Volumes Of Fresh Groundwater Research Articles

Sustainability of fresh groundwater resources in fifteen major deltas around the world

Population growth, urbanization and intensification of irrigated agriculture in the world’s deltas boost the demand for fresh water, with extensive groundwater extraction as a result. This, in turn, leads to salt water intrusion and upconing, which poses a threat to freshwater and food security. Managing fresh groundwater resources in deltas requires accurate knowledge about the current status and behaviour of their fresh groundwater resources. However, this knowledge is scarcely present, especially for groundwater at larger depths. Here, we use three-dimensional variable-density groundwater model simulations over the last 125 ka to estimate the volume of fresh groundwater resources for 15 major deltas around the world. We estimate current volumes of onshore fresh groundwater resources for individual deltas to vary between 1010 m3 and 1012 m3. Offshore, the estimated volumes of fresh groundwater are generally smaller, though with a considerably higher variability. In 9 out of 15 simulated deltas, fresh groundwater volumes developed over thousands of years. Based on current groundwater extraction and recharge rates, we estimate the time until in-situ fresh groundwater resources are completely exhausted, partly leading to groundwater level decline and mostly replacement with river water or saline groundwater. This straightforward analysis shows that 4 out of 15 deltas risk complete exhaustion of fresh groundwater resources within 300 m depth in 200 years. These deltas also suffer from saline surface water which means their groundwater resources will progressively salinize. With a fourfold increase in extraction rates, seven deltas risk a complete exhaustion within 200 years. Of these seven deltas, six suffer from saline surface water. We stress that the groundwater of these six vulnerable deltas should be carefully managed, to avoid non-renewable groundwater use. The progressive exhaustion of fresh groundwater resources in these deltas will hamper their ability to withstand periods of water scarcity.

The three-dimensional groundwater salinity distribution and fresh groundwater volumes in the Mekong Delta, Vietnam, inferred from geostatistical analyses

Abstract. Over the last decades, economic developments in the Vietnamese Mekong Delta have led to a sharp increase in groundwater pumping for domestic, agricultural and industrial use. This has resulted in alarming rates of land subsidence and groundwater salinization. Effective groundwater management, including strategies to work towards sustainable groundwater use, requires knowledge about the current groundwater salinity distribution, in particular the available volumes of fresh groundwater. At the moment, no comprehensive dataset of the spatial distribution of fresh groundwater is available. To create a 3D model of total dissolved solids (TDS), an existing geological model of the spatial distribution and thickness of the aquifers and aquitards is updated. Next, maps of drainable porosity for each aquifer are interpolated based on the sedimentological description of the borehole data. Measured TDS in groundwater, inferred TDS from resistivity measurements in boreholes and soft incomplete data (derived from measurements in boreholes and data from domestic wells) are combined in an indicator kriging routine to obtain the full probability distribution of TDS for each (x,y,z) location. This statistical distribution of TDS combined with drainable porosity yields estimates of the volume of fresh groundwater (TDS < 1 g L−1) in each aquifer. Uncertainty estimates of these volumes follow from a Monte Carlo analysis (sequential indicator simulation). Results yield an estimated fresh groundwater volume for the Mekong Delta of 867 billion cubic metres with an uncertainty range of 830–900 billion cubic metres, which is somewhat higher than previous assessments of fresh groundwater volumes. The resulting dataset can for instance be used in groundwater flow and salt transport modelling as well as aquifer storage and recovery projects to support informed groundwater management decisions, e.g. to prevent further salinization of the Mekong Delta groundwater system and land subsidence, and is available at https://doi.org/10.5281/zenodo.4441776 (Gunnink et al., 2021).

Development of fresh groundwater lens in coastal reclaimed islands

Groundwater serves as a significant freshwater supply for coastal regions. In many coastal areas, extensive islands have been created by filling the sea. These reclaimed islands not only provide valuable space for urban development, but also can serve as valuable aquifers. Recharge of fresh water from rainfall can flush out salt water and eventually create a freshwater lens beneath the reclaimed island. However, how a fresh groundwater lens grows in reclaimed islands and the factors that control its growth are unclear. Here we use a series of three-dimensional density-dependent flow and transport models to investigate the process of fresh-salt water mixing and quantify the potential maximum volume of fresh groundwater that can occur beneath reclaimed islands. Our results show that the recharge rate controls the growth rate and the initialization of the fresh groundwater lens, whereas the permeability of the underlying fill materials and bedrock determines the shape of freshwater lens beneath the island but has a minor influence on freshwater volume. Increasing porosity enlarges the total water volume but diminishes the proportion of fresh water as the larger volume of original salt water in the aquifer requires more recharge to flush and dilute; longitudinal dispersivity promotes the fresh-salt water circulation in a certain range but limits the maximum fresh volume storage, and vertical transverse dispersivity facilitates extending mixing areas to enlarge the freshwater lens. Heterogeneity in the hydraulic conductivity field increases the variability of groundwater flow, restricts the average speed of fresh-salt water circulation, and results in a decrease in the growth rate and maximum volume of the freshwater lens. Our study has large implications for planning and management of reclaimed lands for water sustainability in the future in coastal areas.

Rainfall induced groundwater mound in wedge-shaped promontories: The Strack–Chernyshov model revisited

An analytical solution to the Poisson equation governing Strack's discharge potential (squared thickness of a saturated zone in an unconfined aquifer) is obtained in a wedge-shaped domain with given head boundary conditions on the wedge sides (specified water level in an open water body around a porous promontory). The discharge vector components, maximum elevation of the water table in promontory vertical cross-sections, quantity of groundwater seeping through segments of the wedge sides, the volume of fresh groundwater in the mound are found. For acute angles, the solution to the problem is non-unique and specification of the behaviour at infinity is needed. A “basic” solution is distinguished, which minimizes the water table height above a horizontal bedrock. MODFLOW simulations are carried out in a finite triangular island and compare solutions with a constant-head, no-flow and “basic” boundary condition on one side of the triangle. Far from the tip of an infinite-size promontory one has to be cautious with truncation of the simulated flow domains and imposing corresponding boundary conditions. For a right and obtuse wedge angles, there are no positive solutions for the case of constant accretion on the water table. In a particular case of a confined rigid wedge-shaped aquifer and incompressible fluid, from an explicit solution to the Laplace equation for the hydraulic head with arbitrary time-space varying boundary conditions along the promontory rays, essentially 2-D transient Darcian flows within the wedge are computed. They illustrate that surface water waves on the promontory boundaries can generate strong Darcian waves inside the porous wedge. Evaporation from the water table and sea-water intruded interface (rather than a horizontal bed) are straightforward generalizations for the Poissonian Strack potential.

Salinity of deep groundwater in California: Water quantity, quality, and protection

Deep groundwater aquifers are poorly characterized but could yield important sources of water in California and elsewhere. Deep aquifers have been developed for oil and gas extraction, and this activity has created both valuable data and risks to groundwater quality. Assessing groundwater quantity and quality requires baseline data and a monitoring framework for evaluating impacts. We analyze 938 chemical, geological, and depth data points from 360 oil/gas fields across eight counties in California and depth data from 34,392 oil and gas wells. By expanding previous groundwater volume estimates from depths of 305 m to 3,000 m in California's Central Valley, an important agricultural region with growing groundwater demands, fresh [<3,000 ppm total dissolved solids (TDS)] groundwater volume is almost tripled to 2,700 km(3), most of it found shallower than 1,000 m. The 3,000-m depth zone also provides 3,900 km(3) of fresh and saline water, not previously estimated, that can be categorized as underground sources of drinking water (USDWs; <10,000 ppm TDS). Up to 19% and 35% of oil/gas activities have occurred directly in freshwater zones and USDWs, respectively, in the eight counties. Deeper activities, such as wastewater injection, may also pose a potential threat to groundwater, especially USDWs. Our findings indicate that California's Central Valley alone has close to three times the volume of fresh groundwater and four times the volume of USDWs than previous estimates suggest. Therefore, efforts to monitor and protect deeper, saline groundwater resources are needed in California and beyond.

Recharge estimates for various land uses in the Guarani Aquifer System outcrop area

ABSTRACTThe Guarani Aquifer System (GAS) is a subsurface reservoir that contains the largest volume of fresh groundwater in South America. Despite the relevance of the GAS, a lack of attention has been paid to land use effects on its recharge. We present the most detailed long-term (2004–2011) results of land-use effects on recharge in an outcrop area of the GAS. Water table fluctuations (WTFs) were measured at 11 monitoring wells, which are distributed between different land uses (i.e. eucalyptus, sugarcane, citrus and grassland). Recharge was estimated using a point-scale method (WTF) for each monitored well. The annual recharge estimates for different land uses are eucalyptus forest (135 mm year-1), sugarcane (248 mm year-1), citrus areas (296 mm year-1) and grassland (401 mm year-1). The results indicate that the evapotranspiration seems to be a key parameter in the assessment of recharge in the study area.

Estimating Current and Future Groundwater Resources of the Maldives

AbstractThe water resources of the atolls of the Republic of Maldives are under continual threat from climatic and anthropogenic stresses, including land surface pollution, increasing population, drought, and sea‐level rise (SLR). These threats are particularly acute for groundwater resources due to the small land surface area and low elevation of each island. In this study, the groundwater resources, in terms of freshwater lens thickness, total volume of fresh groundwater, and safe yield are estimated for the 52 most populous islands of the Maldives for current conditions and for the year 2030, with the latter accounting for projected SLR and associated shoreline recession. An algebraic model, designed in previous studies to estimate the lens thickness of atoll islands, is expanded in this study to also estimate volume of groundwater. Results indicate that average current lens thickness, groundwater volume, and per capita safe yield are approximately 4.6 m, 1,300 million liters, and 300 l/day, and that these values will decrease by approximately 10, 11, and 34%, respectively, by the year 2030. Based on results, it is demonstrated that groundwater, in terms of quantity, is a viable source of water for the islands of the Maldives both now and in coming decades, particularly for islands with large surface area and low population. Study results can provide water resource managers and government officials with valuable data for consideration in water security measures.

The impact of salt diapirs on the quality of carbonate karst waters, Bastak, Iran

The Gavbast karstic aquifer located in southern Iran is in direct contact with an exposed salt diapir. To assess the influence of the diapir on the quality of groundwater in the karstic aquifer, electrical conductivity, total dissolved solids, flow rate, temperature and major ion concentrations were measured at 57 sampling sites, including springs, surface waters and wells. A conceptual model of groundwater flow is proposed for the Gavbast karstic aquifer based on the geological setting, water budget, local base of erosion, and hydrochemistry of the sampling sites. The model suggests two subbasins in the Gavbast Anticline draining into two distinct discharging alluvial sections. Unexpectedly, groundwater discharging from the carbonate Gavbast aquifer is saline or brackish and water is of chloride type. The study indicates that the source of salinity of the Gavbast aquifers is infiltration of surface diapir-derived brine into the aquifer. The contribution of the diapir brine in the Gavbast karst aquifer is calculated about 4 L/s, using chloride mass balance. Construction of salt basins to evaporate brine discharging from the diapir springs is proposed to reduce the salinity of karst water. A row of strategically placed wells in the Gavbast karst aquifer would potentially exploit large volumes of fresh groundwater before it is contaminated by the salt. Such low-cost remediation should allow the agricultural exploitation of 40 km2 of currently barren land.

Hydro-geophysical Configuration for the Quaternary Aquifer of Nuweiba Alluvial Fan

Nuweiba is one of Egypt’s strategic marine harbors and tourist sites located on the western coast of the Gulf of Aqaba. The absence of freshwater resources in Nuweiba necessitates investigations of local groundwater aquifers. Determination of the hydro-geophysical parameters of the Nuweiba alluvial fan constitutes the basic foundation for optimizing groundwater exploitation over time. Fifteen Schlumberger vertical electrical soundings (maximum [Formula: see text]), augmented by borehole stratigraphic data, aquifer tests, hydro-chemical analysis, and numbers of measured and/or calculated hydraulic parameters, were carried out in the study area. The fresh to brackish aquifer of Quaternary and pre-Quaternary alluvium sediments was effectively delineated with true resistivity and thickness ranges of [Formula: see text] and [Formula: see text], respectively. The western part of the aquifer has a large volume of fresh ground water for extraction and is characterized by a gradual increase in thickness [Formula: see text], grain size, transmissivity [Formula: see text], and a possible decrease in total dissolved solids levels [Formula: see text] and electrical anisotropy (1.10–1.11). Porosity (30.3–30.4%), tortuosity (1.348), formation factor (6) and hydraulic conductivity [Formula: see text] are essentially constant across the aquifer. The measured storativity of the aquifer is [Formula: see text], which is in accordance with the typical value of the unconfined aquifer at shallow depths.

Groundwater depletion: A global problem

In the past half-century, ready access to pumped wells has ushered in a worldwide “explosion” of groundwater development for municipal, industrial, and agricultural supplies. Globally, groundwater withdrawals total 750–800 km/year (Shah et al. 2000). Economic gains from groundwater use have been dramatic. However, in many places, groundwater reserves have been depleted to the extent that well yields have decreased, pumping costs have risen, water quality has deteriorated, aquatic ecosystems have been damaged, and land has irreversibly subsided. Groundwater depletion is the inevitable and natural consequence of withdrawing water from an aquifer. Theis (1940) showed that pumpage is initially derived from removal of water in storage, but over time is increasingly derived from decreased discharge and/or increased recharge. When a new equilibrium is reached, no additional water is removed from storage. In cases of fossil or compacting aquifers, where recharge is either unavailable or unable to refill drained pore spaces, depletion effectively constitutes permanent groundwater mining. In renewable aquifers, depletion is indicated by persistent and substantial head declines. Excessive groundwater depletion affects major regions of North Africa, the Middle East, South and Central Asia, North China, North America, and Australia, and localized areas throughout the world. Although the scope of the problem has not been quantified globally, on-going analysis by the senior author indicates that about 700–800 km of groundwater has been depleted from aquifers in the US during the 20th century. One of the best documented cases is the 450,000 km High Plains aquifer system in the central US, where the net amount of water removed from storage during the 20th century was more than 240 km—a reduction of about 6% of the predevelopment volume of water in storage (McGuire et al. 2003). In some of the most depleted areas, use of groundwater for irrigation has become impossible or cost prohibitive (Dennehy et al. 2002). In some cases, removing the most easily recoverable fresh groundwater leaves a residual with inferior water quality. This is due, in part, to induced leakage from the land surface, confining layers, or adjacent aquifers that contain saline or contaminated water. In coastal areas, where many of the world’s largest cities are located, the available volume of fresh groundwater is reduced by seawater intrusion and upconing, which in turn are caused by head declines in the aquifer. As depletion continues worldwide, its impacts worsen, portending the need for objective analysis of the problem and its possible solutions. This essay examines future options for evaluating and managing groundwater depletion in a changing physical and social landscape.

The Change in Bioavailability of Organic Matter Associatedwith Clay-Based Buffer Materials as a Result of Heat Andradiation Treatment

ABSTRACTCompacted clay-based buffer surrounds corrosion-resistant waste containers in the Canadian concept for nuclear fuel waste disposal. Clays naturally contain small quantities of organic matter that may be resistant to bacterial degradation. The containers with highly radioactive material would subject the surrounding buffer to both heat (max. 95°C) and radiation (max. 52 Gy/h). Both could potentially break down complex organic material to smaller, more bioavailable compounds. This could stimulate microbial growth and possibly affect gas production, microbially-influenced corrosion or radionuclide migration. Experiments were carried out in which buffer (50 wt.% Na-bentonite, 50 wt.% sand) was heated at 60 and 90°C for periods of 2, 4 and 6 weeks, in some cases followed by irradiation to 25 kGy. Unheated buffer was also irradiated to 25 and 50 kGy at different moisture contents. The treated materials were subsequently suspended in distilled water, shaken for 24 h and centrifuged to remove the solids. The 0.22 μm filter-sterilized leachates were inoculated with equal volumes of fresh groundwater and incubated at room temperature for 10 d to determine the increase in total and viable bacteria compared to a groundwater control. Results indicated that leachates from buffer subjected to heat, radiation or combinations of these, had a stimulating effect on both total and viable cell counts in groundwater, compared to unamended groundwater controls. This stimulating effect was generally most pronounced for viable counts and could be larger than two orders of magnitude. Leachates from untreated buffer material also stimulated the growth of groundwater bacteria, but to a lesser extent than leachates from heat-and radiation-treated buffer material. The effects of heat and radiation on nutrient availability in clay-based sealing materials (at relevant clay/moisture ratios) should, therefore, be taken into account when attempting to quantify the effects of microbial activity on vault performance.

Evolution of pore water chemistry during degradation of cement in a radioactive waste repository environment

Portland cement, used as a solidification, backfill, and construction material, is considered to determine the chemistry of a low and intermediate level radioactive waste repository for very long periods of time. A thermodynamic description of the main constituents of hydrated Portland cement, calcium silicate hydrates (CSH-gels), is presented. The incongruent solubility behaviour of CSH-gel is described by introducing several independent model solids. The thermodynamic properties of these model solids are extracted from literature data and their solubility products are given as a function of the gel-composition. This solubility model is then used to predict the behaviour of cement structures which degrade in groundwater, including the chemical evolution of their pore waters. A volume of fresh groundwater corresponding to 5,000 to 10,000 times the total pore volume is required to completely degrade the cement. The total carbonate concentration of the groundwater is found to be a key parameter as far as the lifetime of the chemical inventory is concerned. As a representative for the antinides, the solubility behaviour of uranium in degrading cement is modelled. Cumulative uranium leaching from the cement is found to be nearly independent of cement and groundwater composition, but strongly dependent on the properties of the solubility limiting phase.

Uranium-series isotopes in the groundwater of the Permo-Triassic sandstone aquifer, Lower-Mersey Basin, U.K.

An investigation of U- and Th-series isotopes in rock and groundwater samples from a Permo-Triassic sandstone aquifer of northwest England is described. The groundwaters sampled range in age and composition from fresh and modern to highly saline and ancient, with at least two sources of salinity. Highest U and Th concentrations are present in the mudrock beds within the sandstone sequences. The 234U/ 238U and 226Ra/ 230Th rock activity ratios suggest that saline waters were once much more extensive throughout the aquifer and have been flushed by several volumes of fresh groundwater since the last glaciation. Uranium contents and activity ratios confirm that the isotope signatures observed in most of the sampled groundwaters are not the result of simple conservative mixing relations between high-U saline waters and low-U fresh groundwaters, but that the isotopic content of the matrix, the matrix mineralogy and fabric, the redox conditions, the salinity, and possibly the carbonate chemistry are all important. Petrographic studies have demonstrated the multiphase nature of the sandstones, and although the isotopic signatures are consistent with the origins of the sampled water as concluded from other hydrochemical and hydrogeological data, quantitative modelling of the processes involved is limited by lack of details of U-bearing phases, pore architecture, and the chemistry of U in high ionic strength solutions. However, Ra concentrations are controlled by sorption processes, and Ra is shown to be mobilised under saline conditions. The mechanisms of Rn release from the aquifer matrix are reviewed and a Rn release factor of 0.11 is calculated. This value is believed to be inappropriate as a measure of the recoil supply of the longer-lived Rn precursors in the U decay series.