Volume Of Groundwater Research Articles

Field Demonstration of the Horizontal Treatment Well (HRX Well®) for Passive In Situ Remediation

AbstractThe horizontal reactive media treatment well (HRX Well®) uses directionally drilled horizontal wells filled with a treatment media to induce flow‐focusing behavior created by the well‐to‐aquifer permeability contrast to passively capture proportionally large volumes of groundwater. Groundwater is treated in situ as it flows through the HRX Well and downgradient portions of the aquifer are cleaned via elution as these zones are flushed with clean water discharging from the HRX Well. The HRX Well concept is particularly well suited for sites where long‐term mass discharge control is a primary performance objective. This concept is appropriate for recalcitrant and difficult‐to‐treat constituents, including chlorinated solvents, per‐ and polyfluoroalkyl substances (PFAS), 1,4‐dioxane, and metals. A full‐scale HRX Well was installed and operated to treat trichloroethene (TCE) with zero valent iron (ZVI). The model‐predicted enhanced flow through the HRX Well (compared to the flow in and equivalent cross‐sectional area orthogonal to flow in the natural formation before HRX Well installation) and treatment zone width was consistent with flows and widths estimated independently by point velocity probe (PVP) testing, HRX Well tracer testing, and observed treatment in downgradient monitoring wells. The actual average capture zone width was estimated to be between 45 and 69 feet. Total TCE mass discharge reduction was maintained through the duration of the performance monitoring period and exceeded 99.99% (%). Decreases in TCE concentrations were observed at all four downgradient monitoring wells within the treatment zone (ranging from 50 to 74% at day 436), and the first arrival of treated water was consistent with model predictions. The field demonstration confirmed the HRX Well technology is best suited for long‐term mass discharge control, can be installed under active infrastructure, requires limited ongoing operation and maintenance, and has low life cycle energy and water requirements.

Risk Control of Karst Collapse----A Case Study of Linyi City

Karst collapse is a common geological disaster, and groundwater level is the main influencing factor of karst collapse in Linyi City. This article uses the established groundwater flow model to propose a method for controlling karst collapse: to make the karst water level not lower than the control level. Based on this method, three different groundwater extraction schemes were designed in the study area, and calculation analysis was performed in conjunction with water level control points. The results show that in the future, the amount of karst water exploitation in the urban area of Linyi City will be reduced by 11.65×104m3/d compared with the current situation, The reduced groundwater volume will be solved by the surface water project of the Andi reservoir in Linyi City, which can control the danger of karst collapse.

Spatiotemporal Analysis of Extracted Groundwater Volumes Estimated from Electricity Consumption.

Land subsidence caused by groundwater overexploitation is a serious global problem. The acquisition of spatiotemporal pumping rates and volumes is a first step for water managers to develop a strategic plan for mitigating land subsidence. This study investigates an empirical formulation to estimate the monthly maximum pumped volume over a 10-year period based on electric power consumption data. A spatiotemporal variability analysis of monthly pumped volume is developed to provide an improved understanding of seasonal pumping patterns and the role of irrigation type. The analysis of regional pumped volume provides an approximation of the spatiotemporal patterns of the variations in pumped volume. Results show the effects of climate, seasonal changes in pumping from irrigation, and the local differences in pumping caused to crop types. A seasonal pumped volume peak occurs annually, with the highest and least pumped volumes occurring in March (highest peak) and September (lowest peak), respectively. However, the majority of the historical maximum pumped volumes have occurred during the last few years. Extracted volumes continue to increase in some locations. The analysis reveals increasing trends in pumping, thereby possibly providing the locations where increased effective stresses may lead to land subsidence.

Hydrogeological evaluation of managed aquifer recharge in a glacial moraine complex using long-term groundwater data analysis

Managed aquifer recharge (MAR) is a strategy to address dwindling water availability that is used to recharge stressed groundwater systems for recovery or adaptation purposes. Glacial moraine complexes can host large groundwater volumes in thick coarse-grained sandy and gravelous aquifers, and therefore are often suitable for MAR. In this research, the impact of 20 years of MAR (infiltration) in the Veluwe glacial moraine complex in the Netherlands is evaluated through time-series analysis and water quality modeling. The research enhances the understanding of hydrological processes in a glacial moraine complex and thus supports effective MAR design. The results show that MAR in the Veluwe area has raised nearby groundwater heads, and that 20 years of infiltration did not significantly deteriorate the groundwater quality. In addition, the analysis revealed an unexpected hydraulic interaction between the glacial complex and the underlying aquifer. The results suggest that MAR in a glacial moraine complex can be an effective strategy for storage of surplus surface water, compensation of groundwater abstraction, or water quality improvement. Monitoring and evaluation of groundwater heads and quality will improve the understanding of the hydrology and hydrochemistry of the water system and the used water resource, which is essential to design effective MAR systems.

A prolific aquifer system is in peril in arid Kachchh region of India

The Bhuj Sandstone, a lithostratigraphic unit of Late Mesozoic - Early Cretaceous age, forms a prolific aquifer system in the Kachchh region of Gujarat State, known for acute water shortage and groundwater dependence. The region forms a part of the great arid zone of western India, marked with low rainfall (annual average 376 mm), which is erratic in space and time. The present research, for the first time, has dealt on the groundwater regime and aquifer configuration of the Bhuj Sandstone, also referred as Bhuj Aquifer. A detailed inventory of water levels in post monsoon 2018 and its comparison with 2014 reveals an average fall of 20.62 m during the period. The area is experiencing intensive groundwater mining. Between 2014 and 2018, the net volume of groundwater mined annually is 955 mcm. Considering 104 mcm as annual recharge in 2018, the total groundwater extraction comes out as 1059 mcm, which is more than 10 times of the annual recharge. To tackle such a dismal scenario a host of interventions are urgently needed, which are mentioned in the paper.

Calculating man-made depletion of a stressed multiple aquifer resource on a national scale

An inexorable depletion of groundwater occurs where groundwater abstraction exceeds the natural recharge, a typical state of (semi-)arid regions, which calls for sustainable management of groundwater resources. This study aims to assess the available storage and recharge rates on a national scale in time and space by modelling the natural recharge in combination with a method to evaluate changing groundwater volumes, which revealed measures to quantify the overdraft of the observed national groundwater resources in Jordan. Applying the combination of hydrological model and method to evaluate changing groundwater volumes, a climate-driven systematic decline of groundwater recharge was eliminated as responsible process, while overdraft leads to dropping groundwater tables.The major findings are, the intensity of groundwater abstraction from a basin becomes visible through the fact, that simulated baseflow exceeds significantly the observed baseflow. About 75% of Jordan's groundwater basins are subject to intense groundwater depletion, reaching annual rates of up to 1 m in some basins. The most affected areas are the basins Zarka, Azraq and the predominantly fossil groundwater reservoirs in Southern Jordan.Contrasting the past, when variable annual precipitation patterns did not negatively influence groundwater recharge, simulations show significantly reduced annual groundwater recharge all over Jordan. Particularly affected is the agricultural backbone in the Jordan Mountains, where recharge rates are predicted to vary between −30 mm/yr and +10 mm/yr in the coming decades, being reflected in the disappearance of freshwater springs and ascending saltwater. The applied methodology is relevant and transferable to other data- and water scarce areas worldwide, allowing (i) a fast estimation of groundwater reservoir development on a national scale and (ii) an investigation of long-term effects of overdraft.

Numerical Treatment of a Two-Dimensional Vertically-Averaged Groundwater Flow Model Using Alternating Direction Explicit Methods

Leachate from a landfill can flow down and contaminate groundwater. Mathematical models are often used to describe groundwater flow, which can help designers to identify an appropriate location for a landfill area. We focused on simulation of hydraulic head of groundwater under landfill construction in a rural area. We used a partial differential equation model called two-dimensional vertically-averaged groundwater flow model to explain groundwater volume, in particular, to explain the hydraulic head of the groundwater which strongly affected the groundwater quality. We applied two alternating direction explicit methods to the groundwater flow model under general boundary conditions and rates of change of boundary conditions to obtain an approximate hydraulic head of groundwater. An alternating direction explicit method for all types of boundaries is used to find approximate solutions. The traditional alternative direction explicit method was modified to form our modified alternative direction explicit method. The results of all method are closed. The traditional alternative direction explicit method and the modified alternative direction explicit method led to more accurate solutions than forward time centered space in heterogeneous aquifer, two methods ensured that a stable solution of all interior points can be obtained by using a fine grid spacing.

Earthquake effects on artificial groundwater recharge efforts in south Japan

The Kumamoto area includes 11 municipalities and a population of about one million. The area represents the largest total use of groundwater in Japan. The main recharging area for this groundwater used to be paddy fields for rice production located along the mid-stream section of the Shirakawa River. In the past few decades, the area of paddy fields has rapidly decreased due to the Japanese government’s rice production adjustment policy and urbanization. In consequence, the groundwater recharge decreased from 656.2 million m3 in 1992 to 606.9 million m3 in 2006. Thus, groundwater recharge from paddy fields only might not be sufficient for groundwater demand in the future. For this reason, an artificial recharge system was established to increase groundwater recharge. In this study, we review the history of groundwater management and results of 14 years’ operation of a large-scale artificial groundwater recharge project in the Kumamoto area. We visualize the resilience of the groundwater management and recharge project by influence of the 2016 Kumamoto Earthquake. It is shown that through an integrated approach of all societal groundwater stakeholders, a sustainable groundwater management can be achieved. The hydrological methods in this integrated approach quantify recharging groundwater volumes for different types of recharge fields by use of 170 local experimental observations. Results and experiences outlined in this paper can be used by planners and managers of dwindling groundwater resources to build resilient systems for groundwater recharge by involving all societal stakeholders through an integrated approach.

Reply to Discussion

AbstractIn the Discussion, Nash et al. (2019) estimate the seasonal change in groundwater volume for a portion of the restored Sierra Nevada Meadow that we evaluated (Hunt et al. 2018) and use this estimate as an upper bound on the possible contribution to flow that is attributable to restoration. The authors conclude that raising the channel bed elevation and reconnecting the meadow floodplain most likely reduced summer streamflow. In contrast, we report at least a fivefold increase in baseflow from the meadow in the years following restoration. In addition, we observed that, after restoration, the previously intermittent stream below the meadow flowed continuously throughout the summer months, despite record drought conditions, and in 2015, the lowest snowpack on record. We suggest that the groundwater budget presented in the Discussion may not adequately represent conditions within the meadow because the authors extrapolate from 5 near‐channel groundwater wells across 62 ha of meadow and assume an area of influence that is approximately one‐sixth of the meadow area. We conclude that the conversion of an intermittent stream to perennial flow during drought conditions is a stronger check on our gauge data than the groundwater budget presented in the Discussion.

Geological Heterogeneity of Coastal Unconsolidated Groundwater Systems Worldwide and Its Influence on Offshore Fresh Groundwater Occurrence

Numerous coastal areas worldwide already experience fresh water shortages due to overexploitation and salt water intrusion. Future climate change and population growth will further intensify this threat in more areas in coming decades. Therefore, it is necessary to explore any potential fresh water source, such as offshore fresh groundwater, that could alleviate this fresh water shortage and provide valuable time for adaptation measures implementation and changes in water management strategies. Recent evidence suggests that a disproportionally large portion of human population living in coastal areas relies on groundwater resources stored in underlying unconsolidated groundwater systems. These systems are often very heterogeneous, combining numerous high permeability aquifers interlaid with low permeability aquitards with varying total thickness. This heterogeneity is a major control on the fresh groundwater volume and groundwater salinity distribution within such systems. Thus, the quantification of geological heterogeneity is often the limiting factor when estimating fresh groundwater volumes, both inland and offshore, along the global coastline. To overcome this obstacle, we combine conceptual geological models with available state-of-the-art global datasets to derive a set of geological heterogeneity parameter distributions quantifying geological heterogeneity of coastal unconsolidated groundwater systems as formed over last 1 Ma. These are then used in an algorithm designed to build synthetic heterogenic parameterizations of coastal unconsolidated groundwater systems along the global coastline. These, in turn, provide key input for modelling variable-density groundwater flow and coupled salt transport to analyze changes in groundwater salinities and offshore fresh groundwater volume. Such an analysis is performed over one full glacial-interglacial cycle (the last 0.13 Ma) to account for oscillating sea-level conditions and shifts in coast-line positions and salinity incursions. Our simulation results show a close match between the modelling scenarios and values presented by literature sources demonstrating the potential of the hereby presented methodology to be applied in similar future studies.

A Dual Delineation Approach to Untangle Interferences in Well Doublets Systems.

Traditional numerical methods for the delineation of wellhead protection areas span deterministic and probabilistic approaches. They provide time-related capture zones. However, none of the existing approaches identifies the groundwater contribution areas related to each source or sink. In this work, the worthiness of the so-called double delineation approach was extended. This task was achieved by simple postprocessing of its dual outputs leading to a highly efficient screening tool. In the particular context of geothermal resources management through the well doublets of the Dogger aquifer in the Paris Basin (France), the approach was extended to forecast the compositional heat breakthrough at production wells. Hence, cold-water breakthrough and temperature decline in production wells are timely assessed in low-enthalpy geothermal reservoirs. The method quantifies how groundwater volumes are moving through space and time between any couple of source and sink. It provides unprecedented tools advancing the enhanced understanding of water resources systems functioning. It is highly recommended to implement the presented concepts in the current and future generations of community groundwater models.

Seasonal and Long-Term Groundwater Unloading in the Central Valley Modifies Crustal Stress.

Changes in terrestrial water content cause elastic deformation of the Earth's crust. This deformation is thought to play a role in modulating crustal stress and seismicity in regions where large water storage fluctuations occur. Groundwater is an important component of total water storage change in California, helping to drive annual water storage fluctuations and loss during periods of drought. Here we use direct estimates of groundwater volume loss during the 2007–2010 drought in California's Central Valley obtained from high resolution Interferometric Synthetic Aperture Radar‐based vertical land motion data to investigate the effect of groundwater volume change on the evolution of the stress field. We show that GPS‐derived elastic load models may not capture the contribution of groundwater to terrestrial water loading, resulting in an underestimation of nontectonic crustal stress change. We find that groundwater unloading during the drought causes Coulomb stress change of up to 5.5 kPa and seasonal fluctuations of up to 2.6 kPa at seismogenic depth. We find that faults near the Valley show the largest stress change and the San Andreas fault experiences only ~40 Pa of Coulomb stress change over the course of a year from groundwater storage change. Annual Coulomb stress change peaks dominantly in the fall, when the groundwater level is low; however, some faults experience peak stress in the spring when groundwater levels are higher. Additionally, we find that periods of increased stress correlate with higher than average seismic moment release but are not correlated with an increase in the number of earthquakes. This indicates groundwater loading likely contributes to nontectonic loading of faults, especially near the Valley edge, but is not a dominant factor in modulation of seismicity in California because the amplitude of stress change declines rapidly with distance from the Valley. By carefully quantifying and spatially locating groundwater fluctuations, we will improve our understanding of what drives nontectonic stress and forces that modulate seismicity in California.

Spent coal mines using for the industrial wastewater disposal

Industrial wastewaters disposal, in particular wastewaters appearing from the coal extraction and enrichment, is an important national economic task of preserving environment. At the same time, the applied wastewater disposal technologies must be ecologically safe and do not requires large financial expenses. This task is quite relevant for Kuzbass due to territory saturation of the coal, metallurgical and chemical industrial enterprises. The paper discusses a mathematical model of one of these technologies -the wastewater treatment in spent workings of closed and flooded coal mines. The proposed mathematical model allows studying the treatment processes occurring in the flooded mine workings, that using numerical experiments and taking into account the slurry water characteristics and changes of the incoming groundwater volume and of the injected wastewater volume. The process of suspended in mine waters coal and rock particles sedimentation and sediment compaction is given special attention in the model. Numerical experiments results, that allowing estimating the safe time mine act as a wastewater treatment plant, are presented.

Estimating groundwater recharge for a freshwater lens in an arid region: Formative and stability assessment

AbstractThe formation of subsurface freshwater lenses on top of brackish groundwater is a fascinating hydrologic phenomenon that creates groundwater supplies of great potential value in arid regions. Information on the recharge quantity and mechanism of these lenses is both scarce and uncertain. This study examines the formation and macroscale stability of the Rawdatain freshwater lens in Kuwait, for which significant pre‐development data are available. The Rawdatain is a large (150 million m3) subsurface freshwater lens overlying brackish groundwater compared to the other freshwater lenses in the Arabian Peninsula. In this study, a three‐dimensional (3D) density‐dependent groundwater flow model is tested against the following data targets to estimate long‐term diffuse and focused groundwater recharge: (i) groundwater head, (ii) total dissolved solids (TDS) groundwater concentration, (iii) volume and vertical thickness of stored groundwater of three different water quality TDS ranges (0–700, 700–1000 and 1000–2000 mg/L) and (iv) geometrical shape features of the lens along cross‐sections. To better represent the spatial variation in TDS, six different recharge zones were assigned to allocate diffuse and focused recharge conditions. Twelve recharge rate scenarios, encompassing a wide range of feasible long‐term average annual recharge values (200,000–5,000,000 m3/year), were tested against the multiple targets and compared with the groundwater age of the Rawdatain lens. Based on comparison with data targets, the long‐term average annual recharge is estimated to be 500,000 m3/year. Scenarios of reduced recharge, which may occur due to changes in land‐use or climate, demonstrate the extremely slow response of the lens, which is in agreement with the slow development and formation of the lens (>2,000 years). Within a 100‐year time frame, a 50% reduction in annual recharge reduces the lens volumes by 21, 17 and 9% for the three water quality categories, respectively. This study demonstrates the stability of freshwater lenses in arid regions and also provides methodology for similar focused rainfall recharge freshwater lenses.

A three-dimensional palaeohydrogeological reconstruction of the groundwater salinity distribution in the Nile Delta Aquifer

Abstract. Holocene marine transgressions are often put forward to explain observed groundwater salinities that extend far inland in deltas. This hypothesis was also proposed in the literature to explain the large land-inward extent of saline groundwater in the Nile Delta. The groundwater models previously built for the area used very large dispersivities to reconstruct this saline and brackish groundwater zone. However, this approach cannot explain the observed freshening of this zone. Here, we investigated the physical plausibility of the Holocene-transgression hypothesis to explain observed salinities by conducting a palaeohydrogeological reconstruction of groundwater salinity for the last 32 ka with a complex 3-D variable-density groundwater flow model, using a state-of-the-art version of the SEAWAT computer code that allows for parallel computation. Several scenarios with different lithologies and hypersaline groundwater provenances were simulated, of which five were selected that showed the best match with the observations. Amongst these selections, total freshwater volumes varied strongly, ranging from 1526 to 2659 km3, mainly due to uncertainties in the lithology offshore and at larger depths. This range is smaller (1511–1989 km3) when we only consider the volumes of onshore fresh groundwater within 300 m depth. In all five selected scenarios the total volume of hypersaline groundwater exceeded that of seawater. We also show that during the last 32 ka, total freshwater volumes significantly declined, with a factor ranging from 2 to 5, due to the rising sea level. Furthermore, the time period required to reach a steady state under current boundary conditions exceeded 5.5 ka for all scenarios. Finally, under highly permeable conditions the marine transgression simulated with the palaeohydrogeological reconstruction led to a steeper fresh–salt interface compared to its steady-state equivalent, while low-permeable clay layers allowed for the preservation of fresh groundwater volumes. This shows that long-term transient simulations are needed when estimating present-day fresh–salt groundwater distributions in large deltas. The insights of this study are also applicable to other major deltaic areas, since many also experienced a Holocene marine transgression.

Estimation of groundwater potential using GIS modeling in Kohistan region Jamshoro district, Southern Indus basin, Sindh, Pakistan (a case study)

The subsurface groundwater potential was calculated through the Geographic Information System (GIS) tool in district Jamshoro, Sindh, Pakistan; research was carried out in the vulnerable and arid climatic regions of the study area, which experienced unpredictable droughts and extreme floods. The primary recharging source of the aquifer is rainfall only in monsoon season from July to September. The current situation of water resources is critical in the region; in the present research work, two principal groundwater aquifers were encountered present in lithology of Laki formation mostly in sand and gravel lithology, which are shallow and deep aquifers. The GIS has been employed as a preservative tool to estimate groundwater potential. The estimated groundwater potential was 29,330.22 million cubic meter (MCM) under the monitoring area of 3207.25 square kilometers (km2); thus, the volume of groundwater was 9.14 MCM/km2 in the selected cluster zone of the study area. It is realized that using a GIS tool combats the susceptible situation and shows authenticated results of groundwater potential.

Satellite-based monitoring of groundwater depletion in California\u2019s Central Valley

Range change data, obtained from Synthetic Aperture Radar satellites, form the basis for estimates of aquifer volume change in California’s Central Valley. The estimation algorithm incorporates a function penalizing changes far from known well locations, linking the aquifer volume changes to agricultural, industrial, and municipal pumping within the Tulare basin. We show that the range changes are compatible with the hypothesis that the source of aquifer volume changes are variations in effective pressure around documented wells. Specifically, inclusion of the well distance penalty does not degrade the fit to the observations, inversions with and without it both give variance reductions of 99.6%. The patterns of aquifer volume change vary significantly from the drought year, between October 2015 and October 2016, to a wet year in 2017, and into 2018, a year with near average rainfall. The 2.3 million acre-feet of estimated volume reduction, a lower bound on the amount of water extracted from the basin between October 2015 and 2016, agrees with independent estimates of 1.8 and 2.3 million acre-feet. The aquifer volume reduction is also compatible with a loss of 3.1 km3 (2.5 million acre-feet) in groundwater volume derived from Gravity Recovery and Climate Experiment (GRACE) satellite data.

The use of noble gas isotopes to constrain subsurface fluid flow and hydrocarbon migration in the East Texas Basin

The migration of hydrocarbons from source-rock to reservoir is a critical stage in petroleum system evolution. However, as migration occurs along confined pathways over geologically rapid timescales it is challenging to place quantitative constraints on migration behaviour in terms of interaction with other basinal fluids, flux, and lengthscales associated with this process. Due to their inert nature, noble gases are ideal tracers of fluid provenance and physical processes in a variety of subsurface systems, and thus can serve as a potential geochemical indicator of migration distance. The recent proliferation of unconventional hydrocarbon production (i.e., production directly from source-rocks) allows for the analysis of fluids from these strata. Here we present noble gas data from natural gas samples produced from 27 related conventional and unconventional wells within the East Texas basin. These data enable characterisation of fluids during migration from source-rock to conventional reservoir. Unconventional gases and fluids are produced from the Haynesville shale (n = 8), and conventional gases and fluids are sourced from the overlying Cotton Valley (n = 5), Travis Peak (n = 9), and James Lime (n = 5) formations. Samples consist primarily of methane (>70%), with small contributions from longer-chain hydrocarbons, and other gases including CO2 and N2. Atmosphere-derived 36Ar, which is introduced into natural gas accumulations via interaction with groundwater during hydrocarbon generation and migration, is consistently higher in samples that have undergone a greater migration distance from the source interval. Thus, calculated volume ratios of gas-water interaction (Vg/Vw) show a far greater incidence of groundwater interaction in migrated samples. Radiogenically-produced 40Ar is also consistently elevated in migrated samples, despite these reservoirs being located in geologically younger strata. We derive the representative volume of rock required to produce the observed radiogenic 40Ar in each sample and show that samples that have migrated further have acquired radiogenic isotopes from larger volumes of rock. Furthermore, our volumetric parameters derived from atmospheric and radiogenic isotopes are consistent with a simple conceptual model of migration. We quantitatively apply this model to demonstrate that migration occurs along relatively localised pathways. Helium (3He/4He) and argon (40Ar/36Ar) isotopes are correlated and show elevated amounts of mantle-derived 3He and radiogenic 40Ar in migrated samples. We interpret this to represent mixing between a pristine source-rock signature and an endmember characterised by elevated 3He/4He and 40Ar/36Ar, likely representative of mantle-enriched groundwater circulating in the wider hydrogeological basin. Finally, we use an advection-diffusion model to show that enriched mantle 3He in the shallower strata and towards the southern edge of the basin can be explained by influx of mantle helium along the fault-bounded southern edge of the basin over timescales of millions of years. Together these approaches represent the first complete noble gas based characterization of a hydrocarbon system, sampling migration from source-rock to trap. We show conclusively that both atmospheric and radiogenic noble gases are entrained in the hydrocarbon phase during migration and that concentrations scale proportionally with migration distance, as greater volumes of groundwater and host rock are encountered.

Contribution to the hydrogeology of Six Hills sandstone aquifer in East El-Oweinat area, south Western desert, Egypt

El-Oweinat area is located in southwestern Egypt and is considered to be one of the new land reclamation projects in the Western Desert. The Nubian Sandstone aquifer has high potentiality and good groundwater quality. The results of geologic and hydrogeologic studies reveal that the Six Hills sandstone aquifer represents the sole groundwater resource used for all purposes (agriculture, drinking, domestic, livestock and poultry) in East El-Oweinat area. The Six Hills sandstone aquifer overlies directly the Precambrian basement rocks. The concerned aquifer exists under unconfined conditions as it is exposed on the surface. The hydrogeological cross sections show that the fully saturated thickness of the Six Hills sandstone aquifer ranges between 150.2 m and 651 m and increases towards the west. The groundwater generally flows towards the northeast direction with an average hydraulic gradient of 0.6 ‰. The calculated groundwater volume of the Six Hills sandstone aquifer in East El-Oweinat area (4,340 km2) reaches 350 bcm of fresh water. The comparison of the depth to water in the same monitoring wells during 14 years (2003 and 2016) reveals that the head decline rate in groundwater depths were ranging between 5 cm/year and 80 cm/year. The pumping rate increased from 600,000 m3/day in year 2003 to 3,600,000 m3/day in 2016. The average transmissivity attains 2,060 m2/day reflecting the high potential of the Six Hills sandstone aquifer in East El-Oweinat area. The groundwater flow model (MODFLOW) has been used to investigate the impact of groundwater withdrawal on groundwater levels for sustainable groundwater management. Four scenarios were applied to predict the probable head changes in the Six Hills sandstone aquifer and their impact on the availability of groundwater. The fourth scenario is recommended in order to sustain the groundwater resources in the study area and keep the drawdown rates in the range of 0.66 m/year through reducing the present discharging rates (10,000 m3/day/well) by about 40%.

A Modeling Approach for Assessing Groundwater Resources of a Large Coral Island under Future Climate and Population Conditions: Gan Island, Maldives

This study assesses the future groundwater supply of a large coral island, Gan Island, Republic of Maldives, under influences of rainfall patterns, sea level rise, and population growth. The method described in this paper can be used to estimate the future groundwater supply of other coral islands. Gan is the largest inhabited island (598 ha) of the Republic of Maldives with a population of approximately 4500. An accurate estimate of groundwater supply in the coming decades is important for island water security measures. To quantify future groundwater volumes in Gan, a three-dimensional, density-dependent groundwater and solute transport model was created using the SUTRA (Saturated Unsaturated Transport) modeling code. The Gan model was tested against observed groundwater salinity concentrations and then run for the 2012–2050 period to compare scenarios of future rainfall (from General Circulation Models), varying rates of population growth (i.e., groundwater pumping), and sea level rise. Results indicate that the total fresh groundwater volume increases approximately 20% if only future rainfall patterns are considered. If moderate pumping is included (2% annual population growth rate), the volume increases only by 13%; with aggressive pumping (9% annual population growth rate), the volume decreases by 24%. Sea level rise and associated shoreline recession leads to an additional 15–20% decrease in lens thickness and lens volume. Results can be used to make decisions about water resource management on Gan and other large coral islands in the Indian and Pacific Oceans. Methods used herein can be applied to any coral island to explore future groundwater security.