Hydraulic Barrier Research Articles

Permeability Data Treatment from Analogous Outcrop for Use in Three-Dimensional Reservoir Model

Over the past forty years, the study of analogous outcrops has become a common practice in the petroleum industry to access more comprehensive geological datasets and better parameterize various geological features. As most reservoirs do not outcrop and all the data obtained are restricted to those collected from wells located in areas of major interest or from indirect measurements, the study of analogous outcrops has brought important contributions to improve the production routines and still represents a helpful information resource. The use of data from analogous outcrops permits, for example, to estimate the sedimentary facies distribution in true reservoirs, and thus to improve on the reliability of fluid flow modeling during the oil field exploration. However, such data must be processed to become equivalent to the data collected under the reservoir conditions and the results obtained to be more realistic. This work presents the statistical method used to make permeability data obtained from an outcrop compatible with the subsurface conditions of poorly consolidated and tectonically deformed sandstone reservoirs. The studied outcrop is predominantly composed of Eocene fluvial sandstones (Resende Formation) of the Volta Redonda Basin, located in the Central Segment of the Continental Rift of Southeastern Brazil. These deposits are affected by deformation bands and faults. From statistical analysis, it was intended that the permeability data become more suitable to be used, in the next phase of the studies, to build a 3D flow model, in which it will be possible to test the potential of different tectonic structures acting as hydraulic barriers in poorly consolidated and deformed reservoirs. The methodological approach applied in this study made it possible to turn compatible the permeability data obtained in conditions of no confining pressures from the outcrop with reservoir conditions under confinement pressures of 2,000 psi. Such pressure conditions are frequently observed in important Maastrichtian reservoirs found in the Brazilian Southeastern Margin.

Analysis of legacy gravity data reveals sediment‐filled troughs buried under Flathead Valley, Montana, USA

AbstractShallow, dominantly silt‐ and clay‐filled erosional troughs in Quaternary sediments under the Flathead Valley (northwestern Montana, USA) are very likely to be hydraulic barriers limiting the horizontal flow of groundwater. Accurately mapping them is important because of increasing demand for groundwater. We used a legacy Bouguer gravity map measured in 1968. The directional derivatives of the map are computed, and the map was enhanced by implementing edge detection tools. We produced generalized derivative, maximum horizontal gradient, total gradient and tilt gradient maps through two‐dimensional Fourier transform analysis. These maps were remarkably successful in locating buried troughs in the northern and northwestern parts of the study area, closely matching locations determined previously from compiled borehole data. Our results also identify hitherto unknown extensions of troughs and indicate that some of the buried troughs may be connected.

Engineered bioclogging in sands: comparison of microbially induced and enzyme-induced biopolymer formation

This study presents two methods of producing an insoluble biopolymer − microbially induced biopolymer formation (MIBF) and enzyme-induced biopolymer formation (EIBF) − and explores their ability to reduce hydraulic conductivity and cause bioclogging in soil from pore to column scales. The batch experiments confirm that insoluble polysaccharidic biopolymers, dextran, are successfully produced either by the model bacteria or by the extracted cell-free enzyme. The results show that the EIBF method is more efficient in producing biopolymer and reducing hydraulic conductivity compared to the MIBF method. This study also uses microfluidic chips, which reveals the pore-filling behaviour of biopolymers produced by both methods. EIBF produces larger dextran flocs than MIBF, and hence EIBF lowers the hydraulic conductivity more than MIBF for a given pore occupancy of dextran. Column experiments demonstrate that both MIBF and EIBF can significantly lower the hydraulic conductivity of coarse sands by two orders of magnitude with only 3% biopolymer pore saturation. The presented results suggest that both methods have the potential to induce well-controlled, engineered bioclogging in coarse-grained soils, and have applications in various geotechnical practices, such as sealing leakage in water-front structures, installing hydraulic barriers and mitigating soil erosion.

Management of saltwater intrusion using 3D numerical modelling: a first for Pacific Island country of Vanuatu

Small island countries like Vanuatu are facing the brunt of climate change, sea level rise (SLR), tropical cyclones, and limited or declining access to freshwater. The Tagabe coastal aquifer in Port Vila (the capital of Vanuatu) shows the presence of salinity, indicating saltwater intrusion (SWI). This study aims to develop and evaluate effective SWI management strategies for Tagabe coastal aquifer. To manage SWI, the numerical simulation model for the study area was developed using the SEAWAT code. The flow model was developed using MODFLOW and the transport model was developed using MT3DMS. Whereby SEAWAT solved flow and transport equations simultaneously. The model was calibrated, and different scenarios were evaluated for the management of SWI. The SLR was also considered in the model simulations. The results indicated that increased population, pumping rates, and SLR affect the SWI rates. To manage the SWI, we introduced hydraulic barriers like barrier wells and injection wells which effectively managed SWI in Tagabe coastal aquifer. The results from this study are significantly important whereby, the water managers, site owners, and governing bodies can use the management strategies presented in this study to create policies and regulations for managing SWI rates in Port Vila. Additionally, the water industry, private businesses, and investors who wish to extract groundwater from the Tagabe can use this study as a reference for daily or yearly freshwater production rates without the risk of SWI.

Utilizing 3D printing for geomembrane fabrication in laboratory-scale geotechnical testing

This research investigates the innovative application of 3D printing technology in the production of geomembranes for laboratory-scale geotechnical experiments. Commonly, geotechnical research relies on the results of model testing. However, due to the high costs and complexity involved, full-scale model testing is rarely carried out. Geomembranes are essential components in various civil engineering projects, particularly in environmental containment and hydraulic barrier systems. In this research, the potential of 3D printing as an innovative method for producing geomembranes is carried out. Using a 3D printer, this research aims to prepare geosynthetic models with accurate geometric scales and tensile properties suitable for laboratory testing. A comparison was made between HDPE geomembranes and 3D printed geomembranes, to highlight their performance in a geotechnical laboratory environment. The research results show that 3D printed geomembranes show potential for applications in geotechnical research.

Optimization of pumping and injection regimes for mitigation of seawater intrusion

Seawater intrusion (SWI) stands as a significant challenge impacting coastal aquifers worldwide. The emergence of SWI is a natural phenomenon owing to the density difference between seawater and freshwater. Even without any pumping, the seawater naturally advances inland until a hydrostatic equilibrium between the two water sources is reached. However, human interventions, such as excessive groundwater extraction, coastal developments, and land use as well as climate change intensify the SWI predicament. Efficient solutions to counteract the SWI problem involve the implementation of hydraulic barriers. By employing the Henry problem as a benchmark, the Finite Element Simulator (FEFLOW), which is a numerical modeling software, was used for simulating the flow and transport processes. An integrated FEFLOW-Python code was developed to optimize hydraulic barriers for mitigating SWI. The methodology aims to achieve the maximum possible reduction in SWI by iteratively adjusting the locations and rates of pumping and injection wells. Unlike other investigations, this study explores the utilization of negative SWI barriers to extract brackish water from the dispersion zone and potentially offer a cost-effective alternative water source for desalination plants’ intakes. For the Henry Problem, to maximize the reduction in of SWI, brackish water extraction wells should be situated near the center-bottom of the intrusion zone, with a pumping rate of 2.5 m3/day. For injection wells, the optimal location varies depending on the injection rate (I). Achieving the highest retardation (61%) occurs when the injection rate is 1 m3/day, positioned at the aquifer’s bottom and close to the coastal boundary. However, for injection rates below 0.7 m3/day, the injection well’s optimal location shifts towards the seawater intrusion toe. The choice of injection well location is intricately tied to the injection rate, offering the potential for a maximum retardation of 61%. This study presents a novel approach by establishing explicit correlations for optimizing hydraulic barriers to effectively manage and mitigate SWI.

Quantifying short-term changes in snow strength due to increasing liquid water content above hydraulic barriers

The rapid weakening of snow layers that accumulate infiltrating liquid water is a well-known, but poorly quantified, mechanism for wet-snow slab avalanche formation. Therefore, quantifying this mechanical process is a crucial part of forecasting these snow avalanches accurately. Currently, studies do not agree on how snow strength should change as a function of volumetric liquid water content (θ) and whether this relationship differs between snow types. Furthermore, strength measurements taken at or above θ=7% in snow are rare, so there is limited understanding at this end of the θ continuum. These levels of saturation can occur in the snow immediately above hydraulic barriers and are considered important for wet-snow slab avalanches to initiate. To address this knowledge gap, a blade hardness gauge (BHG, a.k.a. thin blade penetrometer) and SLF snow sensor were used to take 349 targeted paired measurements of strength and θ, respectively, in the snow above 19 manually wetted hydraulic (capillary) barriers. Using a multiple regression analysis, we developed an expression for the blade hardness of manually wetted snow as a function of θ, crystal form, and blade hardness prior to wetting (R2=0.85). To understand these results in terms of a common measure of snow strength, we also performed a comparison of the BHG and a shear frame in dry snow. The two instruments are highly linearly correlated (R2=0.94), allowing us to interpret our regression in terms of shear strength when compressive stress is low. Our results demonstrate that short-term (< 2 h) changes in snow strength due to increasing θ can differ between snow layers. These differences can be empirically modelled using easily measured dry snow properties (i.e. crystal form and blade hardness), which could allow avalanche forecasters to be more selective when identifying failure layers in advance of a wetting event.

Enhanced air sparging for groundwater remediation using alginate gel-based removable hydraulic barriers

The objective of this study was to investigate the effect of a removable physical barrier on the air sparging performance using a lab-scale aquifer model was investigated. The barrier was installed in water-saturated porous media, prior to the air sparging, by injecting calcium chloride aqueous solution into the aquifer with pre-applied alginate solution. Changes in the air flow direction and air flux at the media surface during air sparging were evaluated. With a hydrogel barrier set at the center of the media, the airflow detoured the barrier resulting in a bimodal air flux distribution at the media surface. While employing two gel-formed barriers positioned away from the media's center, the airflow concentrated specifically on the gap between the barriers. The hydrogel was successfully removed using a sodium bicarbonate solution (1.0 mol/L). Using the hydrogel barrier, the performance of air sparging was significantly enhanced for removing contaminants [tetrachloroethene (PCE) and n-hexane mixture] due to increased air flux; 9.8% of PCE applied (7.8 g) was removed during 120 min air sprging for the gel barrier system whereas no PCE was removed for the control. Alginate gel did not show significant sorption capacity for PCE. It was stable in the contaminant up to 68 days with reasonable loss of its mass. Findings of this study present a promising option for air sparging process specifically targeting the contaminant source zone in the aquifer.

A simulation-optimization integrated framework for subsurface air barrier strategies to mitigate seawater intrusion in a 3D coastal aquifer

Subsurface air barrier, owing to readily accessible and sufficiently clean of injected air, is a more attractive alternative for mitigating seawater intrusion (SWI) in arid and semi-arid coastal regions, compared to hydraulic barrier. Nevertheless, it remains an enormous challenge to recognize the appropriate layout scheme of air barrier system in a 3D coastal aquifer, including the spatial structure of air-injection zone and the adopted injection pressure, simultaneously considering the environmental and economic performances. This study established a simulation–optimization (S-O) integrated framework for determining the optimal air barrier scheme with minimizing total cost while effectively controlling the extent of SWI. Within the proposed S-O framework, the liquid–gas two-phase flow and solute transport (LGST) numerical model was linked with the micro-population genetic algorithm (mGA). Layout parameters of air barriers, salinity at constraint points and air injection rates were continuously exchanged between the LGST model and mGA at each optimization step. A hypothetical 3D confined aquifer was then employed to evaluate the stability of this S-O framework. The total cost of air barriers increasingly declines to a stable minimum value with the evolutionary processes. With the optimal scheme, there forms a complete and continuous air barrier system between adjacent air-boreholes, giving rise to the intrusive seawater decreasing by 69.7 % and the 0.5 isochlor retreating by 43.4 % at 365.0 day. Furthermore, the 3D aquifer system gradually reaches a quasi-steady state with time and the air injection rate cannot indefinitely increase in response to air injection. The proposed S-O framework can therefore provide a decision support system for designing subsurface air barrier with the least cost on the promise of controlling SWI in 3D coastal aquifers.

Effectiveness of self-sealing after gas transport in Boom Clay

The study of gas transport in low permeable materials is becoming a significant focus in energy-related geotechnics, particularly for managing deep geological disposal of long-lived and heat-emitting radioactive waste. Argillaceous rocks, studied to host the disposal, may present induced fractures caused by the excavation activities that increase the permeability to liquid and gas. The generation of gases can also lead to an excessive pressure build-up in these saturated media resulting in the development or reactivation of fractures/fissures creating preferential pathways for the gas flow [1]. Nevertheless, these rocks present the advantage of self-seal (via swelling of clay minerals due to re-saturation, consolidation or creep), reducing fissure permeability and potentially restoring the barrier function [2].&#x0D; This study focuses on Boom Clay, a Cenozoic clay candidate for hosting the repository in the Belgian programme, aiming at characterising the effectiveness of its self-sealing process after gas injection/dissipation tests due to the swelling of clay minerals during re-saturation. This poorly indurated rock presents sedimentary bedding planes that can act as preferential pathways during the gas invasion. In a previous experimental campaign, samples at two bedding orientations (parallel and orthogonal to the flow) were tested under oedometer conditions [3, 4]. The results indicate that gas transport induced slight expansion of the samples in both orientations, increasing their intrinsic permeability, which suggested the opening of preferential paths. Furthermore, the analyses of the pore network using mercury intrusion porosimetry (MIP) and micro-focus X-ray computed tomography (µ-CT) confirmed the development of gas pathways following the bedding direction or interconnecting bedding planes [4, 5].&#x0D; To assess the self-sealing capacity, oedometer tests were carried out at two bedding directions using an oedometer cell with lateral stress measurement to comprehensively understand the stress state and ensure that gas flow occurred through the sample rather than between the sample-ring interface. Prior to the gas injection stage, water permeability was measured. Subsequently, a gas injection/dissipation stage was performed at constant vertical stress. Immediately after, the sample was placed in contact with synthetic water to allow re-saturation. Finally, the water permeability was measured again. The self-sealing capacity of the clay was assessed by comparing the water permeability at both stages. If the obtained values are similar, the barrier function has been restored. Some tests also included a second gas injection to evaluate the potential pathway reopening. MIP and µ-CT data allowed for evaluating microstructural changes due to these processes.&#x0D; The experimental results demonstrate that gas can flow at pressures lower than the minor lateral stress, and the computed effective gas permeability is always higher than the initial intrinsic water permeability, pointing to the opening of preferential pathways (Figure 1a). Water permeability after re-saturation showed values comparable to the initial intrinsic permeability, recovering the hydraulic barrier function thanks to the clay minerals’ swelling (Figure 1a). This indicates the good self-sealing capacity of the Boom Clay. The gas permeability calculated during a second injection/dissipation stage is slightly higher than the initial one, suggesting some memory of the previously opened path (Figure 1a). The self-sealing effect was also observed in the CT images performed under unstressed conditions after water-undrained unloading of the tested samples. The fissures detected after the gas injection (Figure 1b left) are no longer visible after re-saturation within the technique resolution (fissures &gt; 40µm) (Figure 1b middle). However, a small proportion of large and disconnected pores were identified in CT images, likely due to some gas exsolution. Regarding the subsequent gas injection, it again led to the development of fissures following the bedding direction detected with microstructural techniques (Figure 1b right).

Remediation of chlorinated solvents with Electrical Resistance Heating (ERH) at an active industrial site in Italy

Italian legislation defines stringent groundwater chemical quality criteria, to be applied at a site’s downgradient property boundary, irrespective of whether the underlying aquifer is, or could be, used for water resource purposes. In some scenarios, the regulatory authorities may identify less stringent standards, but this rarely occurs. This means that many sites with groundwater contamination are managed using hydraulic barriers, as source zone remediation may not achieve the stringent groundwater standards required due to technology limits or time constraints; therefore, the parties responsible for contamination often decide to continue to operate these hydraulic barriers indefinitely. This article describes the first application in Italy of source treatment using Electrical Resistance Heating (ERH), a remediation technology capable of removing a large percentage of contaminant mass, at a site where a hydraulic barrier is operating within a low yielding aquifer that is not used for water supply. The implementation of this technology was possible since the source zone was far from the downgradient site boundary, thus making achievement of the stringent quality standards at the boundary possible within a reasonable timeframe. The ERH system recovered of about 600 kg of contaminants within a timeframe of 8 months and achieved a reduction of contaminant concentrations in the most impacted areas greater than 90%. This article also emphasizes that, in similar low yielding aquifers, setting less stringent groundwater standards at the site boundary whilst still protecting downgradient receptors may promote more widespread implementation of source remediation activities in Italy.

Self-Sealing of Boom Clay After Gas Transport

AbstractIn the geological disposal of high-level radioactive waste in argillaceous rocks, studying the barrier integrity after gas transport and the pathway closure thanks to self-sealing capacity is a crucial aspect for the safety assessment. This paper presents experimental research in Boom Clay (a potential host rock in Belgium) to evaluate the effectiveness of self-sealing and possible fissure reactivation during a second gas invasion event. Initial water permeability under oedometer conditions was first measured on samples at two bedding orientations, being higher the sample with bedding planes parallel to flow, highlighting marked anisotropy. Then, gas injection tests at a constant volume rate were performed. Results indicated that Boom Clay underwent expansion and degradation during gas injection due to the development of fissures that were quantified using microstructural techniques. The computed effective gas permeability was not significantly dependent on bedding orientation and was slightly larger than the initial intrinsic water permeability. The re-saturation of the samples led to a recovery of the initial water permeability for both orientations, replicating the original anisotropy. The microstructural analyses confirmed the gas pathways’ closure, indicating good self-sealing and the regaining of the hydraulic barrier function. However, a small volume of large unconnected pores was detected on undrained unloading before the microstructural study. An additional gas injection after the self-sealing resulted in a higher effective gas permeability and a larger increase in pore volume, suggesting the reopening of fissures generated during the first injection. Finally, the experimental data were compiled within a multi-scale phenomenological model to relate the microstructural information to macroscopic flow transport properties capturing the intrinsic permeability increase on gas invasion and its recovery during self-sealing.

Investigation of physical properties and suspended particulate matter discharge in a macrotidal estuary at the Amazon-semiarid transition zone

This study evaluated the hydrodynamic circulation and discharge of Suspended Particulate Matter (SPM) in the Arraial-São José Estuarine Complex (ASJEC) located at the Amazon-semiarid interface, northeastern Brazil. Quasi-synoptic data were collected in the rainy and dry seasons of 2017 under spring tide conditions. For each sampling event, two mooring stations (one downstream [S-A] and one upstream [S-B]) were deployed during a complete tidal cycle. The physical characteristics in the rainy and dry seasons enabled dividing the estuarine complex into zones, with a mixing zone/coastal zone interface and a tidal river zone/mixing zone interface. The estuary is vertically quasi-homogeneous, indicating conditions of high vertical instability. The currents at the stations were more intense during the flood tide, demonstrating the importance of the marine component in the region. The estuarine system ranged from partially mixed to well mixed at both stations in the rainy season and was well mixed at both stations in the dry season. Total salt transport was towards the upstream portion of the estuary, suggesting the accumulation of matter in the ASJEC, most likely associated with the entrance of marine waters. The input of coastal water in the ASJEC intensifies the baroclinic pressure gradient, which creates a hydraulic barrier, indicating that SPM discharges from the continental drainage to the estuary and is interrupted from the estuary to the adjacent continental shelf.

Mitigating seawater intrusion in coastal aquifers: Novel approach with treated wastewater injection and groundwater circulation

Seawater intrusion (SWI) is a natural phenomenon that negatively impacts the potability of groundwater and is expected to worsen with rising sea levels due to climate change. Artificial recharge of freshwater is a commonly-used remediation method to mitigate SWI and improve freshwater supply security in affected coastal aquifers. However, limited freshwater availability can limit the effectiveness of this approach. This study proposes a novel mitigation measure, called Inj-GCW, which combines the injection of reclaimed water with the use of groundwater circulation wells (GCW) to enhance the effectiveness of artificial recharge in controlling SWI. GCW is a dual-screened well with isolated screens that extract and inject groundwater into the aquifer. The performance of the proposed measure was quantitatively evaluated using an illustrative simplified unconfined coastal aquifer. Based on the findings, the design parameters were estimated for a field-scale case study of the Nile Delta aquifer (NDA), a large Mediterranean coastal aquifer in Egypt. The study adopts a future scenario that considers Sea-level rise due to climate change and projected population growth by 2100. Results demonstrate that introducing of GCWs into the saltwater wedge, along with injection, effectively retreated the saltwater wedge, due to enhanced velocity, seaward fluxes, and dilution of contamination. Inj-GCW measure led to an 8.9% reduction in SWI and a 5.2% decrease in aquifer salinity compared to the expected intrusion in 2100. Furthermore, the Inj-GCW measure resulted in a 2.2% higher repulsion rate and 0.3% reduction in total salt mass compared to injection alone. The Inj-GCW measure presents a promising solution to SWI challenges in the NDA and other coastal aquifers facing similar issues. The formation of a brackish water bubble at the injection well screen of the GCW and the generated vertical groundwater circulation cells acted as a hydraulic barrier and contributed to the proposed method effectiveness.

Utilization of organoclay amended bentonite for hydraulic barrier applications under adverse electrolyte environment

Inorganic ions such as sodium and calcium are often found in industrial leachates, municipal solid waste leachates and deep geological repositories, which can significantly impact the geotechnical properties of hydraulic barrier materials. Researchers have suggested the utilization of organoclays as landfill liner amendments, especially when the liner is in contact with organic contaminants. This study focuses on the impact of inorganic salts (NaCl and CaCl2) on the geotechnical properties, such as consistency limits, swelling, and hydraulic conductivity of organoclay-amended bentonite (BOC) liners. Different concentrations of salt solutions (0, 0.1 N and 1 N) were selected for the execution of Atterberg limits, free swell tests, and oedometer tests. The surface charge characteristics of bentonite and organoclay were attained with the aid of zeta potential measurements. The variation in the surface topographies of soil samples at different electrolyte conditions was analyzed with the help of scanning electron microscope images. The addition of organoclays significantly reduced bentonite's swelling characteristics and liquid limit due to the unique hydrophobic and organophilic attributes of organoclay but increased the hydraulic conductivity. However, the influence of inorganic salts on the geotechnical parameters of organoclay-amended bentonites showed less variance than that of bentonite. Also, no soil samples exceeded the limiting hydraulic conductivity criteria required for landfill liners, i.e. 1 × 10−9 m/s. Hence, organoclay-amended bentonites could potentially be utilized for hydraulic barrier applications even in an electrolytic environment without compromising the hydraulic conductivity criteria.

Multidirectional Shifts in Hydrological Connectivity Result From Hydraulic Barriers

AbstractThe weakening of hydrological connectivity caused by hydraulic barriers and its impact on the ecological environment has gained worldwide attention. Previous studies have primarily focused on the obstructive effect of barriers, often overlooking its influence on the internal connectivity of regulated water systems. The study employed a new conceptual framework of hydrological connectivity and utilized the connectivity assessment tool to assess the impact of the proposed Poyang Lake Hydraulic Project (PLHP) on the lake's internal connectivity. The estimated impacts were based on various factors, including the connectivity function (CF), the spatial distribution of the connected object (CONNOB), as well as duration and timing of connection. Key findings indicate that the PLHP will enhance the total connectivity and general connectivity involving inundation depth of the lake. This enhancement will result in increased CF values, expanded CONNOBs and prolonged connection durations. Conversely, the PLHP will weaken the general connectivity involving flow velocity and effective connectivity. As a result, CF values will decrease, CONNOBs will shrink, and connection durations will be shorter. It is important to note that the impact of hydraulic barriers varies depending on evaluation metrics, duration, timing, direction, distance, location, hydrological year, and regulation stage. Therefore, it is not appropriate to simply categorize the barrier impact as “weakening” or “enhancing.” This study innovates the knowledge of hydraulic barriers from the perspective of internal connectivity and is expected to provide more comprehensive theoretical guidance for water resource regulation and eco‐environmental protection.

Technical feasibility analysis of using phosphogypsum, bentonite and lateritic soil mixtures in hydraulic barriers

Technical feasibility analysis of using phosphogypsum, bentonite and lateritic soil mixtures in hydraulic barriers

The Role of Stress States on the Hysteric Behavior of Expansive Soil under Multiple Drying-Wetting Cycles

Expansive soils in the field are typically exposed to cyclic wetting and drying due to climatic fluctuations and subjected to a variety of stress conditions in nature or when used as compacted layers for the construction of hydraulic barriers or waste disposal facilities. The hysteric behavior of the soil-water characteristic curve (SWCC) is a key parameter for understanding, modeling, and interpreting the unsaturated behavior of these soils under such conditions. This study investigates the effect of stress states on the hysteresis behavior of soil-water characteristic curves (SWCCs) for compacted highly expansive clay over a range of matric suction between 0 and 1500 kPa. Two test series were performed, the first test series investigated the effect of stress states on the hysteresis of SWCCs during a single drying-wetting (DW) cycle. The second test series studied the combined effect of stress applied and multiple drying-wetting cycles on the SWCC hysteresis. For the sake of comparison, the overall SWCC hysteresis due to drying-wetting cycles was quantified using the average degree of hysteresis in terms of volumetric water content (ADHθ). Furthermore, contributors to the observed hysteresis were defined using two newly proposed measures; namely, average degree hysteresis in terms of gravimetric water content (ADHw) and in terms of volume change (ADHe*). The outcomes of this study indicate that consideration of stress states on the hysteresis of SWCC for expansive clay is of great importance. The results show a dual trend for the variation of ADHθ with applied vertical stress. Furthermore, multiple DW cycles induced a significant reduction in the hysteresis (ADHθ) under low- and high-stress states up to a certain level of DW cycles, then, no further changes in the hysteresis trend were detected. It was also found that hysteresis loops under a low-stress state were concentric in shape while hysteresis loops for specimens under a high-stress state were non-concentric, with a downward shift in hysteresis loops with the increase in DW cycles.

Influence of the water content on the complex conductivity of bentonite

Compacted bentonite is considered as a potential buffer material for deep geological disposals of high-level nuclear wastes. Methodologies to non-intrusively monitor the water content of such sealing materials are important in the context of the safety of these storage facilities and for various engineering applications as well. Induced polarization is a non-intrusive geophysical method sensitive to the water content of porous media. We investigated the complex conductivity spectra of 69 samples made of 2 distinct MX80 bentonites, one in the form of powder from crushed pellets (Type I) and the other in the form of a granulated bentonite mixture (GBM, type II). The samples are prepared at different compaction states and saturations. The pore water conductivity of the porous samples is estimated to be ∼2.5 S m−1 (25 °C) by two different methods. The complex conductivity spectra were obtained at room temperature (25 ± 2 °C) in the frequency range 1 Hz-45 kHz. In-phase and quadrature conductivities reflect conduction (electromigration) and polarization processes, respectively. At a given frequency, both the in-phase and quadrature conductivities increase with the water content along a trend that is independent of the compaction state. An induced polarization model based on the dynamic Stern layer model is used to explain these results. The first Archie's exponent m is inferred from the formation factor F using the in-phase conductivity data versus the pore water conductivity data at different salinities (NaCl, 25 °C). The dynamic Stern layer model applied to polarization correctly predicts the dependence of the in-phase conductivity, quadrature conductivity, and normalized chargeability with the water content and the Cation Exchange Capacity (CEC). This petrophysical work can easily be applied to time-domain induced polarization data collected in field conditions to monitor hydraulic barriers.

Quantifying the potential seepage from porphyry copper tailing impoundments using a multi-isotopic approach

Porphyry-style copper deposits are characterized by low Cu grades and high tonnages, resulting in large mine tailing volumes disposed in impoundments. Due to the mining tailing sizes, waterproofing techniques cannot be applied along the dam base. Therefore, to minimize seepage towards the aquifers, pumping wells are usually installed as hydraulic barriers. Currently, there is a controversy over whether or not the water extracted from hydraulic barriers should be counted as the use of new water rights. Consequently, a growing interest to develop tools to trace and quantify the tailing impacts in groundwater and to determine the water pumped amount subjected to water rights exist. In the present study, isotope data (δ2H-H2O, δ18O-H2O, δ34S-SO42− and δ18O-SO42−) are proposed as a tool to quantify tailings seepage towards groundwater and to assess hydraulic barriers effectiveness. To illustrate this approach usefulness, the Quillayes porphyry Cu tailing impoundment (Chile) case study is presented. The multi-isotopic approach revealed that tailing waters are highly evaporated showing high SO42− content (~1900 mg L−1) derived from primary sulfate ore dissolution, whereas freshwaters, derived from recharge water, have low SO42− contents (10–400 mg L−1) resulting from the interaction with geogenic sulfides from barren host rock. The δ2H and δ18O values of groundwater samples collected downstream from the impoundment suggest a mixing at different proportions of highly evaporated water from the mine tailing waters and non-evaporated regional fresh groundwater. Cl−/SO42−, δ34S-SO42−/δ18O-SO42−, δ34S-SO42−/ln(SO42−) and δ2H-H2O/δ18O-H2O mixing models allowed to determine that groundwater located closer to the impoundment had a mine tailing water contribution from 45 to 90 %, whereas those located farther away had lower contribution (5–25 %). Results confirmed the stable isotope usefulness to determine the water origin and to calculate the hydraulic barrier efficiencies and the pumped water proportions unrelated to the mining tailing subject to the water rights.