturated porous media has important implications for many natural and engineered systems. For the first time, we use spin-echo single point imaging (SE-SPI) of low-field Nuclear Magnetic Resonance (LF-NMR) to monitor nanoparticle transport through a heterogeneous porous medium. The ability of this method to provide information of nano- to micro-scale pore structure and to monitor transient processes is verified by a transport experiment using modified nZVI particles. Early breakthrough of the NP front and large amount of residual NPs (27% of the iron was retained in the core) were observed in the experiment. These observations were due to the presence of preferential flow paths and dead-end pores as demonstrated by micro X-ray Computed Tomography (μCT) imaging. Spatial and temporal data provided by this method enabled numerical simulations to quantify the impacts of spatial heterogeneity in pore structure on NP transport in terms of transport parameters. The simulation that captures the spatial heterogeneity by explicitly considering four regions of distinct porosities outperforms the model assuming a homogeneous pore structure. The maximum of residual sum of squares reduced from 0.328 to 0.138 when considering the impact of pore structure. The transport parameters vary significantly among different regions. For example, in the lowest porosity region of this sandstone, colloid dispersion (D) is 0.10 cm2/min instead of 0.20 cm2/min in the most connected region, while the attachment (kac) increased from 0.007 to 0.025 min−1. Overall, SE-SPI imaging is shown to be an important tool in refining transport processes of NPs in heterogeneous porous media with application to constrain complex natural systems.

Transport of Atrazine and Dicamba through Silt and Loam Soils

The objectives of this research were to determine the role of preferential flow paths in the transport of atrazine (2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine) and dicamba (3-6-dichloro-2-methoxybenzoic acid) through silt and loam soils overlying the High Plains aquifer in Nebraska. In a previous study, 3 of 6 study areas demonstrated high percentages of macropores; those three areas were used in this study for analysis of chemical transport. As a subsequent part of the study, 12 intact soil cores (30-cm diameter by 40-cm height), were excavated sequentially, two from each of the following depths: 0-40cm and 40-80cm. These cores were used to study preferential flow characteristics using dye staining and to determine hydraulic properties. Two undisturbed experimental field plots, each with a 3-m2 surface area, were installed in three study areas in Nebraska. Each was instrumented with suction lysimeters and tensiometers at depths of 10cm to 80cm in 10-cm increments. Additionally, each plot was planted with corn (Zea mays). A neutron probe access tube was installed in each plot to determine soil water content at 15-cm intervals. All plots were enclosed with a raised frame (of 8-cm height) to prevent surface runoff. All suction lysimeters were purged monthly for three months and were sampled immediately prior to pre-plant herbicide application to obtain background chemical concentrations. Atrazine and dicamba moved rapidly through the soil, but only after a heavy rainfall event, probably owing to the presence of preferential flow paths and lack of microbial degradation in these soil areas. Staining of laboratory cores showed a positive correlation between the percent area stained by depth and the subsequent breakthrough of Br- in the laboratory and leaching of field-applied herbicides owing to large rainfall events. Suction lysimeter samples in the field showed increases in concentrations of herbicides at depths where laboratory data indicated greater percentages of what appeared to be preferential flow paths. Concentrations of atrazine and dicamba exceeding 0.30 and 0.05µg m1-1 were observed at depths of 10-30cm and 50-70cm after two months following heavy rainfall events. It appears from the laboratory experiment that preferential flow paths were a significant factor in transport of atrazine and dicamba. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government or GNS.

A comparison of the spatial distribution of vadose zone water in forested and agricultural floodplains a century after harvest

To improve quantitative understanding of the long-term impact of historic forest removal on floodplain vadose zone water regime, a study was implemented in fall 2010, in the Hinkson Creek Watershed, Missouri, USA. Automated, continuously logging capacitance–frequency probes were installed in a grid-like formation (n=6) and at depths of 15, 30, 50, 75, and 100cm within a historic agricultural field (Ag) and a remnant bottomland hardwood forest (BHF). Data were logged at thirty minute intervals for the duration of the 2011, 2012, and 2013 hydrologic years. Results showed volumetric water content (VWC) to be significantly different between sites (p<0.01) during the study, with site averages of 33.1 and 32.8% at the Ag and BHF sites, respectively. Semi-variogram analyses indicate the presence of strong (<25%) horizontal and vertical spatial correlation of VWC at the Ag site, and a relatively short-range (25cm) vertical spatial correlation at the BHF, but only indicate horizontal VWC spatial correlation in the top 30cm of the BHF profile. Likely mechanisms contributing to patterns of observed differences are contrasting rates and depths of plant water use, and the presence of preferential flow paths in the below ground BHF. Results suggest historic forest removal and cultivation of the Ag site lead to an effective homogenization of the upper soil profile, and facilitated the development of strong VWC spatial dependency. Conversely, higher hydraulic conductivity of the more heterogeneous BHF subsurface likely results in a wetting of the deeper profile (75cm) during climatically wet periods, and thus a more effective processing of hydrologic inputs. Collective results highlight the greater extent and degree to which forest vegetation impacts subsurface hydrology, relative to grassland/agricultural systems, and point to the value of reestablishing floodplain forests for fresh water routing, water quality, and flood mitigation in mixed-land-use watersheds.

Single- and Dual-domain Models of Solute Transport in Alluvial Sediments: the Effects of Heterogeneity Structure and Spatial Scale

Fine-scale heterogeneity of alluvial aquifers controls solute transport in groundwater at the scales relevant for practical applications: the architecture of sedimentary structures might create preferential flow paths (PFPs) or hydraulic barriers, which affect the breakthrough curves (BTCs). Objective of this paper was the assessment of the relevance of single- and dual-domain models for different heterogeneity patterns and scale lengths in alluvial sediments. Three case studies have been analysed with a classical single-domain model (SDM) and with three dual-domain models (DDMs): a dual-porosity model (DPorM) and two dual-permeability models (DPerM), which differ for the presence or the absence of solute exchange between the two domains. The first case study includes numerical tracer tests in metre-scale blocks of alluvial sediments; the second is a laboratory experiment of tracer injection in a decimetre-scale column of homogeneous sand; the third is a field tracer test performed at hectometre scale at the Cape Cod site. The relevance of the solute exchange in the DDMs is analysed with the characteristic advection and exchange times and with the Peclet and Damkohler numbers. The SDM is satisfactory for alluvial sediments with unstructured heterogeneity. The uncoupled DPerM is shown to be a better approach than the DPorM in sediments with PFPs; in this case, the coupled DPerM does not improve significantly the results of the uncoupled DPerM. A minor difference between the results of the three DDMs is observed for sediments in which the non-Fickian behaviour is not clearly determined by the presence of PFPs.

Transport of explosive chemicals from the landmine burial in granular soils

The transport of explosive-related chemicals (ERCs) in soils was studied during water infiltration and evaporation processes as a function of soil water content and temperature. The experiments were conducted in two 100 cm uniform cylindrical columns packed with homogeneous sand, and instrumented with air and water pressure sensors and sampling ports to monitor hydraulic conditions and ERCs concentration profiles in soil. TNT and DNT crystals were placed in a porous membrane and buried as a point source near the surface of the soil. Spatial and temporal concentration distributions of conservative solutes were used to evaluate transport behavior of TNT and DNT in soils. Velocity variations and comparison with the numericalmodel HYDRUS-2D indicate the presence of preferential flow paths. Water content and movement near TNT and DNT buried source highly influence their transport in soils and near soil-atmospheric surfaces. The formation of preferential flow paths are attributed to disturbances of soil properties by burial of ERC sources and water content heterogeneities. By the analysis of velocity variations, disturbances near the buried source resulted in hydraulic heterogeneities and preferential flow near the source, which influence their transport away from the source. Preferential flow causes faster movement and greater dispersion of the solutes during infiltration periods, and influences the rate of mixing in the system.

Physicochemical Heterogeneity Controls on Uranium Bioreduction Rates at the Field Scale

It has been demonstrated in laboratory systems that U(VI) can be reduced to immobile U(IV) by bacteria in natural environments. The ultimate efficacy of bioreduction at the field scale, however, is often challenging to quantify and depends on site characteristics. In this work, uranium bioreduction rates at the field scale are quantified, for the first time, using an integrated approach. The approach combines field data, inverse and forward hydrological and reactive transport modeling, and quantification of reduction rates at different spatial scales. The approach is used to explore the impact of local scale (tens of centimeters) parameters and processes on field scale (tens of meters) system responses to biostimulation treatments and the controls of physicochemical heterogeneity on bioreduction rates. Using the biostimulation experiments at the Department of Energy Old Rifle site, our results show that the spatial distribution of hydraulic conductivity and solid phase mineral (Fe(III)) play a critical role in determining the field-scale bioreduction rates. Due to the dependence on Fe-reducing bacteria, field-scale U(VI) bioreduction rates were found to be largely controlled by the abundance of Fe(III) minerals at the vicinity of the injection wells and by the presence of preferential flow paths connecting injection wells to down gradient Fe(III) abundant areas.

Field permeability at shallow depth in a compacted fill

This paper presents a series of field double-ring infiltration tests (DRTs) for investigating the hydraulic conductivity of a compacted silty clay fill that contains some desiccation cracks. These field tests were conducted at three different depths in the fill, to characterise the variations of conductivity with depth. The hydraulic conductivity of the fill was also measured in the laboratory on block samples using a three-directional permeability cell (sample size: 200 mm × 200 mm × 200 mm) and a one-dimensional permeameter (sample size: 70 mm in diameter and 45 mm high). A perched water table was detected in the fill using tensiometers shortly after the start of each DRT, showing the presence of preferential flow paths in the fill. The hydraulic conductivity decreases with increasing depth. The larger hydraulic conductivity of the surface soil indicates the possible presence of macropores and cracks at shallow depths of the fill. The coefficients of variation of the hydraulic conductivity measured from the DRTs (influence volume: 300 mm in diameter and 450 mm high) and the three-directional permeability cell tests are smaller than those measured from the small-size, one-dimensional permeameter tests. The variability of the measured hydraulic conductivity increases as the sample size decreases.

Effects of Design Mix and Porosity of Waste-Derived Paste as Landfill Daily Covers on Lead Retardation

An innovative waste-derived paste of waste tire chips and paper sludge was proposed for daily cover applications in municipal solid waste landfills. In this study, retardation and transport behaviors of lead (Pb) by the proposed paste were investigated using column tests. Breakthrough curves of 16 specimens at various design mixes and porosities were presented. A one-dimensional equilibrium deterministic transport model was used to fit the transport parameters from the breakthrough curves and to verify the homogeneity of the specimens. Analysis of the effluent concentrations from the column tests displayed a retardation effect for Pb in all cases, with fitted retardation factors ranging from 19.9 to 59.0. Lead retardation by the proposed paste was found to be sensitive to the paste’s porosity and design mix. Early arrivals of solute were observed unexpectedly in columns with high paper sludge content under high packing densities and were attributed to the possible presence of preferential flow paths. The ...

Influence of structural and material properties of GCLs on interface flow in composite liners due to geomembrane defects

ABSTRACT: The flow rates along the interface between damaged geomembranes (GM) and geosynthetic clay liners (GCL) placed on top of a compacted clay liner (CCL) were measured by means of laboratory tests performed with an apparatus especially designed for this purpose. The tests performed were aimed at verifying the influence of the structural and material properties of the GCL on the transmissivity along the GM–GCL interface and flow rates through composite liners. Four types of GCLs with two different bonding processes (stitch-bonded or needle-punched) and different bentonites (natural sodium or natural calcium) were tested. The results obtained showed no significant differences among flow rate versus time in most of the tests performed, especially after the steady-state conditions of flow were reached. An analytical solution was employed to estimate the transmissivity of the GM–GCL interfaces. This solution also allowed predictions of flow rates and radius of wetted areas for typical configurations of composite liners in the field. The results obtained showed little influence of the nature of the bentonite and the predominance of the presence of preferential flow paths between the geomembrane and the GCL surface on the transmissivity of GM–GCL interfaces and flow rates through composite liners.

Measurement and modeling of soil water regime in a lowland paddy field showing preferential transport

Paddy is commonly grown under flooded or submerged condition in which substantial amount of water is lost by different processes. Puddling is traditionally done to reduce water loss from lowland rice fields. Since the very step of puddling, rice root zone undergoes structural changes leading to the formation of a layered profile having a hydraulically less-conductive plow sole below the root zone. However, studies have shown that soil cracking and the presence of preferential flow paths in puddle fields defeat this purpose. Description of soil water regime in such a dynamic soil requires an in situ measurement method for soil hydraulic properties. A field experiment was conducted in twelve 30 m 2 plots during the rainy seasons (June to October) of 2004 and 2005 to evaluate a suitable method for estimating soil hydraulic properties of lowland paddy soil. Results showed that piezometric (pressure) heads installed in different soil layers responded to the drying and wetting cycles typically followed in transplanted rice and are observed as a part of monsoon climate in eastern India. The Marquardt–Levenberg algorithm built in the HYDRUS-1D simulation environment was used to inversely estimate soil hydraulic parameters. Estimated parameters revealed larger hydraulic conductivity for the compacted plow sole than those published in literature, which may have resulted from alternate wetting and drying typically observed under monsoon climate and earthworm burrows observed in our experimental field. Results from simulation studies suggest that both the single- and dual-porosity models could simulate water flow considerably well in lowland paddy field although the latter described pressure head time series data slightly better in about 50% of simulations. Similar performance of the single- vs. dual-porosity model may have resulted from estimating a seasonally mean soil hydraulic properties which include the effect of both preferential flow and matrix flow as the specific soil and boundary conditions prevailed. While water may have preferentially transported through the macropores during the wetting cycles in a near-saturated soil, it would have dominantly moved through soil matrix during the drying cycles. This study shows that simple piezometers may be combined with a simulation model to estimate hydraulic properties of different soil layers in a lowland paddy field.

A process‐based transfer function approach to model tile‐drain hydrographs

AbstractTile‐drain response to rainfall events is determined by unsaturated vertical flow to the water table, followed by horizontal saturated water movement. In this study, unsaturated vertical movement from the redistribution of water is modelled using a sharp‐front approximation, and the saturated horizontal flow is modelled by an approximate solution to the Boussinesq equation. The unsaturated flow component models the fast response that is associated with the presence of preferential flow paths. By convoluting the responses of the two components, a transfer function is developed that predicts tile‐drain response to unit amounts of infiltrated water. It is observed that the unsaturated flow component can be cast in a form that is linear in a power function of the infiltrated depth. Since the approach is process based, model parameter definitions are easily identified with soil properties at the field scale. Furthermore, it is demonstrated that the transfer function model parameters can be estimated from moment analysis. Using superposition, the transient tile‐drain response to arbitrary amounts of infiltrated water can be constructed. Comparison with data measured from the Water Quality Field Station show that this approach provides a promising method for generating tile‐drain response to rainfall events. Copyright © 2006 John Wiley &amp; Sons, Ltd.

Mobility and leaching of glyphosate: a review

There is currently concern that glyphosate, a strongly sorbing non-selective herbicide which is widely used in Europe, may be leached from the root zone into drainage water and groundwater. The purpose of this review is to present and discuss the state of knowledge with respect to the mobility and leaching of glyphosate from agricultural soils. Specific attention is given to the adsorption behaviour of glyphosate and the analysis of available studies on glyphosate transport. In addition, there are a number of experimental and numerical studies indicating that other strongly sorbing substances may be transported rapidly to the sub-surface. The experimental studies analysed in the paper encompass column-, lysimeter- and field-scale experiments on glyphosate transport. The experimental findings, combined with transport studies on other strongly sorbing pesticides in the literature, support the hypothesis that transport of glyphosate may be caused by an interaction of high rainfall events shortly after application on wet soils showing the presence of preferential flow paths. Concentrations of glyphosate in European groundwater have been reported occasionally but monitoring is still limited.

Particle‐Size and Element Distributions of Soil Colloids

Transport of colloids (diameter &lt; 1 μm) through soil has implications for both horizon development and contaminant mobility. Colloid physical properties (size, shape), mineralogy, and surface chemistry are known to influence transport and deposition. Sedimentation field‐flow fractionation‐inductively coupled plasma–mass spectrometry (Sd FFF‐ICP–MS) was used to examine size and element composition distributions of colloids collected from the profile of a texture‐contrast soil located in South Australia. The morphology of the colloids was also examined by transmission electron microscopy (TEM). Colloids collected from the soil matrix, the source of mobile colloids, had significant differences in the size distributions among the three horizons sampled. These differences were consistent with long‐term soil formation processes. Colloids mobilized by rainfall, which were collected from overland flow and infiltration through the soil profile, all showed very similar size distributions. This is consistent with the presence of preferential flow paths and suggests that colloids, and colloid‐associated contaminants, can be transported rapidly through the vadose zone with minimal interaction with the soil matrix. The Sd FFF‐ICP–MS analysis showed variation in element ratios (Fe/Al, Mg/Al), which were used to detect changes in surface coatings and mineralogy over the colloid size range. The study demonstrated the utility of Sd FFF‐ICP–MS for examining the influence of colloid size on element composition and on elucidating colloid transport processes in soils.

Experimental Validation of the Tetrahedral Skeleton Model Pressure Drop Correlation for Silica Monoliths and the Influence of Column Heterogeneity

This paper describes the use of computational fluid dynamics for the calculation of the flow resistance through computer-generated models resembling silica monoliths. This study was undertaken to determine the effect of skeleton heterogeneity on the flow resistance and, more precisely, to test the hypothesis that increased skeleton heterogeneity decreases the flow resistance. To evaluate the proposed model, 24 real silica monoliths have been prepared using the same method, covering a wide range of skeleton sizes (2.2 microm < d(s) < 8 microm) and porosities (0.47 < epsilon < 0.66). The permeability of these monoliths was determined by pressure drop measurements, and structural information was obtained by image analysis of laser scanning confocal microscopy-generated 3D images of the skeleton structure. The results indicate that the presence of preferential flow paths due to an increased heterogeneity of the flow through pore space reduces the flow resistance of monolithic media. It is also shown that the pore size is hence a much better suited scaling dimension than the skeleton size to reduce the permeability of monolithic columns.