Formation Of Preferential Flow Paths Research Articles

Meter-scale MICP improvement of medium graded very gravelly sands: Lab measurement, transport modelling, mechanical and microstructural analysis

Microbially induced carbonate precipitation (MICP) is a promising biogrouting method for ground improvement. Most studies to date have focused on MICP treatment of uniform clean sands, with few studies having been conducted at large-scale on well-graded soils more representative of in situ deposits. This study presents a laboratory meter-scale MICP test on medium-graded very gravelly sands. The MICP treatment was conducted in a radial flow cell (diameter: ∼1 m; thickness: ∼0.15 m) with an injection well located at the centre and a constant hydraulic head at the outer boundary to replicate field conditions. Aqueous chemistry of the effluent samples inside the flow cell was continuously monitored. Transport modelling and effluent sampling monitoring of the electrical conductivity and pH show that, in general, there was good delivery and reaction of the bacteria and chemicals in the radial flow cell, with some preferential flow paths being present. The MICP-treated soil was subjected to a series of hydraulic and mechanical tests and microstructural analysis. Interestingly, the biocemented medium-graded very gravelly sands had higher strengths (UCS values of 2.6–7.4 MPa) for a given calcite content (9.2–15.1%) than those in comparable studies where uniform soils have been treated. This can be attributed to the higher initial density of grain-to-grain contact points in the medium-graded soil, and a high grain angularity which resulted in particle interlocking and longer grain-to-grain contact surfaces, as can be seen from Scanning Electron Microscopy images. Consolidated-drained triaxial tests on two cores showed peak deviatoric strengths of 5.9 MPa and 3.7 MPa under an effective confining stress of 100 kPa, with the clear formation of shear bands during shearing, compared to a peak deviatoric strength of 0.5 MPa determined for the untreated soil. MICP treatment clearly enhanced the shear strength and stiffness of the material. The study shows that formation of preferential flow paths may be a challenge for producing uniform biocementation in field applications of MICP. We propose that successful MICP treatment in heterogeneous soils will require a well-designed and well-executed site investigation programme that can identify, a priori, the geometry of any significant high or low permeability features within the soil body to inform the final MICP treatment strategy.

Laboratory hydraulic shearing of granitic fractures with surface roughness under stress states of EGS: Permeability changes and energy balance

To offer key parameters and constitutive laws required for field-scale multiphysics simulations that can accurately predict created fracture network structures in enhanced geothermal systems (EGS) and resulting energy extraction, we explore the full spectral of hydraulic shear process of granite fracture and revisits the linkage between hydraulic and mechanical properties during shear slips. Key results from our novel lab-experiments include the followings: (1) fracture permeability of granite increases due to the hydraulic shear slip even at an effective normal stress of over 50 MPa, (2) shear slip and stress drop are in proportion and increment of the fracture permeability increases with increasing total shear slip displacement, and (3) although hydraulic shear slip tends to make fracture surfaces slightly smoother, the factual characteristics of surface are maintained after slip. By combining our experimental results with the seismological analysis, we first explore energy balance during hydraulic shearing of the preexisting rock fracture and point out the crucial role of the elastic potential energy stored in the surrounding bulk rock masses. Subsequently, we derive a constitutive model related to permeability change of granite fracture during hydraulic shearing under the typical crustal stress of EGS and presume that the maximum change in fracture permeability due to shear dilation is ∼20-fold, although scale effect is not considered. Finally, via the high resolution 2-D mapping of the surface damages, we illuminate the formation of preferential flow paths within rock fracture during hydraulic shearing and its possible role on the improvement of fracture permeability. Thus, we successfully demonstrate both new and sophisticated insights in hydro-mechanical coupled processes during hydraulic shearing to improve the accuracy of fracture network designs in EGS technology.

Evolution of Water Retention Characteristics in Bio-Geochemically Altered Unsaturated Soils: A Pore-Scale Study

Biogeochemical processes in subsurface can dramatically alter the behavior of multiphase fluid flow and the hydrodynamics of porous media by bio-clogging. There exist analytical solutions such as soilsuction – saturation equations which can be used to predict the water retention curve relations in unsaturated soils. However, due to the complexity of various biogeochemical products, their pore-scale behavior and their interplay with pore structure, such analytical solutions would not provide accurate predictions. In this study, a large database of pore-networks were generated by adjusting the statistical and spatial pore and tube size distribution of the networks resembling various levels of bio-clogging. Numerical simulations including the evolution of pore structure, water retention relationship, and air invasion dynamics during desaturationwere explored. The numerical simulations verified that local pore-clogging leads to the development of isolated pore clusters and impermeable zones. The evolution of impermeable zones results in the formation of preferential flow paths towards the mobile zones for the multiphase flow problems. Using parallel computation, the critical predictors of water retention curves in bio-clogged porous media are found.

Visualizing and evaluating wormholes formation dynamics under flow competition in an intermediate-scale dissolution experiment

Wormholes are highly conductive channels that develop in high solubility rocks. They are especially important for environmental and industrial sustainability in saline karst aquifers (e.g. Salar de Uyuni, Salar de Atacama). Wormholes dynamics (i.e., the space and time evolution of these preferential flow paths) depends on the hydrodynamic and geochemical conditions during formation, as well as on wormholes competition for flow. Despite the importance of wormholes interaction for their development, experimental efforts have focused on the evolution of a single flow-path. Direct observation and quantification of wormholes dynamics is still lacking. We propose an experimental set-up to visualize and characterize the dynamics of multiple wormholes, which may help to understand the changes in flow and transport behaviour of aquifers. We performed a dissolution experiment in a 2D synthetic evaporitic aquifer, and simultaneous fluorescent tracer tests before and during wormhole growth. We visualized the growth by sequential photographs, with the fluorescent tracer highlighting the evolving structures. We quantified wormholes dynamics by measuring pressure and mass changes of the aquifer, and by image analysis. On the one hand, results show that wormholes tend to form along areas where flux was fastest prior to dissolution. They also show clear evidence of competition for water between wormholes and represent the first quantitative evidence of the amplifying factor that drives the self-organization in wormhole growth. On the other hand, we found that the competition is slower than predicted by current analytical and numerical theories, but consistent with other laboratory results. We conjecture that the discrepancy between theory and experiments can be attributed to the combined effect of non-linear kinetics and particle dragging during dissolution. We then compared experimental tracer breakthrough curves before and during the formation of preferential flow paths. They reflect wormhole growth by an accentuated non-Fickian behaviour, with reduced first arrival and increased tailing.

Modeling the hydrodynamic behavior of fractures and barriers in porous media using coupling finite elements

Fractures, vugs and barriers are geological structures that strongly affect the fluid flow in porous media. While the first two lead to the formation of preferential flow paths, the latter blocks the flow and induces discontinuities in the pressure field. The difference in scale between these structures and the porous medium can be significant and, therefore, appropriate numerical models are required to take into account their interactions. In the context of the finite element method, this work presents a methodology to couple equi-dimensional and independent meshes (i.e., non-matching meshes) of geological structures and the porous medium by using coupling finite elements (CFEs), which are able to connect the meshes via a penalty method that ensures the continuity of the pressure field. This technique is easy to implement and does not require any special formulation, since the shape functions of the CFE are the ones used in conventional finite elements. Moreover, the addition of the CFE does not increase the number of degrees of freedom of the problem. In order to show that the technique has great potential to capture the main phenomena related to the fluid flow problem in a porous medium, 2D single-phase flow numerical experiments were conducted by considering different geological structures, geometries and boundary conditions, with incompressible fluid. The methodology was validated by comparing the results obtained with several DFM methods available in the literature as well as considering matching meshes between matrix and fracture. The results showed that the proposed coupling scheme applied to equi-dimensional meshes is adequate to reproduce the influence of geological formations such as fractures, vugs and barriers on the hydrodynamic behavior of fluids in porous media.

Fluid Flow Concentration on Preferential Paths in Heterogeneous Porous Media: Application of Graph Theory

AbstractFluid flow through geological formations is often concentrated on distinct preferential flow paths owing to the presence of fractures or large‐scale permeability structures. However, the existence of such structures is not a mandatory condition of preferential paths formation. Pore‐scale spatial fluctuations of pore size and/or pore connectivity in statistically stationary porous media, if sufficiently large, can also lead to the concentration of fluid flow on distinct pathways. In this paper, we attempted to establish the conditions of formation of preferential flow paths in heterogeneous porous media in terms of pore‐size heterogeneity and pore connectivity. We simulated steady‐state flow through stochastically constructed two‐ and three‐dimensional pore networks, in which the width of the pore radius distribution and the pore coordination number (a measure of pore connectivity) were varied. We developed new techniques based on graph theory to identify potential preferential flow paths and characterize them. We observed a gradual transition from approximately uniform flow fields in low heterogeneity/high connectivity networks to flow localization on preferential paths with increasing pore‐size heterogeneity and decreasing connectivity. The transition occurred at lower heterogeneity levels in three‐dimensional than in two‐dimensional simulations and was less influenced by pore connectivity variations. These results were summarized in a phase diagram in pore‐size heterogeneity/pore connectivity parameter space, which we found consistent with relevant real rocks data.

Transient thermo-solutal convection in a tilted porous enclosure heated from below and salted from above

The confinement of CO2 in deep geothermal reservoirs as a means of mitigation of greenhouse gas emissions is continuously motivating research on the retention capacity of these deep aquifers. An important physical containment mechanism is related with CO2 dissolution and thermo-solutal convection. In this context, numerical simulations are performed in this work to assess the effect of inclination, Rayleigh number, and buoyancy ratio on the convective transport in a rectangular porous medium. The porous enclosure is heated from below and cooled from above, whereas a solute is dissolved through the upper boundary with a constant concentration condition and no mass loss through the other boundaries. A set of governing parameters is considered in this assessment: two buoyancy ratios with dominant solute buoyant forces (10 and 100), three Rayleigh numbers (10, 50, and 80), and three inclination angles plus the horizontal case for reference (5°, 10°, and 15°). The solution to the problem is based on a Finite Volume method along with the fixed point iteration for the coupled differential equations, and a Conjugate Gradient algorithm for the algebraic system. The model is validated and tested under mesh analysis. The numerical results show that the inclination angle has a minor effect on the convective mixing properties of the porous medium in comparison with the Rayleigh number and the buoyancy ratio. Increasing the angle slightly decreases the mixing rate as a consequence of the formation of preferential flow paths associated with the inclination, these preferential flow paths make mixing less efficient and give rise to zonation of solute concentration.

Experimental study of the effect of carbonated brine on wettability and oil displacement for EOR application in the Brazilian Pre-Salt reservoirs

ABSTRACT Brazilian Pre-Salt carbonate reservoirs are natural candidates for Enhanced Oil Recovery combined with Carbon Capture and Storage due to their high CO2 production. However, the effect of CO2 on rock wettability still needs to be investigated to build representative reservoir models to predict the ultimate oil recovery. This work applied contact angle measurements to study the consequences of carbonated brine on the initial wettability. Moreover, coreflooding experiments were carried out to evaluate the incremental oil recovery obtained by carbonated brine injection. Finally, nonreactive tracer tests and micro computed tomography were used to observe the effects of mineral dissolution on the sweeping efficiency. Carbonated brine induced a decreased oil-wet behavior caused by rock dissolution and desorption of organic compounds resulting in a contact angle reduction of 55° for the Pre-Salt rock. This contributed to obtain a 27% increase in oil recovery. Moreover, tracer tests and micro computed tomography verified the formation of preferential flow paths by mineral dissolution. The increased oil production and decreased contact angle encourage further studies on the injection of carbonated brine for the Pre-Salt scenario.

Analogue Fracture Experiments and Analytical Modeling of Unsaturated Percolation Dynamics in Fracture Cascades

Core Ideas Mass redistribution at unsaturated fracture intersections depends on free‐surface flow modes. A Washburn‐type flow regime is recovered via an analytical solution for capillary fracture inflow. A Gaussian transfer function predicts mass partitioning for arbitrary‐sized fracture cascades. Infiltration and recharge dynamics in fractured aquifer systems often strongly deviate from diffuse Darcy–Buckingham type flows due to the existence of a complex gravity‐driven flow component along fractures, fracture networks, and fault zones. The formation of preferential flow paths in the unsaturated or vadose zone can trigger rapid mass fluxes, which are difficult to recover by volume‐effective modeling approaches (e.g., the Richards equation) due to the nonlinear nature of free‐surface flows and mass partitioning processes at unsaturated fracture intersections. In this study, well‐controlled laboratory experiments enabled the isolation of single aspects of the mass redistribution process that ultimately affect travel time distributions across scales. We used custom‐made acrylic cubes (20 by 20 by 20 cm) in analog percolation experiments to create simple wide‐aperture fracture networks intersected by one or multiple horizontal fractures. A high‐precision multichannel dispenser produced gravity‐driven free‐surface flow (droplets or rivulets) at flow rates ranging from 1 to 5 mL min−1. Total inflow rates were kept constant while the fluid was injected via 15 (droplet flow) or three inlets (rivulet flow) to reduce the impact of erratic flow dynamics. Normalized fracture inflow rates were calculated and compared for aperture widths of 1 and 2.5 mm. A higher efficiency in filling an unsaturated fracture by rivulet flow observed in former studies was confirmed. The onset of a capillary‐driven Washburn‐type flow was determined and recovered by an analytical solution. To upscale the dynamics and enable the prediction of mass partitioning for arbitrary‐sized fracture cascades, a Gaussian transfer function was derived that reproduces the repetitive filling of fractures, where rivulet flow is the prevailing regime. Results show good agreement with experimental data for all tested aperture widths.

Quantitative analysis of wetting front instabilities in soil caused by treated waste water irrigation

Irrigation with treated waste water (TWW) is a common practice in agriculture, mainly in arid and semiarid areas as it provides a sustainable water resource available at all-season in general and at freshwater shortage in particular. However, TWW still contains abundant organic material which is known to decrease soil wettability, which in turn may promote flow instabilities that lead to the formation of preferential flow paths. We investigate the impact of long-term TWW irrigation on water wettability and infiltration into undisturbed soil cores from two commercially used orchards in Israel. Changes of water content during infiltration were quantitatively analysed by X-ray radiography. One orchard (sandy clay loam) had been irrigated with TWW for more than thirty years. In the other orchard (loamy sand) irrigation had been changed from freshwater to TWW in 2008 and switched back in some experimental plots to freshwater in 2012. Undisturbed soil cores were taken at the end of the dry and the rainy season to investigate the seasonal effect on water repellency and on infiltration dynamics in the laboratory. The irrigation experiments were done on field moist samples. A test series with different initial water contents was run to detect the influence on water movement at different wettabilities. In this study we show that the infiltration front stability is dependent on the history of waste water irrigation at the respective site and on the initial water content.

Modelling capillary hysteresis effects on preferential flow through melting and cold layered snowpacks

Accurate estimation of the amount and timing of water flux through melting snowpacks is important for runoff prediction in cold regions. Most existing snowmelt models only account for one-dimensional matrix flow and neglect to simulate the formation of preferential flow paths. Consideration of lateral and preferential flows has proven critical to improve the performance of soil and groundwater porous media flow models. A two-dimensional physically-based snowpack model that simulates snowmelt, refreezing of meltwater, heat and water flows, and preferential flow paths is presented. The model assumes thermal equilibrium between solid and liquid phases and uses recent snow physics advances to estimate snowpack hydraulic and thermal properties. For the first time, capillary hysteresis is accounted in a snowmelt model. A finite volume method is applied to solve for the 2D coupled heat and mass transfer equations. The model with capillary hysteresis provided better simulations of water suction at the wet to dry snow interface in a wetting snow sample than did a model that only accounted for the boundary drying curve. Capillary hysteresis also improved simulations of preferential flow path dynamics and the snowpack discharge hydrograph. Simulating preferential flow in a subfreezing snowpack allowed the model to generate ice layers, and increased the vertical exchange of energy, thus modelling a faster warming of the snowpack than would be possible without preferential flow. The model is thus capable of simulating many attributes of heterogeneous natural melting snowpacks. These features not only qualitatively improve water flow simulations, but improve the understanding of snowmelt flow processes for both level and sloping terrain, and illuminate how uncertainty in snowmelt-derived runoff calculations might be reduced through the inclusion of more realistic preferential flow through snowpacks.

Effects of olive mill wastewater disposal on soil: Interaction mechanisms during different seasons

Abstract Environmental conditions play a major role for effects of olive mill wastewater (OMW) application to soil. Choosing a different season for OMW application than the commonly practiced winter, may help avoid negative effects. However, understanding of the OMW-soil interaction during different seasons is still incomplete due to the lack of comparative data. In this study, an 18 months field experiment was carried out in an olive orchard in West Bank. Degree and persistence of soil salinization, acidification, accumulation of phenolic compounds and soil water repellency were investigated as a function of soil depth and time elapsed after OMW application, which was performed either in spring, summer (with and without irrigation) or winter. The persistence of negative effects increased with duration of the hot and dry period following the application due to accumulation and polymerization of OMW. On the other hand, leaching of OMW components to groundwater is favored during the rainy season and by formation of preferential flow paths before the rain season starts. The risks of groundwater contamination and persistent negative effects decrease with increasing time under conditions favoring biological activity. Therefore, OMW application in spring if improved by a careful irrigation is considered as the most suitable under semiarid conditions for clay loam soils.

Dynamic Modeling of Channel Formation During Fluid Injection Into Unconsolidated Formations

SummaryInjectivity loss is a common problem in unconsolidated-sand formations. Injection of water into a poorly cemented granular medium may lead to internal erosion, and consequently formation of preferential flow paths within the medium because of channelization. Channelization in the porous medium might occur when fluid-induced stresses become locally larger than a critical threshold and small grains are dislodged and carried away; hence, porosity and permeability of the medium will evolve along the induced flow paths. Vice versa, flowback during shut-in might carry particles back to the well and cause sand accumulation inside the well, and subsequently loss of injectivity. In most cases, to maintain the injection rate, operators will increase injection pressure and pumping power. The increased injection pressure results in stress changes and possibly further changes in channel patterns around the wellbore. Experimental laboratory studies have confirmed the presence of the transition from uniform Darcy flow to a fingered-pattern flow. To predict these phenomena, a model is needed to fill this gap by predicting the formation of preferential flow paths and their evolution. A model based on the multiphase-volume-fraction concept is used to decompose porosity into mobile and immobile porosities where phases may change spatially, evolve over time, and lead to development of erosional channels depending on injection rates, viscosity, and rock properties. This model will account for both particle release and suspension deposition. By use of this model, a methodology is proposed to derive model parameters from routine injection tests by inverse analysis. The proposed model presents the characteristic behavior of unconsolidated formation during fluid injection and the possible effect of injection parameters on downhole-permeability evolution.

Dye tracer visualization of flow patterns and pathways in glacial sandy till at a boreal forest hillslope

Dyes are valuable tracers in visualizing flow patterns and pathways in soil. We applied the dye Acid Blue 9 to unsaturated and saturated soil profiles at a boreal forest hillslope consisting of glacial sandy till, and determined the soil physical properties from soil samples. The objective was to characterize preferential flowpaths, investigate their porosity, extent and connectivity, and complement earlier findings on subsurface flow formation at the site. According to the results, preferential flowpaths were formed by roots, erosion related to soil water flow, freezing–thawing cycles, and soil fauna. The role of roots and stones in the formation of preferential flowpaths was emphasized. Porosity of preferential flowpaths was 5.1±1.8%, and they extended to a depth of about 55cm from the soil surface; the deepest roots reached the same depth. When the soil saturated, individual preferential flowpaths self-organized into continuous and well-connected lateral flow systems along the slope. At the slope shoulder, preferential flow network covering the entire soil profile, as well as preferential flowpaths on the underlying bedrock surface, were considered crucial for runoff generation. In the midslope area, runoff generation was characterized by lateral preferential flow and the transmissivity feedback phenomenon. At the slope foot, preferential flowpaths in soil below the eluvial horizon were the major runoff contributors. A full saturation of the soil profile at the slope is unlikely under natural conditions. However, lateral flow was found to occur also in unsaturated soil.

Green Roofs in the Mediterranean Area: Interaction between Native Plant Species and Sub-Surface Runoff

Green roofs (GRs) may represent a solution for minimizing the impact of urbanization on the hydrological cycle and for sustainably managing water resources in urban environment. Several studies have shown that GRs effectively control the surface runoff conveyed into the combined sewer systems, by reducing overall volumes and peak flow rates. The hydrological/hydraulic performance of a GR is strongly dependent upon vegetation species used. In this study the influence of plants on the sub-surface runoff is investigated by monitoring two experimental GRs in wet weather conditions. It is hypothesized that although plants reduce sub-surface runoff, they may also boost the formation of preferential flow paths. The results showed that the sub-surface runoff is strongly influenced by vegetation. While for smaller storm events the GR highly retains stormwater due to the plant interception, for larger storm events the GR’s performance decreases due to the increase of sub-surface runoff. This may be due to the presence of the plant root which produces preferential macropores flow through the substrate.

Challenges in modeling unstable two‐phase flow experiments in porous micromodels

AbstractThe simulation of unstable invasion patterns in porous media flow is very challenging because small perturbations are amplified, so that slight differences in geometry or initial conditions result in significantly different invasion structures at later times. We present a detailed comparison of pore‐scale simulations and experiments for unstable primary drainage in porous micromodels. The porous media consist of Hele‐Shaw cells containing cylindrical obstacles. By means of soft lithography, we have constructed two experimental flow cells, with different degrees of heterogeneity in the grain size distribution. As the defending (wetting) fluid is the most viscous, the interface is destabilized by viscous forces, which promote the formation of preferential flow paths in the form of a branched finger structure. We model the experiments by solving the Navier‐Stokes equations for mass and momentum conservation in the discretized pore space and employ the Volume of Fluid (VOF) method to track the evolution of the interface. We test different numerical models (a 2‐D vertical integrated model and a full‐3‐D model) and different initial conditions, studying their impact on the simulated spatial distributions of the fluid phases. To assess the ability of the numerical model to reproduce unstable displacement, we compare several statistical and deterministic indicators. We demonstrate the impact of three main sources of error: (i) the uncertainty on the pore space geometry, (ii) the fact that the initial phase configuration cannot be known with an arbitrarily small accuracy, and (iii) three‐dimensional effects. Although the unstable nature of the flow regime leads to different invasion structures due to small discrepancies between the experimental setup and the numerical model, a pore‐by‐pore comparison shows an overall satisfactory match between simulations and experiments. Moreover, all statistical indicators used to characterize the invasion structures are in excellent agreement. This validates the modeling approach, which can be used to complement experimental observations with information about quantities that are difficult or impossible to measure, such as the pressure and velocity fields in the two fluid phases.

Processes affecting the efficiency of limestone in passive treatments for AMD: Column experiments

Experiments using columns packed with calcite grains (size 1–2mm) were carried out to study the behavior of passive treatment systems designed to remediate contaminated water from acid mine drainage (AMD). Two types of synthetic acidic solutions (H2SO4) were injected in the columns: (1) iron-rich with Fe(III) at pH 2, and (2) aluminum-rich at pH 2 and 3. Fluid flow was constant during the experiments with Darcy velocities ranging from 6×10−4 to 1×10−3Lm−2s−1. The columns worked efficiently removing aqueous iron and aluminum as long as calcite dissolved and buffered the solution pH (increasing pH and promoting the precipitation of Fe(III)- or Al-oxyhydroxides). However, Ca released from dissolving calcite, along with the sulfate in solution, led to formation of gypsum coatings on the calcite grain surfaces which eventually prevented calcite dissolution. This passivation process limited the efficiency of the columns. Larger input sulfate concentrations or higher pH led to shorter passivation times. Characterization of the pore structure and composition by X-ray microtomography (mCT) and X-ray microdiffraction (mXRD) showed the precipitation of gypsum coatings on the calcite grains and secondary oxyhydroxides between the grains. This secondary mineral precipitation favored the formation of preferential flow paths, isolating regions of non-reacted limestone. An improved experimental design (mixing limestone grains and glass beads) minimized the formation of these preferential flow paths. Experimental results have been modeled with the CrunchFlow reactive transport code. Fitting of the results required a decrease in the reactive surface area of calcite, which is consistent with the passivation process.

Evidence of preferential path formation and path memory effect during successive infiltration and drainage cycles in uniform sand columns

The formation of preferential flow paths in the partially saturated zone, and in naturally structured media, is well known. This study examines non-uniform flow in uniform sand columns under different pressure and infiltration/drainage conditions. Experiments were carried out in a vacuum box, with applied suction set to three different heads, and with infiltration fixed at two different flow rates. Tailing observed in some conservative tracer breakthrough curves suggests the formation of immobile resident water pockets which slowly exchange mass with the flowing water fraction. The applied suction controlled the degree of water immobilization whereas flow rate had minimal effect on the dynamic behavior. Trapping and exchange of water occurred repeatedly during successive infiltration and drainage cycles, implying a (hysteretic) memory effect of the previously formed preferential flow paths. Flow and solute transport modeling suggests that these dynamics can be described by a mobile–immobile model that corroborates measurements suggesting preferential flow path formation. These findings have implications for the natural attenuation of contaminants in the partially saturated zone, but also for the persistence of a contamination source exposed to repeated conditions of infiltration and drainage.

Impact of treated wastewater irrigation on water repellency of Mediterranean soils

Irrigation with treated wastewater (TWW) is gaining importance due to declining water availability in dry regions. TWW irrigation has various potential adverse effects on soil quality such as hydrophobic effects on soil surfaces, reducing initial sorptivity and promoting the formation of preferential flow paths. In May and June 2010, in situ infiltration measurements using mini disk tension infiltrometer were deployed in five different orchard plots in Israel to assess the impact of different irrigation water qualities on the soil water repellency index R. In most plantations, long-term test sites were accessed to compare adjacent plots irrigated with fresh water (FW) or TWW. Topsoil samples were analyzed for selected physical and chemical characteristics. The mean R values increased at all TWW sites, from +15 up to +55 % compared with FW sites. The water drop penetration time (WDPT) increased up to 30 fold at three of five TWW sites compared with FW sites. Subsequent U tests and multilevel analysis indicated an impact of the type of irrigation water on R and WDPT. Moreover, soil electrical conductivity and exchangeable sodium percentage were consistently higher at all TWW sites. These results show that irrigation water quality clearly influences physical and chemical properties of the soil.

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.