Preferential Flow Paths Research Articles

Experimental Investigation of Two-Phase Flow Properties of Heterogeneous Rocks Based on X-Ray Microfocus Radiography

Summary An uncommon facet of formation evaluation is the assessment of flow-related in-situ properties of rocks. Most of the models used to describe two-phase flow properties of porous rocks assume homogeneous and/or isotropic media, which is hardly the case with actual reservoir rocks, regardless of scale; carbonates and grain-laminated sandstones are but two common examples of this situation. The degree of spatial complexity of rocks and its effect on the mobility of hydrocarbons are of paramount importance for the description of multiphase fluid flow in most contemporary reservoirs. There is thus a need for experimental and numerical methods that integrate all salient details about fluid-fluid and rock-fluid interactions. Such hybrid, laboratory-simulation projects are necessary to develop realistic models of fractional flow in complex rocks, i.e., saturation-dependent capillary pressure and relative permeability. Furthermore, these two crucial properties are usually measured independently. Capillary pressure is typically assessed using static measurements and unrealistic pressure conditions, whereas relative permeability is evaluated dynamically. Consequently, the disparity between the nature of the two experimental procedures often results in a potentially significant loss of information. We document a new high-resolution visualization technique that provides experimental insight to quantify fluid saturation patterns in heterogeneous rocks which allow for the simultaneous and dynamic evaluation of two-phase flow properties. The experimental apparatus consists of an X-ray microfocus scanner and an automated syringe pump. Rather than using traditional cylindrical cores, thin rectangular rock samples are examined, their thickness being one order of magnitude smaller than the remaining two dimensions. During the experiment, the core is scanned quasicontinuously while the fluids are being injected, allowing for time-lapse visualization of the flood front. Numerical simulations are then conducted to match the experimental data and quantify effective saturation-dependent relative permeability and capillary pressure. The experimental results indicate that flow patterns and in-situ saturations are highly dependent on the nature of the heterogeneity and bedding-plane orientation during both imbibition and drainage cycles. In homogeneous rocks, fluid displacement approaches piston-like behavior. The assessment of capillary pressure and relative permeability is performed by examining the time-lapse water saturation profiles resulting from fluid displacement. In spatially complex rocks, high-resolution time-lapse images reveal preferential flow paths along high-permeability sections and a lowered sweep efficiency. Our experimental procedure emphasizes that capillary pressure and transmissibility differences play an important role in fluid-saturation distribution and sweep efficiency at late times. The method is fast and reliable to assess mixing laws for fluid-transport properties of rocks in spatially complex formations.

Road salt intrusion dynamics in an ex-urban native wetland complex

Inland freshwater wetlands throughout the northern U.S. and Canada are experiencing an increase in salinity due to road salt runoff during winter months. Salinization affects soil texture, contaminant transport, microbial activity, and plant growth in wetlands. Therefore, there is a pressing need to understand the dynamics of road salt intrusion in urbanized freshwater ecosystems. We used distributed high-resolution sensors to evaluate the dynamics of road salt intrusion into a wetland complex, Gensburg Markham Prairie (GMP), located in the ex-urban area outside Chicago, Illinois (USA). The in situ sensors measure electrical conductivity (EC), surface and groundwater level, precipitation, water temperature, and air temperature at 30-minute intervals. Water samples were collected monthly from 13 shallow groundwater wells and eight surface water locations and analyzed for Cl-, Mg2+, Na+, Ca2+, and K+. Two-years of continuous data show periodic spikes in EC during winter months, generally by an order-of-magnitude, due to intrusion of road salt applied on nearby roads. However, this behavior was not evident from monthly water samples, indicating that traditional water quality sampling methods likely miss such abrupt salt intrusion dynamics caused by rapid snowmelt runoff events. Higher levels of EC and Cl- occurred at the periphery of GMP near roadways, as well as in a preferential flow path to the interior of the wetland. Spectral analysis of EC time-series in ditches suggests that there is no correlation between salinity dynamics at super-annual timescales. This indicates that the salinity dynamics at GMP are event-driven, and the introduced solutes are rapidly exported from the site. This research supports development of improved de-icing strategies by local agencies and informs site-specific management of wetland ecosystems under anthropogenic stressors.

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.

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.

Modeling rainfall-driven transport of Glyphosate in the vadose zone of two experimental sites in North-East Italy

A vertical one-dimensional analysis of infiltration processes and mobility of a tracer (potassium bromide) and a glyphosate-based herbicide, both subjected to hydrological forcing, was performed. Glyphosate is a widespread herbicide whose potential harmfulness and mobility under hydrological forcing have not been fully understood yet. Here, the spatio-temporal evolution of the two compounds was monitored for one year in two experimental sites (Settolo - Valdobbiadene, Colnù - Conegliano), located within the production area of the Prosecco wine (Treviso, Italy). In each experimental site the activities were carried out on two 25 m2 plots located at distances of 50-100 m from each other. The interpretative analyses considered rainwater infiltration as the driving mechanism of the herbicide transport and allowed us to obtain the calibration of a one-dimensional hydrologic model in each monitored plot. Different scenarios of the tracer evolution were simulated considering the pedologic properties of the shallower soil layers, the status of the plant coverage and of the root apparati, leading to a satisfactory reproduction of the observations in both the experimental sites. Modeling the mobility of the herbicide, considering also the degradation to its metabolite AMPA, proved to be more challenging, due to the tendency of glyphosate to be adsorbed to the soil matrix rather than be dissolved in water and transported toward deeper soil layers. Nevertheless, the analysis of model results for tracer and herbicide, compared with in situ observations, suggests that the transport of the glyphosate can take place even when it is adsorbed to the soil, through the movement, triggered by intense precipitation events, of microscopic soil particles within preferential flow paths.

Modelling of gas-solid heat transfer and pressure drop in a rock-packed bed using pore-scale simulations

The aim of this work is to provide closure laws for a macroscopic model of the heat and momentum transfer of real rock packings. We use X-ray nanotomography to obtain the geometry of two real rock packings (basalt and gravel). To address preferential flow paths on non-periodic REV, we have developed a method to properly extract closure laws on non-periodic REV from thermally fully developed pore-scale flow simulations. Several pore-scale simulations are performed over a range of Reynolds numbers (5–243) with a Prandtl number of 0.7. Then there are proposals for closure laws for real rocks, which are of completely arbitrary shape. These laws are based on a Nusselt number correlation and a dimensionless drag force correlation extracted from the pore scale simulations. The Nusselt number correlation is an extension for arbitrary shapes, i.e. real rocks, of the one proposed by Sun et al. (2015). The dimensionless drag force correlation is an original expression in excellent agreement with the experimental datas of Allen et al. (2013). The results support the hypothesis that the mean Sauter diameter is a key parameter that captures the similar behaviour of real rock packings, either for heat or momentum transfer. As this parameter cannot be replaced by other characteristic lengths in the closure laws, it needs to be properly evaluated, for example using nanotomography. For optimising the design of real rock-packed beds, the proposed closure laws allow accurate upscaling of heat transfer and pressure losses.

Potential groundwater contamination from oil drilling in the Okavango

Canadian oil and gas company, Reconnaissance Energy Africa (ReconAfrica), plans to exploit potential oil reserves in the Cubango Okavango River Basin (CORB) in Namibia and Botswana, where little is known about the local groundwater systems and how contamination and its effects could impact the region. Using borehole data from target oil and gas areas, we calculated hydraulic gradients, flow direction, and flow velocity to map potential groundwater contamination effects from oil and gas drilling in the region. We also plotted the major geological structures, calculated flowpaths, and estimated travel times for contaminated groundwater that may travel along these preferential flowpaths from the drill sites to the Okavango River and Delta. Results indicate that contaminated groundwater from the oil lease areas could take 3–23.5 years to reach the Okavango River system via the shallow sandy aquifer, but in a worst-case scenario, contamination could reach the Delta within four days via structures associated with dykes and faults that serve as primary flowpaths. Such contamination could adversely affect human health and the region's ecosystems and biodiversity. We recommend prohibiting oil exploration and production activities within the CORB until future studies can determine the impacts of hydrocarbon extraction with greater certainty.

Preferential Flow and Preferential Path Characteristics of the Typical Forests in the Karst Region of Southwest China

The ecological environment in the karst region of southwest China is fragile, and vegetation degradation is considered the main cause of this phenomenon. Vegetation growth is affected by preferential flow due to a large number of fissures and macropores in the soil. Therefore, the preferential flow and preferential paths in local typical vegetation need to be studied. The distribution and developmental characteristics of preferential flow and preferential paths in three typical local stands were obtained by conducting field staining tracer experiments and using image processing techniques. The results showed that there was preferential flow movement in the soil moisture of the local understorey and that there was a high degree of preferential flow development and high variability. The degree of preferential flow in several typical forests can be expressed, from highest to lowest, in the following order: secondary forests, Cupressus duclouxiana Hickel (Hickel., 1814) forest, and Pinus massoniana (Lamb., 1803) forest. The preferential paths in the soil were mainly distributed above 15 cm, and the number of preferential paths with a small influence radius was much greater than that with a large influence radius. The preferential paths with an influence radius of 2.5~5 mm accounted for the largest proportion. The total number of preferential paths in the P. massoniana forest was small. The preferential paths in the secondary and C. duclouxiana Hickel forests were similar in number, but the secondary forests had more preferential paths with a large influence radius. This study investigated the characteristics of preferential flow and preferential paths in the typical forests of the karst region in southwest China, and it can provide data and research directions for the study of hydrological processes in local forest vegetation.

Land use effects on surface runoff and soil erosion in a southern Alpine valley

In mountain regions, soil landscapes are highly vulnerable against soil loss. Moreover, these environments are particularly affected by land use changes, which influence soil properties and related processes like surface runoff generation and soil erosion. These processes are in turn amplified by extreme climatic events and intensive geomorphological dynamics. The objective of this study is to quantitatively assess the effects of land use changes on surface runoff and soil erosion in a southern Alpine valley (Onsernone valley, Switzerland) characterized by a former intense land use followed by a progressive abandonment in the last decades. Surface runoff and related sediment transport has been analysed under controlled and reproducible conditions using a portable rainfall simulator device (1 m2). The results show a statistically significant increase in surface runoff when the soil gets water repellent reducing the surface infiltration capacity and generating preferential flow paths, which prevent a homogeneous wetting of the soil. However, the documented high sensitivity of surface runoff to land use changes does not result in an equally high sensitivity to soil erosion processes. Instead, soils display a high aggregate stability leading to very low sediment transports except for abandoned and reforested agricultural terraces. There, the lack of maintenance and progressive collapse of terrace dry walls locally increase slope angles and directly exposes the soil to atmospheric agents and surface runoff, which causes soil erosion rates beyond the customary natural level.

Influence of green roof plant density and redirecting rainfall via runoff zones on rainfall retention and plant drought stress

Green roofs are a promising engineered ecosystem designed to reduce stormwater runoff and restore vegetation cover in cities. Plants can contribute to rainfall retention by rapidly depleting water in the substrate, however, this increases the risk of plant drought stress. This study determined whether lower plant density or preferentially redirecting rainfall to plants on green roofs could reduce drought stress without reducing rainfall retention. Plant density was manipulated, and metal structures were installed above the substrate surfaces to redirect the flow of rainwater towards plants (runoff zones). Green roof modules were used to test three plant density treatments: unplanted, half-planted (10 plants/m2) and fully-planted (18 plants/m2), and two runoff zone treatments which were installed in unplanted and half-planted modules. It was expected that 1) green roofs with greater plant density would experience more drought stress (i.e., lower leaf water status), and 2) green roofs with runoff zones would show higher ET and hence retention compared with those without runoff zones, as water will be directed to plants (run-on zones), facilitating growth. Contrary to the hypothesis, evapotranspiration (ET) and rainfall retention were similar for half-planted and fully-planted modules, such that ∼82 % of applied rainfall was retained. While both vegetation treatments dried out the substrates before rainfall was applied, the fully-planted modules dried out quicker and showed significantly lower leaf water status than half-planted modules. This indicates that planting at lower density may reduce plant drought stress, without reducing rainfall retention. Installing runoff zones marginally reduced ET and rainfall retention, likely due to shading by the runoff zone structures reducing evaporation from the substrate. However, runoff also occurred earlier where runoff zones were installed as they likely created preferential flow paths that reduced soil moisture and therefore ET and retention. Despite reduced rainfall retention, plants in modules with runoff zones showed significantly higher leaf water status. Reducing plant density therefore represents a simple means of reducing plant stress on green roofs without reducing rainfall retention. Installing runoff zones on green roofs is a novel approach that could reduce plant drought stress, particularly in hot and dry climates, albeit at a small cost of reduced rainfall retention.

Channel Network Modeling of Flow and Transport in Fractured Rock at the Äspö HRL: Data‐Worth Analysis for Model Development, Calibration and Prediction

AbstractPerformance assessment of nuclear waste disposal in deep crystalline bedrock demands a thorough understanding of the related flow and transport processes. Uncertainties may arise both from the selection of the conceptual model as well as the estimation of the related model parameters. Discrete fracture network (DFN) models are widely used for such modeling while channel network models (CNM) provide an alternative representation, the latter focusing on the fact that flow and transport in deep fractured media often are dominated by a small number of long preferential flow paths. This study applies the principle of channel networks, implemented in the Pychan3d simulator, to analyze the hydraulic and tracer transport behavior in a 450‐m‐deep fractured granite system at the Äspö Hard Rock Laboratory in Sweden, where extensive site characterization data, including hydraulic and tracer test data are available. Semi‐automated calibration of channel conductances to field characterization data (flow rates, drawdowns, and tracer recoveries) is performed using PEST algorithm. It was observed that an optimal CNM connectivity map for channel conductance calibration can only be developed by jointly fitting flow rates, drawdowns and tracer mass recovery values. Results from data‐calibrated CNM when compared to a corresponding calibrated DFN model shows that the CNM calibrates and adapts better than a DFN model with uniform fracture surfaces. This comparative study shows the differences and uncertainties between two models as well as examines the implications of using them for long term model predictions.

Three-dimensional characterisation of pore networks and fluid flow in segregated heaps in the presence of crushed ore and agglomerates

Due to the differences in ore feeds and stacking procedures, segregated pore networks are commonly observed in industrial heap leaching operations, which strongly affect the pore-scale fluid flow and the resulting metal leaching efficiency. Three segregated models, namely, the agglomeration model or AM with intra-particle pores, the crushed ore model or COM with inter-particle pores and the mixed segregated model or MSM with both intra- and inter-particle pores, are defined in this work. The segregated pore network in the MSM consists of agglomerates and crushed particles. The models were obtained by performing micro-computed tomography scanning experiments in packed beds followed by image processing algorithms. Several fluid flow-related parameters were quantified to understand the effects of segregated pore networks on fluid flow in heap leaching. The isolated and connected pores co-exist in porous ore heaps, and the latter determines heap hydrodynamics. The number of connected pores in the AM is higher than in the COM. However, the connected pores with too small pore throats are very slow in fluid transfer by capillarity, resulting in stagnant regions in ore beds. A smaller tortuosity value was also observed in the COM, resulting in fast-flowing preferential fluid flow paths. These are unfavourable to yield uniform fluid flow distributions in industrial heaps. The results of this work suggest the prerequisites for improving fluid flow distribution in heaps, including well-organised heap construction procedures, controlling the particle size distribution and large-scale studies coupled with non-invasive imaging and flow simulations.

New Insights from Legacy Seismic Data regarding Basalt Elevations and Variability on the Hanford Site

Abstract Migration of groundwater contaminants in the Gable Gap area of the Hanford Site in southeastern Washington State is strongly influenced by the distribution and permeability of basalts that lie beneath an unconfined aquifer. Locally, folding and faulting of the Columbia River Basalt associated with the Yakima fold and thrust belt followed by erosion due to the Lake Missoula floods resulted in a complex basalt surface that represents either an impermeable lower boundary to the unconfined aquifer system or localized regions of increased permeability that potentially promote communication between the unconfined aquifer system and deeper, confined aquifer systems. Paleo-channels carved into the basalt by floodwaters are thought to provide preferential flow paths for groundwater contaminants. In 2011, a seismic landstreamer campaign was carried out to image the basalt surface and produced pre-stack depth migrated p-wave reflection images. The reflection images identified two large troughs that may represent paleo-channels and several areas of possible faulting. Here, the streamer data are re-analyzed using refraction travel-time and Rayleigh wave dispersion analyses to obtain images of compressional and shear wave velocities within the suprabasalt sediment sections and the upper basalt surface. The combined interpretation of reflection and seismic velocity images shows complexity in the basalt velocity and elevation, which varies by 50 m or more within the study area. These results, along with other ongoing geophysical investigations, will be used to inform the site geologic model and potentially guide placement of future boreholes needed to quantify vertical flow between the confined and unconfined aquifers.

4D interpretation of time-lapse electrical resistivity monitoring data to identify preferential flow path in a landfill, South Korea.

Monitoring the leakage of leachate from a landfill is critical in preventing possible contamination in the surrounding area. Time-lapse (TL) electrical resistivity tomography (ERT) has been performed along eleven survey lines at four different time points in a landfill in Korea. The TL data sets were interpreted using an in-house 4D inversion algorithm. Changes in 4D inversion results were analyzed in order to interpret a leachate-contaminant region. Since the rainy season started during obtaining TL ERT data sets, the effects of precipitation on TL ERT data are also analyzed. Changes in electrical resistivity (ER) showed that precipitation increases ER of contaminant zones. As hydrogeochemical data offer contamination information in some areas where boreholes are located, these are helpful to interpret and compare with ERT inversion results to evaluate the extent of the contaminated plume. We also classified soil textures from particle size analysis on soil samples and analyzed electrical conductivity (EC) and dissolved oxygen (DO) using groundwater samples obtained from observation wells in the survey site. The information on soil structure as well as the results of 4D inversion provided insight into the location of a preferential flow path.

Evaluating subsurface flow connectivity in a pine-covered hillslope with stemflow infiltration and ground-penetrating radar surveys

The hydrological response of sloping catchments is strongly conditioned by the connectivity of subsurface preferential flows. The objective of this paper is to investigate the role played by stemflow infiltration in subsurface water flow dynamics, focusing on a forested hillslope located in an Aleppo pine Mediterranean forest (Pinus halepensis, Mill.) located at Sierra Calderona, Valencia province, Spain. We combined stemflow artificial experiments with the ground-penetrating radar (GPR) as a non-invasive technique to investigate stemflow-induced preferential flow paths activated by different trees and the related hydrological connectivity at the hillslope scale. Our observations allowed us to identify different dynamics associated with the initiation of stemflow and then lateral preferential flow, including the activation of connected preferential flow paths that received stemflow water from different trees. These observations provided empirical evidence of the role of stemflow in the formation of lateral preferential flow networks. Our measurements also provided estimations for flow velocities, which provided new insight on the magnitude of stem-induced lateral preferential flow paths. The applied protocol offers a simple, repeatable and non-invasive way to conceptualize hillslope responses to rainstorms.

Effectiveness of the tropical plants Rhynchospora corymbosa and Coix lacryma-jobi in vertical flow constructed wetlands for municipal primary sewage effluent treatment

The performance of two tropical plants, Rhynchospora corymbosa L. (RC) and Coix lacryma-jobi, L (CL) in treatment of primary sewage effluent in lab-scale vertical-flow constructed wetlands (VFCW) along with no plant control wetland was investigated. A batch-flow VFCWs were operated under batch fill and drain hydraulic loading system with hydraulic retention times (HRT) of 0.5, 1, and 2 days and fill rate of 8 L/day. Removal of solids, organics, nutrients, and pathogens were monitored. The volumetric contaminant removal rates were best described by 1st order kinetics except for ammonia and phosphate, which was best described by Stover-Kincannon kinetics. Influent TSS, PO4 3−, COD, BOD5, and total coliform concentration were low but high in NH4 + concentration. CL was better in nutrient removal as HRT increases compared to RC. RC was more efficient at TSS, turbidity, and organics removal. Pathogen removal was independent of plant type but HRT. Solids and organic removal were lower in CL planted CWs due to preferential flow paths created by their bulky root. CL planted CWs removed more nutrients followed by RC planted CWs and then no-plant control CWs. The results of these tests demonstrate that both CL and RC are suitable for the treatment of municipal wastewater in VFCW system.

In-Depth Performance Analysis and Comparison of Monolithic and Particulate Zwitterionic Hydrophilic Interaction Liquid Chromatography Polymer Columns.

The kinetic performance of different zwitterionic hydrophilic interaction liquid chromatography polymer columns is evaluated and compared in-depth. For this purpose, two lab-made monolithic columns, synthesized with different crosslinkers, and a commercial particle packed column are considered. It is found that performance evaluation techniques, such as comparing plate height curves or fitted A-, B- and C-terms, obtained by fitting experimental plate height data to a plate height model, are complicated by the determination of a reliable characteristic length. This is due to the very different morphology of these column types, and the heterogeneity of the monolithic columns. The occurrence of a convective flow through the packed particle column further complicates the interpretation of the obtained fitting parameters, as part of the C-term is wrongfully attributed to the A-term. Therefore, the use of the kinetic plot method is suggested for the comparative evaluation of these columns, as kinetic plots do not require the determination of a characteristic length, nor rely on any fitting parameters. With the kinetic plot method, it is demonstrated that the lab-made monolithic columns outperform the packed particle column for plate counts between 10,000 and 800,000. This is attributed to the higher column efficiency of these columns, due to their small domain and skeleton sizes, and their high permeability, resulting from their high external porosity and the occasional occurrence of preferential flow paths.

In-situ flow visualization with Geo-Positron-Emission-Tomography in a granite fracture from Soultz-sous-Forêts, France

We investigate the fluid flow field in a fractured granite core sample. Sequential imaging with Positron-Emission-Tomography (PET) allows direct reconstruction of flow streamlines, thus providing a unique insight into the fluid dynamics of complex fractured crystalline materials. Pulse migration experiments using the positron-emitting radionuclide 18F− as tracer were conducted on a fractured granitic drill core, originating from a depth of 1958 m of the Enhanced Geothermal System (EGS) reference site at Soultz-sous-Forêts, France. The flow field was analyzed as a function of in- and outlet positions across the fracture, as well as applied flow rates. Different flow path characteristics were identified. Both the fracture aperture variation and the topography of the fracture surface affect the flow field with consequences on flow channeling and preferential flow paths. Furthermore, pulse migration experiments were also numerically simulated with a 2.5D model using COMSOL Multiphysics®.While the higher flow rate experiments show wider and higher dispersion of the flow path, lower velocity results in more localized flow and channeling behavior. This type of study thus yields enhanced experimental insights into the hydrodynamics of fracture flow and its relation to the rough structure of a single fracture, compared to input-output experiments. It can help to validate model simulations and experimentally determine hydrodynamic parameters needed for reactive transport modeling that are otherwise estimated with a high degree of uncertainty.

Effect of a soluble subgrade on leakage through a geomembrane defect

Leakage and erosion of a soluble subgrade overlain by a geomembrane with a 70-mm-long slit defect is examined. The subgrade (gypsum) solubility and its rate of dissolution from a solid state in contact with various solutions are investigated. In the absence of flow, the rate of dissolution is negligible. However, when there is flow there is dissolution, and if the flow is high enough, erosion. Erosion greatly increases fluid migration. The presence of an interface between the subgrade and a dissimilar material (e.g. a geomembrane) facilitates flow, dissolution, and erosion as the interface becomes an ever increasing preferential flow path, leading to a much greater leakage and erosion feature. The findings from this study highlight the risk of having a soluble subgrade below a single geomembrane used alone and exposed for the containment of liquids.

Impact of high‐permeability preferential flow zones on pumping‐induced airflow in unconfined aquifer

AbstractWhen the unsaturated zone of the unconfined aquifer is covered by a low‐permeability upper layer, significant airflow will be generated in the unsaturated zone during water pumping. However, high permeability preferential flow zones (PFZs) such as fractures and macropores are frequently present in the unsaturated zone, forming the preferential fluid flow paths, which may change the original airflow pattern in the unsaturated zone during the pumping test and consequently affect the precision of obtained aquifer hydraulic parameters. The main objective of this paper is to investigate the effect of PFZs in low‐permeability upper layer on pumping‐induced airflow in the unsaturated zone by numerical simulations of transient three‐dimensional air‐water two‐phase flow and to quantify errors in the aquifer hydraulic parameters obtained during pumping test. The results demonstrate that a large amount of air flows quickly from the atmosphere into the unsaturated zone through the PFZs, and that the PFZs can draw some air from the nearby low‐permeability soils as well. The significant influx of air through PFZs also reduces the negative air pressure in the unsaturated zone and decreases the drawdown in the saturated zone at intermediate times, which are nevertheless still greater than the results obtained in the homogeneous aquifer. Estimations of the aquifer hydraulic parameters reveal that errors of these parameters obtained are smaller when the PFZs with favourable combinations of permeability, width and quantities facilitate more air to flow into the unsaturated zone.