Freshwater Lens Research Articles

Permeability of porous media in coral reefs

The formation and storage of fresh groundwater in coral reefs is affected by many factors. The permeability of porous media in island and reef formation is one of the main controlling factors. Coral sand is the only medium for the formation of coral reefs. Its permeability characteristics are very different from the permeability characteristics of quartz sand from land sources. In this study, a series of indoor model tests and microphysical property tests were designed in order to reveal the reasons for the special permeability of coral sand from the microcosmic point of view. The conversion equation of the hydraulic conductivity between quartz sand and coral sand was established. Finally, through numerical simulation, the variation rule and the parameter sensitivity of a freshwater lens were studied when the hydraulic conductivity changed within the reasonable range of natural existence. It was found that the special permeability of coral sand was caused by four factors, including roundness (Φ), roughness (Ψ), surface charge, and the surface hydrophilicity of the particles. Among the four factors, the main factors and the secondary factors were classified according to the influencing degree that changed with the sizes of the particles. The influencing sensitivity of the hydraulic conductivity on the properties of the freshwater lens had the following order: freshwater appearance time > steady-state freshwater reserves > thickness of the steady-state freshwater lens> lens stabilization time.

A pole-to-equator ocean overturning circulation on Enceladus

Enceladus is believed to have a saltwater global ocean, heated at the ocean–core interface and losing heat to the floating ice shell above. This configuration suggests an important role for vertical convection. The ice shell has dramatic meridional thickness variations that, in steady state, must be sustained by the ocean circulation against processes acting to remove these anomalies. This could be achieved through spatially separated regions of freezing and melting at the ocean–ice interface. Here, we use an idealized, dynamical ocean model forced by an observationally guided density flux at the ocean–ice interface to argue that Enceladus’s interior ocean should support a meridional overturning circulation. This circulation establishes an interior density structure that is more complex than in studies that have focused only on vertical convection, including a shallow freshwater lens in the polar regions. Spatially separated sites of ice formation and melt enable Enceladus to sustain significant vertical and horizontal stratification, which influences interior heat transport and is critical for understanding the relationship between a global ocean and the planetary energy budget. On the basis of our model, the presence of low salinity layers near the polar ocean–ice interface implies the ocean’s bulk salinity could substantially exceed values inferred from Cassini plume samples.

Flank margin caves can connect to regionally extensive touching vug networks before burial: Implications for cave formation and fluid flow

AbstractFlank margin caves are extreme endmembers of vuggy porosity that form as diagenesis drives the progressive coalescence of smaller solutional pore spaces. Due to their morphological isolation during formation, the prevailing hypothesis has been that fluid flow in and out of flank margin caves occurs via the matrix permeability and that adjacent chambers only become hydraulically connected through nonmatrix porosity during burial, collapse, and fracturing. To our knowledge, however, no studies have evaluated how flank margin caves are connected to regional flow systems in modern carbonate platforms. In this study, we evaluate the connectivity of wells, boreholes, blue holes, and flank margin caves in increasingly older bedrock on San Salvador Island, Bahamas, using tidal attenuation analysis. Phreatic karst features are not reported in Holocene bedrock, and permeability magnitudes from wells suggest Holocene deposits connect to the ocean along matrix‐dominated flow paths. Permeability magnitudes in bedrock surrounding wells, boreholes, and karst features deposited during Marine Isotope Substage (MIS) 5e suggest connection to the ocean through matrix and touching vug porosity. Boreholes, blue holes, and flank margin caves in pre‐MIS5 bedrock connect to the ocean via touching vugs. We suggest that increasing bedrock permeability, cave number, and cave size observed within progressively older bedrock on San Salvador is a function of the cumulative number of freshwater lenses emplaced over successive sea‐level stillstands. We suggest that the morphologies of the two largest caves are consistent with dissolution in multiple lenses occupying lower elevations, collapse resulting in breakout domes, and overprinting of collapse chambers during subsequent highstands. As a result, some caves may not reflect connectivity of the bedrock surrounding the main chambers but may reflect connectivity of more diagenetically mature bedrock at lower elevations where their antecedent chambers formed.

Tracking changing water budgets across the Bahamian archipelago

Freshwater resources on small island nations are already at risk from sea level rise and groundwater pumping however, increasing aridity due to climate change further stresses water availability. The amount of freshwater availability on small island nations is delicately balanced by incoming precipitation (P) and outgoing evapotranspiration (ET). As climate changes, some island nations may see increasing aridity (ET > P) which can have implications for long term freshwater sustainability. Understanding how P and ET are changing allows small island nations to make more informed planning decisions regarding sustainable management and to adapt contemporary strategies to accommodate future change. The Bahamas is one such nation made up of over 700 small islands, many of which rely on freshwater lenses for potable water and irrigation. We use precipitation obtained from the TerraClimate reanalysis dataset and satellite derived actual evapotranspiration (ETa) to analyze how seasonal and yearly water budgets have changed over the last 16 years. We show that ETa is increasing across the Bahamas at a higher rate than global averages. The cause of ETa increases is driven by both increases in temperature and water availability form shifting precipitation patterns, ultimately driving decreases in the amount of available water to recharge the freshwater lens.

Analytical, Experimental, and Numerical Investigation of Partially Penetrating Barriers for Expanding Island Freshwater Lenses

AbstractFreshwater lenses are of great importance in supporting both the vulnerable ecosystems of small‐to‐medium oceanic islands and the lives of local inhabitants. Recently, an engineering approach that embeds a low‐permeability, fully penetrating barrier along the shoreline was proposed and demonstrated under controlled conditions to be effective in enhancing island lens size. The current study extends that work by investigating the effect of partially penetrating barriers placed at the shoreline on both circular and strip island lenses, using: (1) sharp‐interface analytical solutions, (2) dispersive numerical modeling, and (3) sand tank experimentation. Analytical, experimental, and numerical results are in reasonable agreement in terms of the freshwater‐seawater interface. The findings indicate that partially penetrating barriers of sufficient depth (i.e., the freshwater depth in barrier from the fully penetrating barrier case) produce comparable results to fully penetrating barriers, and therefore, the former is more cost‐effectiveness. Moreover, the minimum barrier depth (i.e., the “critical depth”) required to obtain approximately the same lens as that achieved using the fully penetrating barrier is estimable from the analytical solution, as verified by laboratory experiments and numerical simulations. The sensitivity analysis based on the analytical solution indicates that a lower barrier permeability and a greater barrier thickness, which lead to a higher freshwater storage (as expected), require a larger critical depth. The guidance on the design of partially penetrating shoreline barriers arising from this research adds to existing engineering techniques for enhancing freshwater resources on circular and strip islands.

Molecular Traits of Dissolved Organic Matter in the Subterranean Estuary of a High-Energy Beach: Indications of Sources and Sinks

Advective flows of seawater and fresh groundwater through coastal aquifers form a unique ecohydrological interface, the subterranean estuary (STE). Here, freshly produced marine organic matter and oxygen mix with groundwater, which is low in oxygen and contains aged organic carbon (OC) from terrestrial sources. Along the groundwater flow paths, dissolved organic matter (DOM) is degraded and inorganic electron acceptors are successively used up. Because of the different DOM sources and ages, exact degradation pathways are often difficult to disentangle, especially in high-energy environments with dynamic changes in beach morphology, source composition, and hydraulic gradients. From a case study site on a barrier island in the German North Sea, we present detailed biogeochemical data from freshwater lens groundwater, seawater, and beach porewater samples collected over different seasons. The samples were analyzed for physico-chemistry (e.g., salinity, temperature, dissolved silicate), (reduced) electron acceptors (e.g., oxygen, nitrate, and iron), and dissolved organic carbon (DOC). DOM was isolated and molecularly characterized via soft-ionization ultra-high-resolution mass spectrometry, and molecular formulae were identified in each sample. We found that the islands’ freshwater lens harbors a surprisingly high DOM molecular diversity and heterogeneity, possibly due to patchy distributions of buried peat lenses. Furthermore, a comparison of DOM composition of the endmembers indicated that the Spiekeroog high-energy beach STE conveys chemically modified, terrestrial DOM from the inland freshwater lens to the coastal ocean. In the beach intertidal zone, porewater DOC concentrations, lability of DOM and oxygen concentrations, decreased while dissolved (reduced) iron and dissolved silicate concentrations increased. This observation is consistent with the assumption of a continuous degradation of labile DOM along a cross-shore gradient, even in this dynamic environment. Accordingly, molecular properties of DOM indicated enhanced degradation, and “humic-like” fluorescent DOM fraction increased along the flow paths, likely through accumulation of compounds less susceptible to microbial consumption. Our data indicate that the high-energy beach STE is likely a net sink of OC from the terrestrial and marine realm, and that barrier islands such as Spiekeroog may act as efficient “digestors” of organic matter.

Evolution of Salinity and Water Table Level of the Phreatic Coastal Aquifer of the Emilia Romagna Region (Italy)

The coastal aquifers of the Mediterranean region are highly susceptible to seawater intrusion due to a combination of challenges such as land subsidence, high aquifer permeability, urbanization, drainage, and an unsustainable use of water during the dry summer months. The present study is focused on a statistical analysis of groundwater data to evaluate the spatial changes of water level and electrical conductivity in the coastal phreatic aquifer of the Emilia-Romagna (Northeast Italy) for the period from 2009 to 2018. Data from 35 wells distributed across the entire regional coastal area are used to establish a temporal trend, as well as correlations between salinity, water table level, and rainfall. Water table and salinity distribution maps for the entire study area are discussed regarding surface geology and water management. Most of the wells are in the beach wedge sand unit, which allows for easy connectivity between groundwater and surface water. Surface water and groundwater salinization are enhanced along the surface water bodies connected to the sea. The lowest water table level occurs in the western and northern parts of the study area, because of the semiconfined behavior of the aquifer. Only in the northernmost, close to the Po River, and in the southernmost parts of the study area does the groundwater remain fresh for the whole period considered due to river aquifer recharge. In the rest of the region, the thickness of freshwater lenses, where present, is less than 4.5 m. The existence of a water table level below sea level and high saline water at the bottom of the aquifer in most of the study area suggest that the aquifer is in unstable hydrodynamic conditions and groundwater quality is not fit for human consumption or for irrigation. This study is the first to provide a regional overview of the state of groundwater level and salinization within the coastal aquifer of the Emilia-Romagna Region; it also suggests that, overall, the salinization trend has slightly decreased from 2009 to 2018.

Cycling of redox-sensitive trace metals in barrier island freshwater lenses

Freshwater lenses connect the terrestrial and marine realm via groundwater discharge at the edges of islands and serve as drinking water resources. We studied the redox-sensitive metals U, Mo, V, and Tl along the redox gradient of fresh groundwater lenses on Spiekeroog Island, northern Germany. Groundwater solute concentrations were linked to groundwater age and redox characteristics. We further quantified the contribution of precipitation, sea spray, and aquifer matrix to the groundwater metal concentrations and evaluated the sink and source function of the aquifer under oxic and reducing conditions.We found that biogeochemical processes altered the concentrations of the trace metals. In young, oxygen to nitrate reducing zones, the aquifer matrix represented the major metal source to the groundwater. For Tl, rain was an additional important (anthropogenic) source. Under manganese and iron oxide to sulfate reducing conditions, U and Tl were sensitive to redox dependent removal, whereas Mo and V were less affected by reductive precipitation/adsorption. In detail, 99% of dissolved Tl, 88% of U, 66% of Mo, and 44% of V were removed to the solid phase in comparison to values from less reducing zones. Large parts of the western freshwater lens on Spiekeroog were anoxic. For this reason, the delivery of aquifer derived metals to the ocean via fresh groundwater discharge appeared to be limited. Higher U, Mo, V, and Tl concentrations were observed in the presently developing young freshwater lens in the east of Spiekeroog Island. This suggests that less reducing groundwater lenses may be a source of these metals to the adjacent beach/coastal seawater. Especially for V, freshwater discharge from sandy coastal aquifers may be important, as groundwater concentrations exceeded seawater concentration under oxic as well as anoxic conditions. Regarding the suitability of the freshwater as drinking water, all measured trace metal concentrations were classified as uncritical.

An analytical methodology to estimate the changes in fresh groundwater resources with sea-level rise and coastal erosion in strip-island unconfined aquifers: illustration with Savary Island, Canada

Closed-form analytical solutions for assessing the consequences of climate change on fresh groundwater oceanic island lenses have been developed by hydrogeologists during the last decade. Based on existing equations, this study focuses on the case of strip oceanic islands when three combined effects of climate change are observed to affect the freshwater lens volume and its groundwater resource renewal: sea-level rise, erosion, and change in groundwater recharge rates. New equations, integrating these combined effects of climate change on fresh groundwater resources are provided. These equations are solved by a novel methodology based on a Dupuit-Forchheimer groundwater flow model that allows for determination of the hydrogeological parameters included in the equations. The approach is illustrated with the strip island of Savary, which is located along the Pacific Coast of Canada in the province of British Columbia. This example illustrates, on the one hand, the volume depletion of the island freshwater lens and, on the other hand, the decrease of the renewal rate of groundwater. The proposed approach can be applied to any strip islands worldwide to assess the cumulative effects of sea-level rise and shore erosion on groundwater resources, depending on the predicted climate change scenarios. The results can then help decision-makers to anticipate the effects of climate change on the groundwater availability in strip oceanic islands and plan future groundwater use accordingly.

Groundwater age distribution in a highly dynamic coastal aquifer

Besides other environmental tracers, the use of the 3H-3He method enables the estimation of modern groundwater ages. Widely used in fresh groundwater studies, the combination of tritium (3H) and its stable daughter product tritiogenic helium-3 (3Hetrit), however, has so far only been applied in a few studies to date groundwater in coastal aquifers. The coastal aquifer presented in this study is located at the very young eastern part of the North Sea barrier island Spiekeroog, the so-called ‘Ostplate’, that is characterized by a currently developing freshwater lens. Previously field and numerical modeling studies showed that the groundwater salinity distribution within this coastal aquifer is largely determined by seawater infiltration during storm tides. The main aim of this study was to characterize the groundwater age distribution in a highly dynamic coastal aquifer using the 3H-3He age dating method. In addition, a transient 2-D density-dependent flow and transport model that was calibrated to groundwater heads and salinity was utilized to assess whether measured 3H-3He ages can be reliably simulated despite the mixing of distinctly different and time-variable 3H endmembers, i.e., freshwater and seawater. The numerical calculation of the 3H-3He age distribution, thereby, was based on the simulation of the fictive tracer ‘stable tritium’ (sum of 3H and 3Hetrit) and 3H. Even though the freshwater lens is very young, both field observations and simulation results indicate notably lower ages within the freshwater lens as compared to the age of the lens itself. Simulation results show that frequent seawater infiltration, most importantly during winter storm tides, and discharging water from the freshwater lens lead to a disturbance of the vertical age stratification below the beach. A comparison with direct ages shows that the chosen 2-tracer approach improves system understanding, especially with regard to dispersion effects.

Dynamic analysis of freshwater lens in Laura Island

Dynamic analysis of freshwater lens in Laura Island

IMPLICATIONS OF CORAL REEF MINING PITS ON 2D HYDRODYNAMICS

Small Island Developing States (SIDS) are among the most vulnerable areas to the impact of natural hazards and climate change. Flooding due to storm surges and extreme waves, coastal erosion, and salinization of freshwater lenses are already a serious threat and could lead to irreversible consequences in the coming decades. On the other hand, reef flat mining is one of the most common practices to source the required material for the implementation of coastal protection measures on SIDS. There are strong concerns about this practice, because partial removal of the protective reef could increase wave impacts on the islands. However, the available data and knowledge on the effects of these mining pits are currently very limited. Klaver et al. (2019) studied the effects that pits may have on nearshore hydrodynamics and wave runup. Results obtained via numerical modeling indicate that excavation pits cause a decrease in infragravity wave energy around the fundamental mode of the reef, which is partly caused by reduced wave transmission. Similarly, changes in sea and swell (SS) wave energy are attributed to reduced transmission, as well as a decrease in wave dissipation and (triad) wave-wave interaction. Earlier studies by Ford et al. (2013) and Yao et al. (2016) point out that mining pits may lead to additional two-dimensional effects. In this presentation, we present the potential 2D effects of reef flat excavation pits on nearshore hydrodynamics. More specifically, we aim to assess the impact of excavation pits on nearshore circulation patterns, which could be an indicator of coastal morphological changes.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/85-IMM66eWE

A Growing Freshwater Lens in the Arctic Ocean With Sustained Climate Warming Disrupts Marine Ecosystem Function

AbstractOne of the most robust changes in the hydrological cycle predicted by Earth System Models (ESMs) during the remainder of 21st century is an increase in the difference between precipitation and evapotranspiration (P‐E) in arctic and boreal regions. We explore the long‐term consequences of this change for marine ecosystems in the Arctic Ocean using the Community Earth System Model forced with a business as usual scenario of future greenhouse gas concentrations. We find that by the year 2300 increases in freshwater delivery considerably reduce Arctic Ocean surface salinity, creating a freshwater lens that has far‐reaching impacts on marine biogeochemistry. The expanding freshwater lens limits vertical nutrient supply into the euphotic zone by enhancing vertical stratification and accelerating surface lateral mixing with surface waters in the North Atlantic, which become increasingly nutrient depleted from weakening of the Atlantic Meridional Overturning Circulation (AMOC). The resulting increase in nutrient stress reduces marine export production in the Arctic Ocean by 53% in 2300 relative to the 1990s and triggers a shift in community composition with small phytoplankton replacing diatoms. At the same time, the seasonal timing of export production undergoes a 2‐month forward shift, with the peak advancing from July to May. This suggests that the threat to food webs and higher trophic levels may intensify after the year 2100 as gains in productivity from sea ice loss saturate and freshwater impacts on nutrient stress continue to strengthen. Our analysis highlights the critical importance of changing terrestrial hydrology and land‐ocean coupling as drivers of long‐term biogeochemical change in the Arctic Ocean and the necessity of multi‐century climate change projections.

Climatogenic Restrictions of Arid Forestry

The creation of protective forests is especially difficult in arid regions of Russia, and global warming can further affect their ability to persist. In this study climatogenic changes in forest growing conditions are examined in the Northern Caspian region, taking into account that the functioning of forest stands is limited on historically treeless lands: authomorphic meliorated solonetz soils and calcisoles (by the extra moisture of winter snowfalls and the humidity levels during the vegetational period), as well as intrazonal hydromorphic gleyic kastanozems of the local relief depressions (by the inexhaustibility of freshwater lenses in salty subterranean waters based on its periodic resupply by the spring surface flow every 2–4 years). For forest stands established on automorphic soils, a prolonged period of annual draughts and mild winters with little snow deposit in forest lands has proven to be the most dangerous. During such periods, the large-scale death of trees is observed and many forest stands are ruined. For forest stands on hydromorphic soils, such periods prove to be just as dangerous because of the 15-year-long absence of spring surface flow. Such a climatogenic scenario of long-termed deterioration of forest growing conditions in the Northern Caspian region is similar to the southernmost regions of Russia that are currently totally unable to sustain forests, with unstable snow cover and frequent draughts.

Applying resistivity (dipole-dipole, Schlumberger, and Wenner) joint inversion to detect endokarst features in Quintana Roo, México

The state of Quintana Roo is part of the Yucatan Peninsula (YP), a platform formed by calcareous sediments. The joint action of erosion, transport, and sedimentation processes by surface and underground water on limestone results in a structure known as karst. Karst systems are typically characterized by lack of surface water due to high infiltration, presence of sinkholes and caves, and groundwater flow through carbonates that over time create long channels and caverns by rock dissolution. According to the literature the main source of freshwater in the Yucatan Peninsula is a regional aquifer. Hydrogeological models explain this aquifer as a thin freshwater lens floating over seawater, and the thickness of this freshwater lens varies with proximity to the coast. Ongoing population growth in Quintana Roo has increased demand for freshwater, and the only source of water there is found underground. Geophysical exploration methods are important for obtaining information on the structure of the subsoil, which is fundamental for developing flow models and estimating water availability, thus enabling us to carry on with sustainable water management. In this study, we apply a two-dimensional joint and separate inversion of data with dipole-dipole, Schlumberger, and Wenner configurations on real and synthetic data. We show that joint inversion is better than using separate inversions because the joint inversion uses a higher density of data and different current distribution in the subsoil. The high heterogeneity of karstic environments can be better detected if more information on the subsoil is obtained, allowing for more precise interpretations.

Source of saline groundwater on tidally influenced blue holes on San Salvador Island, Bahamas

Stable hydrogen and oxygen isotope signatures of waters within Church and Inkwell blue holes are measured on San Salvador Island (Bahamas) to identify the origin of their fresh and saline waters. Stable isotope data, paired with a suite of physicochemical water parameters measured throughout the blue holes, as a function of both time and depth, provide a detailed understanding of the tidally influenced groundwater interactions on the island. Blue holes are prominent karst features in carbonate environments which serve as windows into subterranean hydrologic processes. Carbonate island hydrology is often complicated by complex interactions between the marine and meteoric water systems, as tidal pumping and water mixing result in diagenetic alteration of the bedrock, that in turn influence dissolution rates and preferential flow paths. Although the blue holes on the island are physically influenced by tidal forcing, the stable isotope data indicate that both their fresh and saline waters are of a meteoric origin rather than seawater, where the meteoric water is likely becoming saline through enrichment by aerosol-derived sea salts. Additionally, the physical profiles of each blue hole indicate differences in mixing processes driven by wind and tidal forcing, where stronger mixing can result in a disruption of the freshwater lens. The implications of this study are important for assessing mixing corrosion processes and dissolution effects, but more research and longer data sets are needed to show whether these results are applicable to other coastal carbonate environments.

Evaporation and Salt Accumulation Effects on Riparian Freshwater Lenses

AbstractWhile previous studies conclude that riparian freshwater lenses are the result of buoyancy forces that drive river water circulation in the adjacent aquifer, only highly idealized conceptual models have been assessed. A numerical study is conducted to investigate the influence of evaporation from unsaturated bare soils, including accumulation and precipitation of salt, on riparian freshwater lenses, assuming otherwise similar conditions to those adopted in recent studies. Non‐evaporating simulations that account for the unsaturated zone are first compared to previous saturation‐only numerical and analytical results, and reasonable matches are obtained. The modeling analysis of evaporative simulations includes three different approaches to the treatment of solute in evaporated water: (a) no evaporative salt accumulation, (b) evaporative salt accumulation, and (c) as for (b) except resistance to evaporation caused by salt accumulation is considered. Significant evaporative fluxes caused gaining river conditions to change to losing conditions, leading to the cessation of buoyancy‐driven freshwater circulation and to freshwater bodies that are not lenticular. Rather, evaporation reduces regions where the water table and/or unsaturated zone are fresh relative to the freshwater lens volume. Evaporation rates depend on the hydraulic connection between the floodplain surface and water table, which is limited where the water table is deep, the soil hydraulic conductivity is low, and under higher potential evaporation rates. The results show that floodplain evaporation may control the freshwater‐saltwater distribution and lead to hypersaline groundwater under certain conditions, although earlier buoyancy‐driven conceptual models of riparian lenses appear to remain valid where the thickness of the unsaturated zone is sufficiently large.

Small atoll fresh groundwater lenses respond to a combination of natural climatic cycles and human modified geology

Freshwater lenses underlying small ocean islands exhibit spatial variability and temporal fluctuations in volume, influencing ecologic management. For example, The Palmyra Atoll National Wildlife Refuge harbors one of the few surviving native stands of Pisonia grandis in the central Pacific Ocean, yet these trees face pressure from groundwater salinization, with little basic groundwater data to guide decision making. Adding to natural complexity, the geology of Palmyra was heavily altered by dredge and fill activities. Our study based at this atoll combines geophysical and hydrological field measurements from 2008 to 2019 with groundwater modeling to study the drivers of observed freshwater lens dynamics. Electromagnetic induction (EMI) field data were collected on the main atoll islands over repeat transects in 2008 following ‘strong’ La Niña conditions (wet) and in 2016 during ‘very strong’ El Niño conditions (dry). Shallow monitoring wells were installed adjacent to the geophysical transects in 2013 and screened within the fresh/saline groundwater transition zone. Temporal EMI and monitoring well data showed a strong contraction of the freshwater lens in response to El Niño conditions, and indicated a thicker lens toward the ocean side, an opposite spatial pattern to that observed for many other Pacific islands. On an outer islet where a stand of mature Pisonia trees exist, EMI surveys revealed only a thin (<3 m from land surface) layer of brackish groundwater during El Niño. Numerical groundwater simulations were performed for a range of permeability distributions and climate conditions at Palmyra. Results revealed that the observed atypical lens asymmetry is likely due to more efficient submarine groundwater discharge on the lagoon side as a result of lagoon dredging and filling with high-permeability material. Simulations also predict large decreases (40%) in freshwater lens volume during dry cycles and highlight threats to the Pisonia trees, yielding insight for atoll ecosystem management worldwide.

Water Quality Study of Freshwater of Bekaa Valley to the Delta at Mediterranean Seawater in the region of Lebanon

The water quality of fresh water, springs and wells in Bekaa valley, mountain district, a region of Republic of Lebanon in connection/delta of Mediterranean sea (basin) is surveyed along the Lebanese north, central and south coast at Tripoli, Jounieh and Saida deltas respectively in which, Sannine Aquifer at Bekaa valley, the major groundwater source for the coastal area is in-direct contact with the sea. The thin freshwater lens (delta) is being heavily exploited in this area, where a large number of wells exist in the area passing coastal areas of Lebanon. Many of those wells are saline and are not suitable for direct domestic use or even for irrigation. This study deals with water quality study, general assessment of level of pollution and potential influence on seawater through analysis of Physico-Chemical parameters and selected Inorganic substances of water sample collected from south, central and north Lebanese coastal water among with major water natural resources at Bekaa valley.

Control of fracture networks on a coastal karstic aquifer: a case study from northeastern Yucatán Peninsula (Mexico)

The Yucatan Peninsula karst aquifer in southeastern Mexico is important because it is the only source of freshwater supply in the region. Along the eastern coast, the aquifer behaves as a shallow unconfined aquifer, and one of its main characteristics is the development of a complex network of karstic conduits. Electrical resistivity tomography (ERT) is a geophysical technique that provides reliable information on aquifer properties that can be applied to karst. With this in mind, ERT surveys (with dipole-dipole and Wenner arrays) together with vertical profiles of groundwater electrical conductivity were obtained in the northeast of Yucatan Peninsula in the Akumal zone. The objective was to evaluate the characteristic structure of the unsaturated and saturated areas, and to determine the importance of factors at a local level that affect the thickness of the freshwater lens, as well as groundwater flow directions near the coast. The results of the ERT sections show a thin vadose zone with resistivity values greater than 1,052 Ωm and a saturated zone with resistivity values that vary from 1 to 1,052 Ωm, and show the presence of dissolution conduits and fracture zones that are affecting the freshwater lens of the aquifer. In addition, water-table measurements show the role of fracture networks in the groundwater flow. The overall results indicate conditions for a possible microbasin in the Akumal zone, highlighting the importance of local fractures, which could work as groundwater divides along the eastern coast of the Yucatan Peninsula.