Shallow Water Table in Arid Urban Zone: Preliminary Study at Sultan Qaboos University Campus, Oman

A shallow water table (SWT) rises up to several centimeters below the soil surface and even exfiltrates making surface impoundments that are common in humid climates. Recently, SWTs emerged even in arid and semi-arid regions of several metropolitan areas of the Arabian Gulf Cooperation Council (GCC) countries with a pernicious impact on the urban infrastructure, health, ecohydrology of parks and gardens, among others. Quantifying SWT’s dynamics and understanding the associated phenomena of water motion and solute transport are vital for finding the proper mitigation techniques and reducing the risks associated with SWT. In this preliminary study, SWT was delineated, its seasonal dynamics was monitored, evaporation losses were quantified, and groundwater chemistry was measured at Sultan Qaboos University (SQU) campus (Oman). The study sites were near the building of the College of Nursing (Wadi N, 4 soil excavations thereafter indicated as S1, S2, S3 and S4) and near the SQU Mosque (Wadi M, 3 locations were labeled 1,2 and 3). Field measurements of SWT in soil excavations, laboratory analysis of collected groundwater samples, and numerical simulation of evaporation using HYDRUS-1D were conducted. The results showed that the water table depth in all excavations in Wadi N increased during June compared to January-March by 24 times (from 0-6 cm to 130-145 cm), while the water salinity increased by 50% (from 4,563 mg/L to 9,164 mg/L). Unlike Wadi N, the water table in Wadi M remained steady (within 4.5-6 cm), while the water salinity of the locations 1 and 2 increased by nearly 40% to 22% (7,948-13,632 mg/L and 7,372-9,075 mg/L), respectively, whereas it decreased by nearly 30% in location 3 (from 9,177 to 6,374 mg/L). These variations in water chemistry suggest that the studied SWT is patchy and caps perched aquifers, which are likely to be disconnected hydraulically from each other and the regional aquifer located at the depth of about 15 m under the local ground surface. Considering the seasonal variation in weather conditions, the results of HYDRUS-1D simulations showed that the evaporation rate in June was higher by about 50% than that for January-March. The discharge from the water table due to evaporation from the whole area of Wadi N was estimated by the model to be around 6600 m3. Further studies should determine the source of groundwater that would make possible a hydrological balance for perched aquifers. This would drilling and installing nested piezometers with regular record of SWT and regional groundwater, geophysical surveys of the subsurface, detailed analysis of soil pedogenesis and morphology of the vadose zone, isotope analysis of groundwater, and detailed numerical simulations of groundwater and soil moisture motion

The impact of geological heterogeneity on horizontal well-triplet performance in CO2-circulated geothermal reservoirs

CO2 circulated geothermal production can be integrated with CO2 geological sequestration as a utilization method to offset cost. Investigation of heterogeneity impact is limited to CO2 sequestration and its effect on CO2 circulation and associated heat recovery is unclear. This study is aimed to improve the understanding of this problem by numerical experiments. A set of spatially correlated heterogeneous porosity fields is generated using a variety of geostatistical parameters, i.e., variance, correlation lengths, anisotropy and azimuth. Heterogeneous fields of intrinsic permeability and initial/residual water saturation are derived from porosity using equations regressed from a field dataset. Twenty combinations of injection pressure and well space obtained by Latin-Hypercube sampling are deployed in each heterogeneous field, generating a suite of numerical geothermal reservoir models. Performance indicators, including lifespan, net stored CO2 , produced heat flux, and total recovered heat energy in lifespan, are calculated from each model simulation. The simulation results suggest that geologic heterogeneity could develop high-permeable CO2 flow paths, causing bypass of the hot low-permeable zones, shortened lifespan and reduced total recovered heat energy. Depending on the azimuth, anisotropy can create either flow barriers or preferential flow paths, increasing or decreasing heat sweeping efficiency. The relative angle between horizontal wells and the axis of maximum continuity of the heterogeneity can be optimized to maximize heat recovery efficiency. These finds provide useful insights of interplay between geological heterogeneity, well placement and operation of CO2 circulated geothermal production.Cited as: Chen, M., Al-Saidi, A., Al-Maktoumi, A., Izady, A. The impact of geological heterogeneity on horizontal well-triplet performance in CO2-circulated geothermal reservoirs. Advances in Geo-Energy Research, 2022, 6(3): 192-205. https://doi.org/10.46690/ager.2022.03.0

Numerical evaluation of hydrogen production by steam reforming of natural gas

Industry-scale hydrogen is mainly produced by steam methane reforming (SMR), which uses natural gas as the feedstock and fuel and co-produces CO2. This study aims to numerically evaluate hydrogen production by SMR under various reacting conditions. Unlike the previous studies with limited scenarios, the performance of SMR is continuously evaluated in a high-dimensional input-parameter space. The SMR plant including a combustor, a reformer, and a water-gas shifter is modeled in Aspen HYSYS software. The four key parameters, including methane fraction of the feedstock, reformer pressure and temperature, and shifter temperature, are treated uncertain and 50 samples are drawn from a four-dimensional parameter space defined by their ranges. Each sample is input to HYSYS model and mass ratio of each component in product streams is obtained as the output variables. Based on the 50 pairs of input-output data, response surfaces of the outputs are developed to surrogate HYSYS models. The fast response surface models are then used to calculate global sensitivity indices and evaluate SMR processes. Results show the reformer performance is controlled by temperature rather than pressure, and a temperature higher than 900 °C can maximize the reaction rate. The water-gas shifting reaction is inhibited in the reformer but significantly enhanced in the shifter. Hydrogen is mainly produced in the reformer while the major function of the shifter is to convert CO to nontoxic CO2.Cited as: Chen, M., Al-Subhi, K., Al-Rajhi, A., Al-Maktoumi, A., Izady, A. Al-Hinai, A. Numerical evaluation of hydrogen production by steam reforming of natural gas. Advances in Geo-Energy Research, 2023, 7(3): 141-151. https://doi.org/10.46690/ager.2023.03.0

Modelign of seepage through embankments of earth-filled recharge dams in Oman: steady and transient regimes

Аналитически исследована линейная фильтрация через ядро плотины и примыкающие к нему сильнопористые присыпки. Течение в ядре анализируется с помощью конформного отображения пентагона в области годографа скорости на треугольник в физической плоскости

Moisture and temperature in a proppant-enveloped silt block of a recharge dam reservoir: Laboratory experiment and 1-D mathematical modelling

Mosaic 3-D cascade of parallelepiped-shaped silt blocks, which sandwich sand- lled cracks, has been discovered in the eld and tested in lab experiments. Controlled wetting-drying of these blocks, collected from a dam reservoir, mimics field ponding-desiccation conditions of the topsoil layer subject to caustic solar radiation, high temperature and wind, typical in the Batinah region of Oman. In 1-D analytical modelling of a transient Richards’ equation for vertical evaporation, the method of small perturbations is applied, assuming that the relative permeability is Avery-anov’s 3.5-power function of the moisture content and capillary pressure is a given (measured) function. A linearized advective dispersion equation is solved with respect to the second term in the series expansion of the moisture content as a function of spatial coordinates and time. For a single block of a nite thickness we solve a boundary value problem with a no- ow condition at the bottom and a constant moisture content at the surface. Preliminary comparisons with theta-, TDR- probes measuring the moisture content and temperature at several in-block points are made. Results corroborate that a 3-D heterogeneity of soil physical properties, in particular, horizontal and vertical capillary barriers emerging on the interfaces between silt and sand generate eco-niches with stored soil water compartments favourable for lush vegetation in desert conditions. Desiccation significantly increases the temperature in the blocks and re-wetting of the blocks reduces the daily average and peak temperatures, the latter by almost 15°C. This is important for planning irrigation in smartly designed soil substrates and sustainability of wild plants in the region where the top soil peak temperature in the study area exceeds 70°C in Summer but smartly structured soils maintain lash vegetation. Thee layer of dry top-blocks acts as a thermal insulator for the subjacent layers of wet blocks that may host the root zone of woody species

Control of Topology of Water Fluxes in Arid Agriculture: Amalgamation of Subsurface Irrigation, Managed Aquifer Recharge and Engineered Soil Substrat

We present the results of field-, farm- observations and experiments, as well as mathematical modeling of optimal control of descending, ascending and lateral water fluxes (viz., evapotranspiration, infiltration, seepage from/to subsurface emitters/drains, losses/gains from/to a deeper confined aquifer commingled via a leaky layer with the irrigated one, and water uptake by roots)

Potentiel du radar de pénétration du sol pour la caractérisation de la nappe phréatique peu profonde dans la région de Mnasra au Maroc

Morocco is a water-scarce country confronted with a severe dependence on rain-fed agriculture and dwindling groundwater reserves. Since 1995, new water regulation laws and management strategies have been promulgated providing a comprehensive framework for an integrated management. Moreover, water managers should have precise data on the current state of water tables depth in strategic aquifers. Unfortunately, the main source of these data are sporadic wells with no automated monitoring systems making the assessment of  water table dynamics, costly, time consuming and out-phased with decision maker needs. In this respect, this paper focuses on the capability of Ground Penetrating Radar to determine the depth of shallow water table in Mnasra region, located in the Gharb region of Morocco as a pilot study to generalize its use in the future for groundwater dynamic monitoring purposes in Morocco. The experiment was undertaken using Mala 800 MHz shielded antennas and was able to probe the depth of the upper fresh water table at 3.75 m deep in the Mnasra aquifer in semi-arid conditions. Data collected by GPR can be used as substitute for well logs to enhance the monitoring of water tables in stressed areas during droughts and excessive recharges during rainy season. Keywords : Ground Penetrating Radar, Shallow water table, Mnasra, Gharb Region, Mala 800 Mhz Antenna, Monitoring of water table.  Le Maroc est un pays à ressources en eau limitées confronté à une dépendance à l’agriculture pluviale et une diminution des réserves des eaux souterraines. Depuis 1995, de nouvelles lois de règlementation d’eau et des stratégies de gestion ont été promulguées fournissant un cadre global de gestion intégrée. Par ailleurs, les gestionnaires doivent disposer de données précises et actualisées sur le niveau des nappes stratégiques. Malheureusement, la principale source des données est celle des puits sporadiques sans systèmes de suivi automatisés rendant l’évaluation des dynamiques des nappes coûteuse, exigeante en temps et en déphasage avec les besoins des décideurs. Ainsi, cet article se focalise sur la capacité des sondeurs radar (GPR) à déterminer la profondeur de la nappe souterraine de Mnasra, zone pilote située dans la région du Gharb au Maroc pour généraliser son utilisation dans le suivi des nappes au Maroc. Il était possible de sonder, en utilisant des antennes Mala 800 MHz, le niveau de la nappe à une profondeur de 3.75 m dans des conditions semi-arides. Les données GPR collectées peuvent être utilisées comme substitut des puits pour rehausser le suivi des nappes dans les zones arides pendant les périodes de sécheresse et de recharge intense. Mots-clés : Sondeur Radar, nappe non profonde, Mnasra, Région de Gharb, Antenne Mala 800 Mhz, suivi des nappe

Potentiel du radar de pénétration du sol pour la caractérisation de la nappe phréatique peu profonde dans la région de Mnasra au Maroc

Morocco is a water-scarce country confronted with a severe dependence on rain-fed agriculture and dwindling groundwater reserves. Since 1995, new water regulation laws and management strategies have been promulgated providing a comprehensive framework for an integrated management. Moreover, water managers should have precise data on the current state of water tables depth in strategic aquifers. Unfortunately, the main source of these data are sporadic wells with no automated monitoring systems making the assessment of  water table dynamics, costly, time consuming and out-phased with decision maker needs. In this respect, this paper focuses on the capability of Ground Penetrating Radar to determine the depth of shallow water table in Mnasra region, located in the Gharb region of Morocco as a pilot study to generalize its use in the future for groundwater dynamic monitoring purposes in Morocco. The experiment was undertaken using Mala 800 MHz shielded antennas and was able to probe the depth of the upper fresh water table at 3.75 m deep in the Mnasra aquifer in semi-arid conditions. Data collected by GPR can be used as substitute for well logs to enhance the monitoring of water tables in stressed areas during droughts and excessive recharges during rainy season. Keywords : Ground Penetrating Radar, Shallow water table, Mnasra, Gharb Region, Mala 800 Mhz Antenna, Monitoring of water table.  Le Maroc est un pays à ressources en eau limitées confronté à une dépendance à l’agriculture pluviale et une diminution des réserves des eaux souterraines. Depuis 1995, de nouvelles lois de règlementation d’eau et des stratégies de gestion ont été promulguées fournissant un cadre global de gestion intégrée. Par ailleurs, les gestionnaires doivent disposer de données précises et actualisées sur le niveau des nappes stratégiques. Malheureusement, la principale source des données est celle des puits sporadiques sans systèmes de suivi automatisés rendant l’évaluation des dynamiques des nappes coûteuse, exigeante en temps et en déphasage avec les besoins des décideurs. Ainsi, cet article se focalise sur la capacité des sondeurs radar (GPR) à déterminer la profondeur de la nappe souterraine de Mnasra, zone pilote située dans la région du Gharb au Maroc pour généraliser son utilisation dans le suivi des nappes au Maroc. Il était possible de sonder, en utilisant des antennes Mala 800 MHz, le niveau de la nappe à une profondeur de 3.75 m dans des conditions semi-arides. Les données GPR collectées peuvent être utilisées comme substitut des puits pour rehausser le suivi des nappes dans les zones arides pendant les périodes de sécheresse et de recharge intense. Mots-clés : Sondeur Radar, nappe non profonde, Mnasra, Région de Gharb, Antenne Mala 800 Mhz, suivi des nappe

Deterioration of Layered Coastal Aquifer Water Due to Density Dependent Flows

Seawater intrusion in coastal aquifers threatens the development of coastal areas in many countries around the globe. In spite of the number of studies that have examined seawater intrusion in different heterogeneous coastal systems and hydrogeological settings, a situation where the coastal aquifer consists of two permeable layers separated by an aquitard with major abstraction from the upper aquifer (as is the case in the Al-Batinah area of Oman) has not been studied yet. This thesis focuses on such a situation and aims to extend existing knowledge about seawater intrusion dynamics and the consequent quality deterioration in such coastal aquifers. A good understanding of the seawater intrusion patterns is essential for improved management of coastal water resources. Shallow groundwater abstraction is expected to speed up seawater intrusion in the upper aquifer compared with that for the lower aquifer. As a consequence, an unstable condition with a complex flow and transport pattern develops. With time, a saline boundary layer (SBL) forms as salt is transported across the aquitard into the upper part of the lower aquifer, in time inducing instability and convective mixing. The system remains stable when the SBL is confined entirely within the aquitard. The extent and sensitivity of this unstable mixing and the consequential lower aquifer contamination are expected to depend on the hydrological and hydrogeological characteristics of the aquitard and of both upper and lower aquifers. Intensive two-dimensional (2D) numerical simulations have been performed for different aquitard properties and selected characteristics of upper and lower aquifers to gain an insight into the dynamics of the densitydriven flow. The impacts of different remedial strategies on the transport process are examined. The transport pattern in a three-dimensional (3D) system is also explored for selected cases. Both qualitative and quantitative measures have been used to analyse the results. These include selected salinity levels (standards for domestic and irrigation purposes), maximum depth of penetration (MDP), and total mass present in the lower aquifer (MtL). The literature presents several numerical codes for solving variable density groundwater flow and transport problems. In this thesis, a finite difference simulator SEAWAT-2000 is selected and sensitivity of its results to spatial and temporal discretizations and numerical solution techniques is first tested against the Elder-Voss and Souza benchmark problem (EVS). The results are shown to be sensitive to the level of spatial and temporal resolutions for the chosen numerical scheme. A high-resolution mesh is necessary to minimize incorrect seeding of fingers and consequential artificially induced salinization due to numerical errors. The numerical simulations show that the pattern of contaminant-spread from buoyancy induced mixing in the layered coastal aquifer has a complex structure, described as a moving zone of instability. The instability occurs first in the seaward part of the SBL and then moves landward. With time the seaward part of the system begins to stabilize as the vertical density gradient reduces, but instability develops in the landward direction. As a result, previously adopted dimensionless criteria such as Rayleigh Number (Ra), wavelength (@) and wave number (µ) are not readily suited to analyse system stability. Factors that affect the timing of instability onset include SBL thickness and its solute concentration (C), and the upward vertical (Vv) and horizontal flow velocities (VH) within the lower aquifer. Large upward Vv retards the formation of the SBL while large VH increases the dispersion term, which smooths out the initial perturbations and thus impedes the occurrence of the instability. Investigation of the influence of aquitard parameters shows that increases in some (e.g., aquitard dispersivity, and porosity) promote the occurrence of instability and intensify the convective mixing by promoting the growth of the SBL. On the other hand, decreases in some lower aquifer parameters (e.g., smaller lower aquifer hydraulic conductivity and dispersivity) enhance the growth of the initial perturbations of the SBL into fingers. The growth of the SBL alone does not imply occurrence of instability as large flow velocity or high dispersion within the lower aquifer can refresh the bottom part of the SBL, and hence impede the trigger of instability. Factors or conditions that speed up seawater intrusion within the lower aquifer decrease the degree of convective mixing. This is because the unstable vertical density contrast between the upper and lower aquifers does not then develop, and the system regains stability more rapidly compared with cases where the seawater intrusion in the lower aquifer is slow. A higher abstraction rate (Q) speeds up the seawater intrusion and promotes SBL development, and hence an early occurrence of instability. For a specific set of aquifer parameters, the time for the lower aquifer to become unsuitable for domestic or irrigation purposes shortens as the abstraction rate increases. Different management options like discontinuous abstraction, alternating abstraction from the two aquifers, and short-term abstraction regimes are examined for their effects on the convective mixing. For a management option to be effective in reducing the degree of convective mixing, the seawater intrusion in the upper aquifer should be retarded or reversed so that the salt supply to the SBL is reduced. Subjecting both aquifers to continuous abstraction of approximately equal rates results in relatively similar intrusion rates, thereby impeding the onset of instability. While the work in this thesis is not based on an actual case-study, the Al-Batinah region in Oman is used as a reference for model conceptualisation since seawater intrusion in layered coastal aquifers is of major concern there. Given that the rainfall rate is very small in such arid regions, natural recharge (direct from rainfall) is not effective in reversing seawater intrusion in a useful timeframe. However, artificial recharge (through injection wells) can improve the groundwater quality when abstraction is stopped. The dispersion zone in the upper aquifer decreases, thereby slowing the growth of fingers. If abstraction from the upper aquifer continues, obviously a higher recharge rate is needed to reduce the degree of the convective mixing as well as the seawater intrusion in the upper aquifer. When the lower aquifer is recharged, the fingers are smoothed of and the system can become stable even if abstraction continues. With higher recharge rates, the system becomes stable more quickly. Abstraction from the saline wedge is also investigated as a remediation strategy. With cessation of the inland abstraction, saline water abstraction retards the seawater intrusion and so diminishes the size of the developed fingers. Alternating inland abstraction and saline water abstraction does not assist in effective reduction of the convective mixing given that the isochlors recede toward the sea during saline water abstraction and intrude again during inland abstraction. For a management or remedial strategy to be effective in stabilizing, or reducing the intensity of, buoyancy-induced mixing in the lower aquifer, it must induce strong enough horizontal velocity, VH, in the lower aquifer (to eliminate the developed fingers and prevent perturbations at SBL bottom boundary), large upward velocity, Vv (to resist SBL development), or decreases in the seawater intrusion rate in the upper aquifer so that the unstable condition (dense fluid overlying less dense fluid) does not develop. Although the convective mixing patterns are different in 2D and 3D simulations, results show that there are similarities in the basic features of finger growth, coalescence, and their response to an increase in the abstraction rate. Based on the limited simulations, the 2D results are broadly similar to those of the 3D case and, with appropriate caution; 2D modelling can provide useful information for water resources management in the aquifer situations studied. The study provides an improved understanding of the fingering process in layered coastal aquifers and thereby contributes to the rational management of layered coastal aquifers