Low Hydraulic Conductivity Research Articles

Effects of desiccation-induced cracking and leachate infiltration on the hydraulic conductivity of natural and olivine-treated marine clay

Clayey soils rich in smectite are commonly utilized for the construction of liner materials in engineered landfills owing to their low hydraulic conductivity. However, due to the high swelling potential of such clayey soils, their hydraulic conductivities are adversely influenced by desiccation cracking. In addition to cracks, the hydraulic conductivity of the clayey soils might also be affected by the complex composition of landfill leachate. The current study aims, therefore, to assess the effects of desiccation-induced cracks and leachate infiltration on the hydraulic conductivity of natural and 30% olivine-treated marine clay specimens. Clay specimens were prepared using three compactive energies and subjected to cycles of drying–wetting and hydraulic conductivity testing using tap water and field leachate as the permeants. Test results demonstrate significant improvements in the hydraulic conductivity of the olivine-treated clay samples after cycles of drying–wetting upon permeation with water and leachate. These improvements are attributed to the formation of magnesium–aluminate–hydrate and magnesium–silicate–hydrate gels as confirmed using microstructural analyses. The gels are responsible for minimizing the adverse impact of cracks on the hydraulic conductivity of the treated clay specimens. The findings indicate that olivine can be employed as a suitable additive in decreasing the hydraulic conductivity of soils prone to cracking.

Durability of water-affected paste backfill material and its clean use in coal mining

Backfilling, which is an example of the clean use of mineral resources, allows a safe work environment and provides a disposal means for mine tailings. Safe mining is significantly influenced by the durability of paste backfill material, which can be affected by water. In this study, the effect of water on the mechanical properties and permeability was experimentally examined with increasing curing time in the Taiping Coal Mine, China. The interaction of paste backfill material with water results in an exponential increase in the weakening coefficient with increasing curing time, and a linear decrease with higher water content. Slaking, a process in which cohesion is lost under the effects of water and physical disintegration, occurs in paste backfill material. Low hydraulic conductivity is found, which contributes to the durability of paste backfill material. Microscopic analysis shows that porous and nonuniform distributed structures account for the weak permeability and stable strength. On this basis, a durability classification chart for clean use is proposed; this classification scheme identifies three grades of durability affected by water. The engineering geological characteristics of each grade are provided, providing guidance for identifying the type and grading of durability of paste backfill material affected by water in coal mining. The approach was validated by successful application.

Geomorphic controls of perched groundwater interaction with natural ridge‐top depressional wetlands

AbstractGeographically isolated wetlands (GIWs) are commonly reported as having hardpan or low hydraulic conductivity units underneath that produce perched groundwater, which can sustain surface water levels independently of regional aquifer fluctuations. Despite the potential of GIW‐perched aquifer systems to provide important hydrological and ecological functions such as groundwater storage and native amphibian habitat, little research has studied the hydrologic controls and dynamics of these systems. We compared several ridge‐top depressional GIW‐perched groundwater systems to investigate the role of watershed morphology on hydroregime and groundwater‐surface water interaction. Ridge‐top depressional wetlands in the Daniel Boone National Forest, Kentucky were chosen because they offer natural controls such as lack of apparent connection to surface water bodies, similar climate, and similar soils. Three wetlands with different topographic slopes and hillslope structures were mapped to distinguish key geomorphic parameters and monitored to characterize groundwater‐surface water interaction. Wetlands with soil hummocks and low upland slopes transitioned from infiltration to groundwater discharge conditions in the spring and during storm events. The magnitude and duration of this transition fell along a continuum, where higher topographic slopes and steeper uplands produced comparably smaller and shorter head reversals. This demonstrates that ridge‐top GIW‐perched groundwater systems are largely sensitive to the runoff‐recharge relationship in the upland area which can produce significant groundwater storage on a small‐scale.

¿Pueden los rasgos hidráulicos ayudar a explicar los límites de distribución actual en dos especies de Nothofagus en los Andes de Chile?

The Andes of central Chile is a geographical gradient with a marked variation in its climatic conditions. Along its slopes, are distributed the evergreen Nothofagus dombeyi and deciduous Nothofagus pumilio species that show differences in their leaf habits, range extensions, and in their limits of latitudinal distribution. Plant ecology proposes that unfavorable climatic conditions are limiting factors that determine the tree species distributions and that the functional hydraulic traits responses allow understanding the mechanisms underlying the current distribution of them. We hypothesize that both species will have lower mean values of KL and KS in populations near to distribution limits compared with middle populations due that unfavorable climatic conditions are predominant in the latitudinal range limits. We quantify in situ the leaf (KL) and xylem (KS) specific hydraulic conductivities in populations of N. dombeyi and N. pumilio near their northern and southern limits of distribution as well as in a middle population along Chilean Andes. Results showed that both species had lower mean values in populations near northern and southern limits compared to populations distributed in middle sites. Also, we found that the hydraulic performance population of N. pumilio distributed in the middle site had higher than N. dombeyi. We concluded that lower hydraulic conductivity associated with distribution limits for both study species implies a lower probability of being affected by embolism, independently of their leaf habits, showing a functional hydraulic convergence to low water availability or cold.

Slope Stability of Landfill with Waste Degradation

Nowadays, a large amount of municipal solid waste (MSW) is generated due to the rapid urbanisation in developing countries leads to the demand for larger and higher capacity landfills. Bioreactor landfill technology has been introduced to accelerate the stability of landfill and to solve the issue of limited landfill area. However, the accelerated degradation of the refuse in bioreactor landfills also considerably changes the geotechnical characteristics of the waste in the landfill and thereby increases the concern for waste stability. Hence, this study aims to analyse the stability of both conventional and bioreactor landfill slope with the effects of waste degradation. Finite element method has been used in the slope stability analysis and the stability is presented by the factor of safety. The objectives of this study are i) to determine and assess the main parameter which influences the stability of the waste slope, ii) to determine the effects of waste degradation to the waste properties and iii) to obtain the factor of safety of the landfill slope using numerical analysis by finite element method. From the literature review, it is found that slope stability of a landfill mainly depends on the geotechnical properties of waste, such as moisture content, unit weight, shear strength parameters and hydraulic conductivity of waste. After the degradation process, engineering properties of field refuse are affected which includes the increased pore-water pressure and unit weight, decreased strength and lower hydraulic conductivity. Based on the analysis of conventional landfill slope stability by using Plaxis software, slope ratio of 1:3, 1:4 and 1:5 calculated safe with 1.69, 2.3 and 2.8 whereas the analysis of bioreactor landfill slope stability calculated safe only for slope ratio of 1:4 and 1:5 with 1.60 and 1.97. Moreover, the factor of safety for steeper slopes is lower and vice versa. From the parametric analysis, it is found that the full height of slope and unit weight of waste input affect the result analysis. This study is significant to evaluate the landfill slope stability with the effects of waste degradation and to ensure both conventional and bioreactor landfill slope stability for long periods.

Hydrogeological features affecting spatial distribution of glyphosate and AMPA in groundwater and surface water in an agroecosystem. Córdoba, Argentina

The study area is located in the eastern slope of Las Peñas Mountain and its adjacent oriental fluvio-aeolian-plain. Agriculture is the main activity (soybean, maize, wheat, peanuts and alfalfa) with no-tillage farming and intensive use of agrochemicals (pesticides-fertilizers). Glyphosate (N-phosphono-methylglycine) is the most common used herbicide which suffers microbial biodegradation giving aminomethylphosphonic acid (AMPA), its main metabolite. The objective of this work is to evaluate hydrogeological features which influence the presence of glyphosate and AMPA in waters. In the study area, the main flow direction of surface and groundwater is NW-SE. The unsaturated zone thickness decreases in the same direction from 60 to 0 m, so groundwater surges in low areas in the eastern sector. From the total water samples collected, glyphosate was detected in 66% of surface water samples (0.2 to 167.4 μg/L), in 15.8% of the groundwater samples (1.3 to 2 μg/L) and in the harvested precipitation sample (0.2 μg/L). AMPA was found in 33% of surface water and 15.8% of groundwater. The herbicide detection was related to areas with the shallowest water table (< 4 m), low hydraulic conductivity in the aquifer (K = 1.5 m/d), low hydraulic gradient (i = 0.16%) and very low flow velocity (0.02 m/d). The most outstanding result is that the groundwater presents higher values in comparison with the surface water samples, which can be explained by the greater dilution capacity of streams. The detection of glyphosate and AMPA in the unconfined aquifer shows that the application for decades under the prevailing agricultural model exceeds the degradation potential of the soil and the unsaturated zone, causing groundwater contamination.

Submarine groundwater discharge in an anthropogenically disturbed, semi-arid estuary

Quantification of submarine groundwater discharge (SGD) remains a challenge due to its large spatial and temporal variability exacerbated by natural heterogeneity (e.g., climatic conditions and hydrogeologic settings) and anthropogenic disturbances (e.g., dredging, oil/gas extraction, and oil-field brine discharges). This study investigates the spatial and temporal variability of SGD during different hydroclimatic conditions in a semi-arid, anthropogenically disturbed estuary using Darcy’s law (i.e., fresh/terrestrial SGD) and radon (222Rn, total SGD) and radium (226Ra, saline/recirculated SGD) isotope mass balances. Continuous electrical resistivity imaging and 222Rn surveys revealed potential subsurface influences on SGD with the highest SGD rates in areas with sandier substrates and near transitions from low hydraulic conductivity to higher hydraulic conductivity bottom sediments. Darcy estimates ranged over two orders of magnitude and were slightly higher following the flooding recession (0.09–8.28 m·d−1) than following a period of low precipitation (−0.02–7.84 m·d−1). Mobile continuous 222Rn estimates (0.79–1.81 m·d−1) support higher and more variable SGD rates, similar trends to those previously reported from time-series 222Rn measurements (0.13–3.85 m·d−1 across seasons). Radium-226 derived SGD (1.3 × 10−2–2.7 × 10−2 m·d−1 using the average groundwater endmember) fall short of 222Rn-derived SGD due to inability to account for radium tracer reactivity within the sediment. However, local Darcy estimates agree well with the range of 222Rn, likely due to the steeper gradients near shore. Radium activity ratios and SGD rates reflect mixing of shallow and deep groundwater beneath the bay, likely due to anthropogenic disturbances with a greater influence from deep groundwater 3–6 months following major precipitation events. This study strongly suggests that semi-arid systems receive significant SGD, which in highly anthropogenically disturbed systems are derived from both shallower and deeper groundwater flowpaths and lag the climatic conditions by weeks (shallow inputs) and months or longer (deeper inputs).

Influence of Slope Angle on Water Flow in a Three‐Layer Capillary Barrier Soil Cover under Heavy Rainfall

A three‐layer capillary barrier soil cover has been recently proposed for minimizing rainwater percolation in humid climates. Its performance has been preliminarily investigated by both physical and numerical modeling at a fixed slope angle of 10°. However, the slope angle of soil cover is varied in engineering practice and hence it is an essential design parameter. In this study, the influence of slope angle on water flow in the three‐layer soil cover was investigated via flume model tests subjected to heavy rainfall. A 3‐m long flume model of soil cover was instrumented to monitor matric potential and volumetric moisture content as well as components of water balance at slope angles of 3°, 10° and 18°. Furthermore, numerical simulations of transient seepage were conducted to back‐analyze the experiments. Both experimental and numerical results showed that the infiltration ratio increased as the slope angle increased because of the increase in the soil water potential gradient normal to the slope gradient. Meanwhile, the lateral water flow in the upper two layers was enhanced by the increase of slope angle due to water backfill effect at the top of clay layer. Correspondingly, the steeper the slope, the lower the matric potential, volumetric moisture content and soil water potential in the upper two layers were. Due to low hydraulic conductivity of the bottom clay layer, the percolation through the cover remained virtually nil at each slope angle. The above findings could have significant implications for the design of sloping three‐layer capillary barrier soil cover.

Impact of the wetting process on the hydro-mechanical behavior of unsaturated residual soil from flysch rock mass: preliminary results

Slope instabilities in the form of shallow and deep-seated landslides in flysch formations throughout Europe typically occur after prolonged periods of heavy rainfall. The Rjecina River Valley, Croatia, is characterized by the presence of flysch material in the lower part of the Valley, where numerous historical and recent landslides have occurred. The weathering process and climate conditions result in a complex engineering geological profile of flysch slopes in the Valley, with unsaturated residual soil covering the slopes. To investigate the behavior of residual soil existing on the flysch slope under increasing water content due to the rainfall infiltration process, undisturbed soil samples collected at natural water content were tested in the modified direct shear apparatus. Under imposed stress conditions, samples of low hydraulic conductivity were subjected to a prolonged wetting process simulating the rainfall infiltration process in the field. The obtained results suggest that a gradual decrease of matric suction and an increase of water content resulted in an increase of displacement rates under constant shear stress, which was interpreted as a failure of samples in partially saturated conditions. A unique shear strength envelope expressed in terms of Bishop’s effective stress equation was found to be able to predict stress conditions at the slip surface at the time of failure, while the relationship between measured matric suction and water content closely matched with the main wetting curve. Although the testing results did not point out any special characteristics of residual soil from flysch rock mass behavior, the data about hydro-mechanical behavior of unsaturated residual soil from flysch rock mass, as well as similar fine-grained soils, are very rare and presented results would be valuable for further research. The presented testing procedure and obtained results are useful for studies of rainfall-induced landslides triggered in fine-grained soil materials in zones above the phreatic line, such as shallow landslides occurring in natural flysch slopes or in physical landslide models built in laboratories.

Analysis of Land Use Change and Its Impact on the Hydrology of Kakia and Esamburmbur Sub-Watersheds of Narok County, Kenya

Narok town is one of the places in Kenya which experience catastrophic floods. Many lives have been lost and valuable property destroyed in recent years. Change in land use/land cover upstream of the town area may have contributed significantly to the severity and frequency of flooding events. Runoff, which contributes to floods in Narok town, comes from Kakia and Esamburmbur sub-catchments of Enkare Narok watershed. The objective of this study was to assess the impact of land use/land cover change on the hydrology of Kakia and Esamburmbur sub-watersheds. To detect land use/land cover change, Landsat satellite images from 1985 to 2019 were used. Using supervised classification in Erdas Imagine 2014, land use of the study area was classified into four classes, i.e., forest, rangeland, agriculture and built-up areas. Five land use maps (1985, 1995, 2000, 2010, and 2019) were developed and used to perform land use change analysis. There was rampart conversion of forest to other land uses. Between 1985 and 2019, the forest and rangeland declined by 40.3% and 25.6% of the study area, respectively, while agriculture and built-up areas increased by 55.2% and 10.6% of the study area respectively. Analysis of soil hydrological properties indicate that the infiltration rate and soil hydraulic conductivity were greatest in forest than in other land use types. The basic infiltration rate in forest land was 89.1 cm/h while in rangeland and agricultural land, it was 7.9 cm/h and 15 cm/h respectively. At the top-soil layer, average soil hydraulic conductivity under forest was 46.3 cm/h, under rangeland, 2.6 cm/h and under agriculture, 4.9 cm/h. The low hydraulic conductivity in rangeland and agriculture was attributed to compaction by farm machinery (tractors) and livestock respectively. An interesting observation was made in rangelands where the top layer (0–20 cm) had a higher bulk density and a lower hydraulic conductivity as compared to the next deeper layer (20–40 cm). This was attributed to the combined impact of compaction and localised pressure by hooves of livestock which only have an impact on the top layer. The findings of this study show that land use has a major impact on soil hydrological properties and imply that the observed land use changes negatively affected the soil hydrological properties of the watershed. The decreased infiltration in the increasing areas of degraded land (mainly agriculture and rangeland) and increase in built-up area in Narok town are the possible causes of the increased flood risk in Narok town. It is recommended that flood risk management strategies in Narok town include watershed management to enhance water infiltration.

Exhumation and performance of an Antarctic composite barrier system after 4 years exposure

An Antarctic biopile using a composite liner (high-density polyethylene geomembrane (GMB) over a geosynthetic clay liner (GCL)) was constructed on a coarse granular subgrade to contain hydrocarbon-contaminated soil and leachate. The soil was remediated after 4 years and the biopile was decommissioned. The liner was exhumed to assess the properties and performance of the GMB and GCL. There was no significant change in the GMB index properties. Although cobbles and coarse gravel of the subgrade had left indentations in the GMB, implying tensile strains that could impact long-term performance, there were no holes. There was significant variability in the hydration of the GCL (from 10% to 220%) and in the underlying subgrade soil water content (from 5% to 30%). This reflects the complexity of the subgrade and groundwater flow in the Antarctic environment. The exhumed GCL specimens had low hydraulic conductivity (1 × 10−11 to 7 × 10−11 m/s) at 13 kPa. Soil samples from below the composite liner showed no detectable hydrocarbons and confirmed no migration through the barrier. It is concluded that the composite barrier contained the leachate and biopile soil over the 4 years in service in the extreme Antarctic conditions.

20 years performance measurements of a landfill cover system with components constructed from pre-treated dredged sediments

The water balance and effectiveness of a landfill cover containing a liner from fine-grained dredged material (‘METHA-material’) and a drainage layer from sandy dredged material (‘METHA-sand’) from the Port of Hamburg (Northern Germany) was investigated for 20 years using two test fields (lysimeters), each 500 m2 in size. The layering within the standard design test field (FS) is identical to the layering of the cover on the landfill, whereas the alternative design test field (FA) was designed to provoke desiccation of the liner. The setup abstained from continuous side walls cutting through the liners in order to allow for material properties representative for the landfill cover, at the cost of possible minor lateral inflows or outflows, which were, however, considered negligible. By example of the lysimeter FA the risk of desiccation-induced failure of a liner from METHA-material under the given climate could be shown. In contrast, the discharges and hydro-chemical parameters measured for the lysimeter FS indicate continuous high efficiency of the liner until today. The average discharge below the liner was 14.9 mm/yr. The entire cover contributes to the good liner performance. Reason are (1) the low hydraulic conductivity of the liner (5 × 10−10 m/s) and its large thickness (1.5 m); (2) the increased stresses on the liner due to the 2.5 m thick overlaying cover; (3) the 1.0 m thick drainage layer from slowly-draining METHA-sand, allowing for a continuous nearly water-saturated fringe above the liner; and (4) the sufficient amount of plant available water in the recultivation layer.

Improving the engineering properties of a soft marine clay with forsteritic olivine

The possibility of improving the engineering properties of a marine clay by adding forsteritic olivine is explored. Strength improvement was recorded with ascending values of olivine contents and curing time. An olivine content of 30% was identified as the optimum value to achieve the targeted improvement for all curing durations. Formation of magnesium-aluminate-hydrate and magnesium-silicate-hydrate were found to be the key components responsible for this strength improvement based on morphological and mineralogical analyses. Comparative studies with identical lime-treated marine clay samples revealed that olivine treatment achieved higher mechanical strength and lower hydraulic conductivity relative to lime treatment for a limited curing duration.

How spatiotemporal variation of soil moisture can explain hydrological connectivity of infiltration-excess dominated hillslope: Observations from lesser Himalayan landscape

This study demonstrated the spatiotemporal variation in runoff generating areas in grassed and agro-forested hillslopes of Lesser Himalaya using soil moisture, soil hydraulic conductivity and rainfall datasets. Nine rainfall events of infiltration-excess dominated hillslopes were analysed and found that only high-intensity rainfall events were able to convert 1–5% of runoff at the outlet. The extremely low patch of soil hydraulic conductivity (<5 mm/h) was located at the outlet in agro-forested hillslope whereas similar low soil hydraulic conductivity patch is located at the end portion in a grassed hillslope. Therefore, the grassed hillslope generated less runoff than the agro-forested hillslope due to its surface resistance and heterogeneity in soil hydraulic conductivity. The runoff generated from the upper part of hillslope re-infiltrated into the middle part due to higher soil hydraulic conductivity. During low and medium intensity rainfall conditions, major runoff contribution was observed from low conductivity zones of the hillslope. Moreover, we analysed the correlation of spatial variation in soil moisture with topographic wetness index (TWI) and soil hydraulic conductivity in two different landuse conditions to examine the predictive potential of these attributes during the wet and dry season. The optimal formulation of TWI was obtained from 72 different combinations using linear mixed effects modelling. The correlation between optimal TWI formulation and soil moisture was found to be negative as the main streamline is located near low conductivity zones. Furthermore, the correlation between TWI and soil moisture is stronger in the dry season than the wet season. In comparison to grass cover hillslope, agro-forested landuse system shows better negative correlation between soil moisture and TWI during the wet season as the agro-forested hillslope produced more runoff than the grassed hillslope. The correlation strength of soil hydraulic conductivity and soil moisture was strongest after mid of the wet season which is directly correlated to the moisture content of hillslopes.

A facile method to enhance the performance of soil bioelectrochemical systems using in situ reduced graphene oxide

Bioelectrochemical systems offer a potential solution for the treatment of a broad variety of environmental contaminants. Unfortunately, when applied to the remediation of soil and sediments, the low electrical and hydraulic conductivities of these media limit their effective applicability in full-scale installations. Interestingly, these drawbacks may be overcome by including conductive particles within the soil porosity in order to maximize the outreach of the electrode through the contaminated medium, thereby minimizing electron- and mass-transfer limitations.Herein, we increase the electrical conductivity of a model porous aquifer using amendments of graphene oxide (GO), followed by its reduction to produce reduced graphene oxide (rGO) by means of microbial or electrochemical reduction methods. Both approaches promoted the formation of rGO-sand composites with superior electrical features compared to controls not amended with GO, with conductivity being positively correlated to the GO application rates, within the applied range of 10–2000 mgGO kgsand−1. The electrochemical reduction yielded significantly higher conductivity than the biological method. This result is putatively ascribed to a higher degree of reduction achieved by the former approach. When applied to laboratory scale soil bioelectrochemical systems fed with sodium acetate as a model contaminant, the GO-amended reactors delivered 32x higher anodic current compared to unamended controls. We conclude that GO amendments to porous soils improve the outreach of the electrochemical process to include microbial cells in distal soil locations.

The selection of sealing technologies of the subsoil and hydrotechnical structures and quality assurance

Abstract Various types of trenchless methods are extensively used to create engineering barriers for the purpose of achieving relatively low hydraulic conductivity of the subsoil and hydrotechnical structures. The most commonly used technologies of the grouting curtain creating are the DSM (Deep Soil Mixing), WIPS (Vibro Injcted Thin Wall), low-pressure injection and jet-grouting. Vertical barriers are widely used in environmental control systems to restrict the lateral spreading of liquid or gaseous contaminants and for seepage control through and beneath a levees. The barrier walls are constructed in a single or two phases using a slurry composed mainly of bentonite, cementitious materials and water. Often to solve complex geotechnical problems the technologies are combined (e.g. combining DSM and jet grouting). In the paper some examples of practical application of chosen types of technology dedicated different problems solving are presented. The results of geotechnical investigations of hydraulic permeability, shear strength and depth control of cut-off wall are given. The tests were carried out using BAT system, Cone Penetration Tests (CPT) and drillings. The presented test results should help define the technical and financial criteria that should be used to compare grouting based methods and other techniques applicable to the same geotechnical special works. These recommendations should permit an adequate application of grouting techniques and contribute to finding solutions for the delicate problem of ground sealing, notably when proceeding with hydrotechnical or environmental structures placed below water table.

Effects of bentonite heating on U(VI) adsorption

Engineered barrier systems designed to isolate high-level radioactive waste utilize bentonite, a montmorillonite-rich material, to restrict contaminant transport due to its low hydraulic conductivity and high adsorption capacity. High temperatures (100–200 °C) near waste canisters resulting from radioactive decay may alter the clay's ability to adsorb contaminants. In this study, we examine U(VI) adsorption onto two different bentonite samples subjected to (1) 18 years of in situ heating during an underground experiment and (2) short-term (7-week), high temperature (300 °C) heating in the laboratory. Results show that U(VI) adsorption was lower for field-heated bentonite located closest to the heater, which experienced temperatures of approximately 95 °C, compared to a control sample, which experienced temperatures of approximately 20 °C, over a range of aqueous chemical conditions. On average, Kd values for U(VI) adsorption were 31% lower for 95 °C heated samples. By contrast, U(VI) adsorption onto intermediate-heated bentonite (50 °C) was indistinguishable from the adsorption onto the cold-zone (20 °C) sample. U(VI) adsorption onto lab-heated bentonite was also lower than onto control bentonite over the pH range 4.5–8.0, with an average decrease in Kd values of 50% after heating. Lower U(VI) adsorption to field-heated bentonite persisted after bentonite was purified to isolate the clay fraction. This allows us to rule out changes in pore-water chemistry or accessory mineral composition as causes of the lower adsorption. No evidence of montmorillonite illitization was observed in the heated samples. While some of the lower U(VI) adsorption in the lab-heated bentonite can be explained by changes in aqueous U(VI) speciation, we propose that lower U(VI) adsorption to field-heated bentonite may be primarily due to changes in the montmorillonite edge structure. The observed changes in U(VI) adsorption to bentonite after heating have implications for U(VI) diffusive transport through engineered barriers and must be considered when designing radioactive waste disposal repositories.

The Dynamics of Sea Tide-Induced Fluctuations of Groundwater Level and Freshwater-Saltwater Interface in Coastal Aquifers: Laboratory Experiments and Numerical Modeling

Laboratory experiments were conducted in a vertical, two-dimensional, rectangular flow tank, simulating the response of a phreatic coastal aquifer to a sea tide. Imposed sinusoidal fluctuations of the saltwater level at one side of the flow tank caused three types of fluctuations: (a) hydraulic head throughout the aquifer, (b) saturation degree within the capillary fringe, and (c) salt concentration surrounding the freshwater-saltwater interface (FSI), all recorded by head, saturation, and salinity sensors, respectively. Significant time lags were observed both in the saturation degree within the unsaturated zone and in the salinity within the FSI. All measured values, recorded by the three types of sensors, were simulated and reproduced using a numerical model. The calibrated model was used for mapping the time lags throughout the aquifer. It was found that the time lag of saturation fluctuations within the unsaturated zone increased upward from the groundwater level as the unsaturated hydraulic conductivity decreased. Similarly, the time lag of salinity fluctuations within the FSI increased downward, with distance from the groundwater level. We interpret the low hydraulic conductivity at the capillary zone as the source of attenuation of both saturation and salinity, because both are controlled by the vertical advection of the whole freshwater body. This advection is significantly slower compared to the dynamics of pressure diffusion. The uniqueness of this study is that it provides quantitative data on the attenuation at the capillary zone and its effect on the salinity time lag in coastal aquifer systems.

Development of a conceptual groundwater model for a complex basement aquifer system: The case OF OSUN drainage basin in southwestern Nigeria

Sound knowledge of hydrogeological setting is required for effective groundwater resource management and protection. In the basement environment where groundwater occurrence is discontinuous and highly heterogeneous, hydrogeological conceptual model is required to understand the occurrence and flow pattern of groundwater. This study presents an integrated 2D and 3D GIS method for conceptual modeling of the hydrogeological setting of a basement complex aquifer system in Osun Drainage Basin, Southwestern Nigeria. The efficiency of conjunctive use of RockWorks, Suffer and ArcGIS for surface and subsurface 3D hydrogeological modeling of a basement aquifer system was evaluated.Three distinct aquifers (Weathered Layer Aquifer (WLA), Unconfined Fractured Aquifer (UFA) and Confined Fractured Aquifer (CFA)) were delineated. WLA is characterized by low hydraulic conductivity (K = 0.34–27.54md−1, mean = 4md−1), hence low yield potential. Transmissivity (T) range and mean (0.6–1368 m2d−1, 85 m2d−1) indicate restricted flow within WLA. UFA is hydraulically connected to the underlying multi-layered CFA in the contact zones. The occurrence of UFA is localized within meta-sedimentary environment and it acts as the recharge redistributing medium, connecting the deep seated aquifers with the highly porous surface flaggy rubbles. CFA is characterized by relatively high mean K (0.2–91md−1, mean = 11md−1) and relatively high T (1.6–7185 m2d-1, mean = 483 m2d−1). Comparative analyses of aquifers’ hydraulic properties and spatial pattern of occurrence affirm that regional groundwater flow only takes place within CFA. The variation of hydraulic properties along fault lines is attributed to alternating occurrence of bedrocks of different structural and physico-chemical characteristics. Three distinct groundwater flow systems were identified in the study basin: i) the radially concentric localized shallow groundwater flow systems that occur within the overburden, ii) the W-E oriented regional flow system that feeds the source of Ikogosi Warm Spring in the adjourning basin, and iii) the NNE-SSW trans-current flow system that traverses the study basin at the center. While the first flow system is controlled by surface topography and the hydraulic properties of the vadose zone, the second and third flow systems are structurally controlled. The efficiency and efficacy of the methodology presented in this study is sufficient to develop hydrogeological conceptual model for highly heterogeneous, discontinuous basement aquifer system. Therefore, the developed conceptual model could be transformed to a numerical model of the study basin.

Hydraulic performance and chemical compatibility of a powdered Na-bentonite geosynthetic clay liner permeated with mine drainage

The hydraulic and chemical compatibility of a geosynthetic clay liner (GCL), containing powdered Na-bentonite, was evaluated against artificial acid rock drainage (ARD) in terms of the swell index, hydraulic conductivity and heavy metal retention. Six artificial ARDs with an approximate pH of 3 and different metal concentrations (electrical conductivity, EC, ranging between 75 and 1000 mS/m; ionic strength ranging between 8 and 400 mM) were used in the experiments. The results of free swelling tests showed that high metal concentrations (EC higher than 70 mS/m) negatively impact the swell volume by lowering it. The hydraulic conductivity of the GCL permeated with distilled water was 1.2 × 10−11 m/s, falling in the range of 7.9 × 10−12 to 1.1 × 10−10 m/s when prehydrated with distilled water and permeated with ARDs. The ion exchange and metal precipitation appeared to be the main mechanisms controlling the metal attenuation on the bentonite. The ion exchange mechanism starts with the release of Na from the bentonite and the sorption of the bi- and tri-metals present in the ARDs onto the bentonite. After the depletion of Na, the ion exchange reaction proceeds with the desorption of Ca and Mg from the bentonite and the sorption of cations present in the ARDs onto the bentonite layers. The depletion of Na from the bentonite and the subsequent release of Ca and Mg correlate to the sudden drop in pH and a gradual increase or equilibration of the hydraulic conductivity. It is possible to say that, after this point, hydraulic and chemical equilibrium is reached. From the overall results, the tested GCL showed acceptably low hydraulic conductivity and the potential to attenuate heavy metals present in ARDs.