Groundwater Lenses Research Articles

Modelling Groundwater Hydraulics to Design a Groundwater Level Monitoring Network for Sustainable Management of Fresh Groundwater Lens in Lower Indus Basin, Pakistan

The over-extraction of groundwater from thin fresh groundwater lenses is a threat to the livelihood of farmers in the Lower Indus Basin (LIB). It is essential to monitor and regulate this pumping to sustain fresh groundwater lenses. In this study, we applied a modelling approach in combination with geostatistical analysis to identify the critical locations to monitor the groundwater levels for sustaining fresh groundwater in the LIB. Our approach included four steps: (i) simulating temporal heads using a calibrated hydrogeological model; (ii) sampling monitoring locations using a hexagonal pattern of sampling; (iii) applying principal component analysis (PCA) of the temporal head observations, and selecting high scoring locations from the PCA; and (iv) minimizing the observation points to represent the water level contours. The calibrated model was able to replicate the hydro-dynamic behavior of the study area, with a root mean square of 0.95 and an absolute residual mean of 0.74 m. The hexagonal pattern of spatial sampling resulted in a 195 point network, but PCA reduced this network to 135 points and contour classification reduced it even further to 59 points. The 195, 135, and 59 point networks represented the water levels with average standard errors of 0.098, 0.318, and 0.610 m, respectively. Long-term simulations with increased pumping showed that the water levels would best be assessed by 195 monitoring points, although 135 and 59 points would represent the depleting area but would not capture the water logging area.

Development of groundwater lens for transient recharge in strip islands

Groundwater in surficial aquifers in small islands is stored in the form of freshwater lenses floating on the seawater due to density difference. Recharge from rainfall infiltration is one of the primary mechanisms that control the lens growth and decay. Unlike previous studies either assuming steady-state groundwater lenses or relying on numerical simulations, we derive an approximate analytical solution to describe the transient change of groundwater lenses for a time-dependent, fluctuating recharge rate. The approximate analytical solution is validated by numerical simulations of variable density flow and transport. Based on this analytical solution and dimensional analysis, we investigate the impact of periodic recharge, extended drought events and random time-series recharge on the time-dependent behavior of the lens thickness and volume. Results show that only slowly changing recharge patterns with long periods such as seasonal and yearly changes can cause significant fluctuations of groundwater lenses. In specific, 10%, 50% and 100% fluctuations correspond to dimensionless periods of 200, 500 and 1300, respectively, which are 69, 174 and 451 days for our specified hydrogeological parameters. Thus, assuming an average constant recharge rate may be valid for simulating the transient lens profile and volume for recharge patterns with short periods such as daily fluctuations. We further analyze the approximate analytical solution using the method of low-pass filter. The complex physical process of the lens development is fitted by a second-order system on the frequency domain. Such a filter passes sine-wave recharge components with frequencies smaller than the break frequency and attenuates those higher than the break frequency, yielding smooth profiles of the lens thickness and volume. By analyzing the recharge rate spectrum and comparing the component frequency with the break frequency, we demonstrate how the development of a groundwater lens is affected by the transient recharge in reported laboratory and field observations.

Modelling the effect of high level of total dissolved solids (TDS) for the sustainable utilization of brackish groundwater from saline aquifers in Kuwait

Groundwater resources in Kuwait are almost brackish to saline. There is a concern that the shallow aquifers in Kuwait are polluted with high levels of salinity. The brackish groundwater lenses of the utilized aquifers in Kuwait are strategically important for the agricultural and industrial sectors. These lenses float above saline water bodies. It is the aim of this study to simulate the impact of skimming brackish lenses on the vertical movement of saline water in the utilized aquifer. In order to achieve this aim, pumping tests were conducted on the wells of the study area using Tartakovsky–Neuman method. The results showed that the average transmissivity and specific yield are 330 m2/day and 12%, respectively. The results of the pumping tests were fed into a density-dependent flow with solute transport model to simulate the fate of saline water (with TDS values from 10,000 to 43,000 mg/l) in the utilized aquifer under pumping conditions. The modelling results show that controlling the vertical movement of deeper saline water can be achieved by locating the well screen alongside most of the saturated thickness that includes both brackish and saline water bodies. Only the salinity of the area around well W-2 exceeded the limit of 10,000 mg/l when the total pumping rates from all the wells were 6624 m3/day. This means that the sustainable yield of the aquifer in the utilized zone of the study area is about 5746 m3/day (2.1 Mm3/year) after eliminating the pumping rate of well W-2. The elevated final salinity of the pumped water at this well is influenced by the value of the initial salinity of this well, which was high at 9519 mg/l prior to pumping.

Evaluating dynamic mechanisms and formation process of freshwater lenses on reclaimed atoll islands in the South China Sea

Groundwater on remote coral atoll islands in tropical oceans occurs in the form of freshwater groundwater lenses (FGLs) that serve as an important water resource for local inhabitants and ecosystems. Continuous recharge of freshwater from rainfall can flush out salt water and eventually create a FGL beneath the natural or reclaimed island. However, the process whereby a FGL grows in reclaimed islands, and the dynamic mechanisms and formation process are unclear. This study used numerical modeling for a reclaimed island in the South China Sea to evaluate the process of mixing between freshwater and saltwater, dynamic mechanisms, and the formation process of FGLs influenced by tidal action. Our results revealed that the long-term average flow lines of FGLs were superimposed by short-term tidal fluctuations beneath the atoll island. The tidal signals move rapidly inland from the lower aquifer and are then propagated upwards to drive the water level in the Holocene sediments. This causes an oscillation of the FGLs and increased mixing between salt and fresh water. The formation of the FGLs can be divided into three stages: preparatory, formation, and pseudo steady-state. Yongshu Island (a reclaimed island) was used as a case study. The island’s preparatory phase lasted approximately 2.5 years, and will take 20 years to form a stable FGL (with a thickness of about 15 m). Recharge rates, and the nature of the contact between the Holocene and the underlying Pleistocene aquifers, determine the shape of the FGLs beneath the island. Abstraction during the formative period of the FGLs will increase the time required to reach a stable state. The results obtained enhance the understanding of the formation of FGLs, and can provide a reference for the management of freshwater resources on atoll islands.

ИЗМЕНЕНИЕ ПАВОДКООБРАЗУЮЩИХ СВОЙСТВ ВОДОСБОРОВ НА РЕКАХ СЕВЕРО-ЗАПАДНОГО КАВКАЗА В 2000–2014 гг.

The rivers of the North West Caucasus are characterized by a flood runoff regime. Floods leading to material damage occur on the rivers of the region every year. However, in the XXI century. the development of both automatic monitoring systems and the capabilities of mathematical modeling of hydrological processes make it possible to fill this gap. After the flood of 2012, a network of automatic level gauges is developing in the region, recording water levels every 10 minutes. Analysis of the accumulated archive of observations allows us to determine the features of the formation and passage of floods on the rivers of the region. Among the features of catchments that determine the transformation of precipitation into runoff in the catchment, vegetation and soil cover are distinguished. When it rains in a multi-layer forest, each layer is sequentially moistened from top to bottom. After the vegetation has been moistened, the precipitated water reaches the soil surface and some of it infiltrates into deeper soil horizons up to the waterproof layers, where it replenishes the groundwater lenses. The vegetation cover of the North West Caucasus in the foothill part is represented mainly by plowed steppes with ravine forests, and in the mountainous part – almost entirely by forests. From west to east of the region, mainly oak forests give way to beech and hornbeam forests with a higher water retention. High forest cover and widespread in the region gray, brown forest and soddy-calcareous soils have a water-holding capacity sufficient to absorb intense precipitation. A large proportion of anthropogenic landscapes in the western part of the region causes a lower capacity for absorbing precipitation and, accordingly, a greater flood hazard. Further decrease in forest cover and development of the region will increase the flood hazard.

Regional and well-scale indicators for assessing the sustainability of small island fresh groundwater lenses under future climate conditions

Salinization of the freshwater lens and well fields under future climate and groundwater demands threaten the sustainability of groundwater abstraction in small islands. In this paper, two simple indices are proposed for a comprehensive assessment of freshwater salinization in small islands. Values of these indices are computed based on a sharp-interface finite element numerical model with the interface matched to the lower limit of freshwater. The first index relates rainfall percentiles with the computed freshwater volume and provides regional assessment of changes in the volume stored in the freshwater lens over time. The second is a sustainability index, based on computed saltwater ratios in pumping wells as the performance indicator, which represents the well salinization risk at a spatial scale. The proposed methodology is illustrated with the example of Tongatapu Island. Freshwater lens dynamics and well salinization under various scenarios of dry, median, and wet general circulation model predictions-based unsteady recharge, sea level rise, and pumping demands were evaluated for the period 2010–2099. The freshwater lens of Tongatapu is dependent on rainfall, and the 60-month rainfall percentile is highly correlated with the total freshwater volume. Public well field indicates sustainability indexes of 45% and 100% under the current pumping conditions for the driest and the wettest scenarios, and with increased pumping and sea level rise, they reduce to 30% and 90%, respectively. Thus, management of pumping rates is essential for future freshwater sustainability. This methodology can be used for first-hand temporal and spatial estimations of small island freshwater lens salinization considering both the regional and well scales.

Applying satellite‐derived evapotranspiration rates to estimate the impact of vegetation on regional groundwater flux

AbstractFreshwater resources are at a premium across the world, including many water‐limited areas across Australia. Ongoing water level decline in some groundwater systems suggests that the rate of loss may have reached unsustainable levels . However, identifying which factors are primarily responsible for the trend (lack of rainfall–recharge, presence of vegetation, and groundwater pumping) remains challenging using groundwater observations alone. We applied satellite‐derived estimates of evapotranspiration (ETa), which, when combined with local rainfall data and field‐based groundwater level observations, established a regional water balance spanning 1,500 km2 and seven groundwater lenses over a 10‐year period (2000 – 2010). Assuming that the extent of the freshwater isohaline represents the recharge area for a lens, the water balance suggests that the median annual groundwater recharge rate varied between 226 ± 92 mm year−1 (Mikkira) and −162 ± 194 mm year−1 (Uley East). Uley South is the most regionally significant lens in the system and recorded a median annual recharge rate of 91 ± 182 mm year−1. Overlaying vegetation highlighted the impact of woodland areas on groundwater recharge, where the trees were accessing groundwater to support ETa, provided the water table was <10 m. Areas of grassland demonstrated the highest median groundwater recharge rates of 151 mm year−1, followed by cropping (133 mm year−1) and pasture (95 mm year−1). Exploring the resilience of the groundwater system to variations in extraction (pumping) and woodland coverage suggests that resource managers must consider both systematic losses in order to maintain groundwater equilibrium.

The Effect of Hurricane Irma Storm Surge on the Freshwater Lens in Big Pine Key, Florida using Electrical Resistivity Tomography

Animals and plants on low elevation oceanic islands often rely on a thin lens of fresh groundwater and this lens is vulnerable to seawater contamination from storm surge. Documentation of the impact of the storm surge on the freshwater lens and its subsequent recovery is limited. In September 2017, Hurricane Irma made landfall in the Florida Keys as a category 4 storm with storm surge heights in excess of 2 m. This study used Electrical Resistivity Tomography (ERT) to investigate the effect of the storm surge on the freshwater lens of Big Pine Key, FL. The study compared ERT images along three profiles ranging between 220 and 280 m length collected in 2011 with post storm data collected about 3 to 4 months (November 2017/January 2018) and eight (May 2018) months after Irma. The post storm data documented that the storm surge impacted the freshwater lens on all three profiles with low resistivity (i.e., high salinity) zones in the upper 2 m of the groundwater. The increase in salinity was most pronounced in the lower elevations of the profiles. The May 2018 data were collected immediately after 2 weeks of intense precipitation. These data showed 40% recovery of the freshwater lens, most pronounced in the lower elevation of the profiles. This suggests that both the impact of storm surge and the freshwater recovery due to precipitation are most pronounced in low elevation regions where both saline and freshwater can collect at the surface.

Analytic solutions for fresh groundwater lenses floating on saline water under desert dunes: The Kunin-Van Der Veer legacy revisited

A fully-saturated lens of steady fresh groundwater floating in a homogeneous and isotropic desert sandy aquifer is analytically studied based on a hydrological model by Kunin and interface solution by Van Der Veer. A static saline groundwater is beneath the lens. A phreatic surface of moving fresh water inside the lens is partially recharged (either naturally or by managed aquifer recharge) from the vadose zone and partially exfiltrates to it. A spatially focused recharge and intensive evapotranspiration preserve a steady downward-upward topology of fresh water motion. In terms of the 1-D Dupuit-Forchheimer approximation in a horizontal in-lens saturated flow a boundary value problem (BVP) for an ODE for the Strack potential is solved. The shape of the water table and, based on the Ghijben-Herzberg assumption, the interface are found. The total volume of the positive-pore pressure water flowing within the lens is evaluated. Constant infiltration and evaporation rates as well as evaporation linearly decreasing with depth of the water table (counted from the ground surface) are considered. The case of 2-D flow is tackled by the Toth model. A triangular analytic element approximates a half of the flow domain and consists of an isobaric side and two no-flow sides. Conformal mapping of this triangle onto a reference plane and solution of the Dirichlet BVP in a half-plane deliver the distribution of infiltration-exfiltration intensity along the water table, total flow rate and locus of the hinge point. A mathematically more cumbersome approximation of the flow domain assumes the water table to be a tilted straight line but the interface to be found as a free boundary. Solution of the corresponding BVP uses a curvilinear triangle in the hodograph plane.

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.

Up-coning Caused by Excess Pumping during Drought Had Reduced Storage Volume of Fresh Groundwater Lens on Atoll Island

Up-coning Caused by Excess Pumping during Drought Had Reduced Storage Volume of Fresh Groundwater Lens on Atoll Island

Groundwater geochemistry fluctuations along a fresh-saltwater gradient on the carbonate islands of the lower Florida Keys

Climate change will have long-lasting effects on the availability of fresh water on small, carbonate islands that have isolated fresh groundwater lenses, particularly as sea level rises and rainfall regimes shift. The carbonate islands of the Florida Keys provide an ideal location to study the effect of variable rainfall on the aqueous geochemistry of the islands' groundwater. In a rainfall-driven carbonate system, the expectation is that limestone dissolution will occur within the vadose zone resulting in increased ions in the groundwater. However, geochemical processes are also affected by the salinity of groundwater and the extent of the mixing zone between fresh and salt water. We chose two islands to conduct the study of the shallow groundwater: the largest island in the lower Florida Keys, Big Pine Key (BPK), and a smaller island, Upper Sugarloaf Key (SLK). From May 2011 through April 2012, monthly groundwater samples were collected from 24 shallow (1 m deep) wells located along a fresh to saline gradient on both islands. Groundwater chemistry was compared with rainfall amounts from a weather station on BPK. Saturation indices for aragonite and calcite, generated with geochemical modeling in PHREEQC, were compared to conservative mixing between Gulf of Mexico water and freshwater. Equilibrium to supersaturated conditions with respect to carbonate minerals dominated in all of the groundwater samples. Saturation indices varied with rainfall with the most supersaturated samples observed after a large rain event and samples approaching equilibrium after the longest period without rainfall. Calcium in excess of what would be expected from conservative mixing of fresh water and seawater was observed in all groundwater samples and was elevated at near-shore locations, especially on BPK. Contrary to expectations, dissolution resulting from mixing of freshwater and seawater was not supported in the shallow groundwater. Instead, dissolution within the narrow vadose zone from rain events likely resulted in the excess calcium in the groundwater. Seasonal fluctuations in groundwater composition were primarily observed on the smaller island and were related to the fresh water balance, changing rapidly after a heavy rain event, and suggest that a size threshold has been surpassed for a stable lens. Rising seas will further decrease lens extent and vadose zone depth, reducing the potential for future limestone dissolution.

Estimating the freshwater-lens reserve in the coastal plain of the middle Río de la Plata Estuary (Argentina)

Drinking-water supply is one of the main issues that populations face in many coastlands. Shallow coastal aquifers are often characterized by the presence of lens-shaped freshwater floating on the saline groundwater plume of marine origin. These groundwater lenses are commonly associated with landforms, such as littoral ridges and dunes and in many cases they represent the main source of water supply in remote coastal areas. At the right side of the middle Río de la Plata estuary (Argentina) the aquifer system is generally saline. Elongated and thin sandy beach ridge systems emerging from the general flat morphology of the marsh-flood plain are capable of storing precipitations forming freshwater lenses, which to date are the main freshwater supply for inhabitants. The aim of this study is to identify and delimitate the presence of such valuable freshwater reserves in order to provide the first necessary guidelines for the water management plan in this area, which has never been implemented since, to the Authors' knowledge, no specific investigation had been carried out before this study. To achieve this goal, Vertical Electrical Sounding, groundwater electrical conductivity measurements, water balances and groundwater chemical analyses were performed and interpreted together. The whole dataset was processed to define the electro-stratigraphic model of the study area and to produce the map of the electrical conductivity of the shallow aquifer. In addition, a three-dimensional model of the fresh water reservoir has been implemented for a better understanding of the relationship between geomorphology and groundwater. Results point out that a total freshwater volume of 78,259,700m3 is stored into a continuous lens and the annual average recharge from precipitation amounts to 6,303,500m3. Although preliminary, this work provides the basic knowledge on the potential fresh groundwater lenses and provides important information for addressing a sustainable use of the freshwater resource.

Impact of coastal forcing and groundwater recharge on the growth of a fresh groundwater lens in a mega-scale beach nourishment

Abstract. For a large beach nourishment called the Sand Engine – constructed in 2011 at the Dutch coast – we have examined the impact of coastal forcing (i.e. natural processes that drive coastal hydro- and morphodynamics) and groundwater recharge on the growth of a fresh groundwater lens between 2011 and 2016. Measurements of the morphological change and the tidal dynamics at the study site were incorporated in a calibrated three-dimensional and variable-density groundwater model of the study area. Simulations with this model showed that the detailed incorporation of both the local hydro- and morphodynamics and the actual recharge rate can result in a reliable reconstruction of the growth in fresh groundwater resources. In contrast, the neglect of tidal dynamics, land-surface inundations, and morphological changes in model simulations can result in considerable overestimations of the volume of fresh groundwater. In particular, wave runup and coinciding coastal erosion during storm surges limit the growth in fresh groundwater resources in dynamic coastal environments, and should be considered at potential nourishment sites to delineate the area that is vulnerable to salinization.

Hydrogeology and management of freshwater lenses on atoll islands: Review of current knowledge and research needs

On atoll islands, fresh groundwater occurs as a buoyant lens-shaped body surrounded by saltwater derived from the sea, forming the main freshwater source for many island communities. A review of the state of knowledge of atoll island groundwater is overdue given their susceptibility to adverse impacts, and the task to address water access and sanitation issues within the United Nations’ Sustainable Development Goals framework before the year 2030. In this article, we review available literature to summarise the key processes, investigation techniques and management approaches of atoll island groundwater systems. Over fifty years of investigation has led to important advancements in the understanding of atoll hydrogeology, but a paucity of hydrogeological data persists on all but a small number of atoll islands. We find that the combined effects of buoyancy forces, complex geology, tides, episodic ocean events, strong climatic variability and human impacts create highly dynamic fresh groundwater lenses. Methods used to quantify freshwater availability range from simple empirical relationships to three-dimensional density-dependent models. Generic atoll island numerical models have proven popular in trying to unravel the individual factors controlling fresh groundwater lens behaviour. Major challenges face the inhabitants and custodians of atoll island aquifers, with rising anthropogenic stresses compounded by the threats of climate variability and change, sea-level rise, and some atolls already extracting freshwater at or above sustainability limits. We find that the study of atoll groundwater systems remains a critical area for further research effort to address persistent knowledge gaps, which lead to high uncertainties in water security issues for both island residents and surrounding environs.

Closed-form analytical solutions for assessing the consequences of sea-level rise on unconfined sloping island aquifers

Closed-form analytical solutions for assessing the consequences of sea-level rise on fresh groundwater oceanic island lenses are provided for the cases of both strip and circular islands. Solutions are proposed for directly calculating the change in the thickness of the lens, the changes in volume and the changes in travel time of fresh groundwater within island aquifers. The solutions apply for homogenous aquifers recharged by surface infiltration and discharged by a down-gradient, fixed-head boundary. They also take into account the inland shift of the ocean due to land surface inundation, this shift being determined by the coastal slope of inland aquifers. The solutions are given for two simple island geometries: circular islands and strip islands. Base case examples are presented to illustrate, on one hand, the amplitude of the change of the fresh groundwater lens thickness and the volume depletion of the lens in oceanic island with sea-level rise, and on the other hand, the shortening of time required for groundwater to discharge into the ocean. These consequences can now be quantified and may help decision-makers to anticipate the effects of sea-level rise on fresh groundwater availability in oceanic island aquifers.

Groundwater salinisation on atoll islands after storm‐surge flooding: modelling the influence of central topographic depressions

AbstractFresh groundwater lenses (FGLs) are of utmost importance for human survival on small and isolated atolls. This article examines saline damage to atoll FGLs from wave washover caused by storm surge and studies the particular influence of central topographic depressions (CTDs). We model storm surge over atoll islets of contrasting widths (400 and 800 m), both with and without CTDs of various sizes. Three key findings emerge. First, under equilibrium undisturbed conditions, the CTD slightly reduces the size of the FGL compared to atoll islets without this feature. Second, during marine flooding, prior saturated conditions at the base of a CTD impede seawater infiltration into the substrate, thereby limiting saline damage in that location. Third and most crucial, however, the amount of salt accumulated within the CTD is significant, ranging from 2 to 10 times higher than the net subsurface infiltration during the period of the storm inundation.

Conceptualization of a fresh groundwater lens influenced by climate change: A modeling study of an arid-region island in the Persian Gulf, Iran

Understanding the fresh groundwater lens (FGL) behavior and potential threat of climatic-induced seawater intrusion (SWI) are significant for the future water resources management of many small islands. In this paper, the FGL of Kish Island, an arid-region case in the Persian Gulf, Iran, is modeled using two-dimensional (2D) and three-dimensional (3D) simulations. These simulations are based on the application of SUTRA, a density-dependent groundwater numerical model. Also, the numerical model parameters are calibrated using PEST, an automated parameter estimation code. Firstly a detailed conceptualization of the FGL model is completed to understand the sensitivity of the FGL to some particular aspects of the model prior to analysis of climate change simulations. For these investigations, the FGL system is defined based on Kish Island system to accomplish the integrated comparison of features of a conceptual model that are representative of real-world systems. This is the first study which adopts such an approach. The comparison of cross-sectional simulations suggests that the two-layer properties of the Kish Island aquifer have a significant influence on the FGL while the impacts of lateral-boundary irregularities are negligible. The impacts of sea-level rise (SLR), associated land-surface inundation (LSI), and variations in recharge rate on the FGL salinization of Kish Island are investigated numerically. Variations of SLR value (1–4 m) and net recharge rate (17–24 mm/year) are considered to cover a possible range of climatic scenarios in this arid-region island. The 2D and 3D simulation results demonstrate that LSI caused by SLR and recharge rate variation impacts are more important factors in the FGL in comparison to estimated SLR impacts without LSI. It is also shown that climate change impacts on the FGL are long-term to reach a new FGL equilibrium in the case of Kish Island’s aquifer system. The comparative analysis of 2D and 3D results shows that three-dimensionality is a significant factor, especially in large-scale 3D systems of small islands. The results of this study are expected to have implications for the understanding and management of the fresh groundwater resources of Kish Island and are also expected to be relevant to the study of the impact of climate change on groundwater resources on islands worldwide.

Enabling Successful Aquifer Storage and Recovery of Freshwater Using Horizontal Directional Drilled Wells in Coastal Aquifers

Aquifer storage and recovery (ASR) of freshwater surpluses can reduce freshwater shortages in coastal areas during periods of prolonged droughts. However, ASR is troublesome in saline coastal aquifers as buoyancy effects generally cause a significant loss of injected freshwater. The use of a pair of parallel, superimposed horizontal wells is proposed to combine shallow ASR with deep interception of underlying saltwater. A shallow, fresh groundwater lens can thereby be enlarged and protected. This freshmaker setup was successfully placed in a coastal aquifer in The Netherlands using horizontal directional drilling to install 70-m-long horizontal directional drilled wells (HDDWs). The freshmaker prototype aims to inject a specific volume of freshwater and abstract the same volume of water (consisting of injected water and ambient native groundwater) within the targeted water quality. Groundwater transport modeling preceding ASR operation demonstrates that this set up is able to abstract a water volume of 4,200 m3 equal to the injected freshwater volume without exceeding strict salinity limits, which would be unattainable with conventional ASR. This is the first study to demonstrate the potential benefits of HDDWs for a field ASR application. The model outcomes indicate that the feasibility perspectives of ASR in coastal aquifers worldwide require revision thanks to recent developments in hydrologic engineering.

Trace element geochemistry of groundwater in a karst subterranean estuary (Yucatan Peninsula, Mexico)

Trace element cycling within subterranean estuaries frequently alters the chemical signature of groundwater and may ultimately control the total chemical load to the coastal ocean associated with submarine groundwater discharge. Globally, karst landscapes occur over 12% of all coastlines. Subterranean estuaries in these regions are highly permeable, resulting in rapid infiltration of precipitation and transport of groundwater to the coast, and the predominant carbonate minerals are readily soluble. We studied the chemical cycling of barium (Ba), strontium (Sr), manganese (Mn), uranium (U), calcium (Ca) and radium (Ra) within the carbonate karst subterranean estuary of the Yucatan Peninsula, which is characterized by a terrestrial groundwater lens overlying marine groundwater intrusion with active submarine discharge through coastal springs. Terrestrial groundwater calcium (1–5mmolkg−1) and alkalinity (3–8mmolkg−1) are enriched over that predicted by equilibrium between recharging precipitation and calcite, which can be accounted for by groundwater organic matter respiration and subsequent dissolution of calcite, dolomite and gypsum. There is a close agreement between the observed terrestrial groundwater Sr/Ca, Mn/Ca, Ba/Ca and Ra/Ca and that predicted by equilibrium dissolution of calcite, thus the trace element content of terrestrial groundwater is largely determined by mineral dissolution. Subsequent mixing between terrestrial groundwater and the ocean within the actively discharging springs is characterized by conservative mixing of Sr, Mn, Ba and Ca, while U is variable and Ra displays a large enrichment (salinity: 1.9–34.9, Ba: 60–300nmolkg−1, Sr: 15–110μmolkg−1, U: 0.3–35nmolkg−1, Mn: 0.3–200nmolkg−1, Ca: 4.3–12.9mmolkg−1, 226Ra: 18–2140dpm 100L−1). The deep groundwater sampled through cenotes, local dissolution features, is typified by elevated Ba, Sr, Ca, Mn and Ra and the absence of U within marine groundwater, due to enhanced dissolution of the aquifer matrix following organic matter degradation and redox processes including sulfate reduction (salinity: 0.2–36.6, Ba: 7–1630nmolkg−1, Sr: 1.3–210μmolkg−1, U: 0.3–18nmolkg−1, Mn: 0.6–2600nmolkg−1, Ca: 2.1–15.2mmolkg−1, 226Ra 20–5120dpm100L−1). However, there is no evidence in the spring geochemistry that deep marine groundwater within this reaction zone exchanges with the coastal ocean via spring discharge. Total submarine groundwater discharge rates calculated from radium tracers are 40–95m3m−1d−1, with terrestrial discharge contributing 75±25% of the total. Global estimates of chemical loading from karst subterranean estuaries suggest Sr and U fluxes are potentially 15–28% and 7–33% of total ocean inputs (8.2–15.3moly−1 and 4.0–7.7moly−1), respectively. Radium-226 inputs from karst subterranean estuaries are 34–50 times river inputs (6.7–9.9×1016dpmy−1).