Freshwater Lens Research Articles

Combining resistivity and frequency domain electromagnetic methods to investigate submarine groundwater discharge in the littoral zone

Abstract. Submarine groundwater discharge (SGD) is an important gateway for nutrients and pollutants from land to sea. While understanding SGD is crucial for managing nearshore ecosystems and coastal freshwater reserves, studying this discharge is complicated by its occurrence at the limit between land and sea, a dynamic environment. This practical difficulty is exacerbated by the significant spatial and temporal variability. Therefore, to capture the magnitude of SGD, a variety of techniques and measurements, applied over multiple periods, is needed. Here, we combine several geophysical methods to detect zones of fresh submarine groundwater discharge (FSGD) in the intertidal zone, upper beach, dunes, and shallow coastal area. Both terrestrial electrical-resistivity tomography (ERT; roll-along) and marine continuous resistivity profiling (CRP) are used from the shallow continental shelf up to the dunes and combined with frequency domain electromagnetic (FDEM) mapping in the intertidal zone. In particular, we apply an estimation of robust apparent electrical conductivity (rECa) from FDEM data to provide reliable lateral and vertical discrimination of FSGD zones. The study area is a very dynamic environment along the North Sea, characterized by semi-diurnal tides between 3 and 5 m. CRP is usually applied in calmer conditions, but we prove that such surveys are possible and provide additional information to primarily land-bound ERT surveying. The 2D inversion models created from ERT and CRP data clearly indicate the presence of FSGD on the lower beach or below the low-water line. This discharge originates from a potable freshwater lens below the dunes and flows underneath a thick saltwater lens, present from the dunes to the lower sandy beach, which is fully observed with ERT. Freshwater outflow intensity has increased since 1980, due to a decrease of groundwater pumping in the dunes. FDEM mapping at two different times reveals discharge at the same locations, clearly displays the lateral variation of the zone of discharge, and suggests that FSGD is stronger at the end of winter compared to the beginning of autumn. ERT, CRP, and FDEM are complementary tools in the investigation of SGD. They provide a high-resolution 3D image of the saltwater and freshwater distribution in the phreatic coastal aquifer over a relatively large area, both off- and onshore.

Tidal pumping and biogeochemical processes: Dissolution within the tidal capillary fringe of eogenetic coastal carbonates

AbstractGrowing evidence suggests microbial respiration of dissolved organic carbon (DOC) may be a principal driver of subsurface dissolution and cave formation in eogenetic carbonate rock. Analyses of samples of vadose zone gasses, and geochemical and hydrological data collected from shallow, uncased wells on San Salvador Island, Bahamas, suggest tidally varying water tables may help fuel microbial respiration and dissolution through oxygenation. Respiration of soil organic carbon transported to water tables generates dysaerobic to anaerobic groundwater, limiting aerobic microbial processes. Positive correlations of carbon dioxide (CO2), radon‐222 (222Rn) and water table elevation indicate, however, that tidal pumping of water tables pulls atmospheric air that is rich in oxygen, and low in CO2 and 222Rn, into contact with the tidal capillary fringe during falling tides. Ratios of CO2 and O2 in vadose gas relative to the atmosphere indicate this atmospheric oxygen fuels respiration within newly‐exposed, wetted bedrock. Deficits of expected CO2 relative to O2 concentrations indicate some respired CO2 is likely removed by carbonate mineral dissolution. Tidal pumping also appears capable of transferring oxygen to the freshwater lens, where it could also contribute to respiration and dissolution; dissolved oxygen concentrations at the water table are at least 5% saturated and decline to anaerobic conditions 1–2 m below. Our results demonstrate how tidal pumping of air to vadose zones can drive mineral dissolution reactions that are focused near water tables and may contribute to the formation of laterally continuous vuggy horizons and potentially caves. © 2020 John Wiley & Sons, Ltd.

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.

Mitigating drought impacts in remote island atolls with traditional water usage behaviors and modern technology

Adaptation to drought is particularly challenging on remote island atolls, such as those found in the Republic of the Marshall Islands (RMI), a nation of 58,000 populating 29 low-lying coral atolls spread over >2 million km2. Exposure to consecutive atmospheric hazards, such as meteorological floods and droughts diminish scarce water resources and erode the resilience of island communities. Drought impact mitigation measures must supply emergency drinking water to stricken communities, while simultaneously conserving natural sources in order to reduce their vulnerability to subsequent events. Household surveys (n = 298) and focus group discussions (n = 16) in eight RMI communities revealed that 86% of households have experienced drought and 88% reported using multiple water sources to meet normal household needs. With no surface water and a thin freshwater lens (FWL), rainwater collected from rooftops is the most common household water source. The traditional use of carved hollows in the base of coconut trees to collect rainwater (“Mammaks”) appears to have been displaced by large rainwater tanks. However, rationing of rainwater for consumption only during drought was widely reported, with private wells supporting non-consumptive uses. Reverse osmosis (RO) desalination units have provided relief during drought emergencies but concerns have been raised around dependency, maintenance challenges, and loss of traditional water practices. Most notably, RO use has the potential to change the anthroposhpere by adversely affecting the FWL; 86% of RO units were installed at island-centre where excessive pumping can cause upconing, making the FWL brackish. Balancing the introduction of desalination technology to mitigate water shortages with maintenance of traditional water conservation practices to preserve the quantity and quality of the FWL is a promising strategy on island atolls that requires further investigation.

Freshwater lens oscillation induced by sea tides and variable rainfall at the uplifted atoll island of Minami-Daito, Japan

In order to evaluate the behavior of a freshwater lens in the uplifted atoll island of Minami-Daito, Japan, groundwater level and electrical conductivity were simultaneously measured in six monitoring wells. The monitoring allowed determination of the position and variability of the top and bottom positions of the freshwater lens. The freshwater lens periodically oscillates with phase lags (delay time) every 3 h following sea tides. Recharge from local rainfall temporarily increases the volume of the freshwater lens but is disturbed by the low-permeability muddy sediments deposited on the central lowland of the island. Changes in the groundwater levels correlate well with rainfall, after first removing semi-diurnal, diurnal, and long-term components of the sea tides from the groundwater level data using a multiple regression analysis. Changes in the water electrical conductivity provide information on the temporal and spatial fluctuation of the freshwater lens. The monitoring scheme for this freshwater lens could be applied to other uplifted atolls, contributing to better evaluation of the potable groundwater resources and to making freshwater use sustainable on other islands.

Airborne Electromagnetic, Magnetic, and Radiometric Surveys at the German North Sea Coast Applied to Groundwater and Soil Investigations

The knowledge of the subsurface down to about one hundred meters is fundamental for a variety of economic, ecological, and geoscientific tasks, particularly in coastal zones. Marine and terrestrial processes influence coastal zones and both seawater intrusion and submarine freshwater discharges may occur. The Federal Institute for Geosciences and Natural Resources (BGR) conducted airborne geophysical surveys in the coastal region of the German Bight between 2000 and 2014. The helicopter-borne system used simultaneously collected electromagnetic (HEM), magnetic (HMG), and radiometric (HRD) data. An area of about 5900 km2 was covered with parallel flight lines at 250 m line separation and additional tie-lines at larger separations. In total, about 25,000 km of data at sampling distances of 4 m (HEM, HMG) and 40 m (HRD) were acquired. The electrical resistivity (HEM), the anomalies of the magnetic field (HMG), and the exposure rate (HRD) are the resulting geophysical parameters derived from the data. The results are displayed as maps of the geophysical parameters as well as vertical resistivity sections (only HEM). Both data and products are publicly available via BGR’s product center. The airborne geophysical results helped to outline the fresh–saline groundwater interface, freshwater lenses on islands, submarine groundwater discharges, buried tunnel valleys, mires, and ancient landscapes.

Transient Evolution of Inland Freshwater Lenses: Comparison of Numerical and Physical Experiments

Brackish to saline groundwater in arid environments encourages the development and sustainability of inland freshwater lenses (IFLs). While these freshwater resources supply much-needed drinking water throughout the Arabian Peninsula and other drylands, little is understood about their sustainability. This study presents a numerical model using the SEAWAT programming code (i.e., MODFLOW and the Modular Three-Dimensional Multispecies Transport Model (MT3DMS)) to simulate IFL transient evolution. The numerical model is based on a physical laboratory model and calibrated using results from simulations conducted in a previous study of the Raudhatain IFL in northern Kuwait. Data from three previously conducted physical model simulations were evaluated against the corresponding numerical model simulations. The hydraulic conductivities in the horizontal and vertical directions were successfully optimized to minimize the objective function of the numerical model simulations. The numerical model matched observed IFL water levels at four locations through time, as well as IFL thicknesses and lengths (R2 = 0.89, 0.94, 0.85). Predicted lens degradation times corresponded to the observed lenses, which demonstrated the utility of numerical models and physical models to assess IFL geometry and position. Improved understanding of IFL dynamics provides water-resource exploration and development opportunities in drylands throughout the Arabian Peninsula and elsewhere with similar environmental settings.

Groundwater flow and residence times below a meso-tidal high-energy beach: A model-based analyses of salinity patterns and 3H-3He groundwater ages

In this study, 3H-3He ages in the intertidal zone based on both field measurements and numerical simulations are presented to provide insights into the flow and transport behavior within the subterranean estuary of a meso-tidal, high-energy beach. In a systematic modelling approach, important hydrogeological parameters (dispersivity, hydraulic conductivity and fresh groundwater flux) were varied in a cross-shore transect at a beach on Spiekeroog Island in north-western Germany. Both measured and simulated salinity and age distribution within the intertidal zone suggest the existence of two– at least temporally occurring- saltwater recirculation cells accompanied by two fresh discharge location. This is in contrast to the state-of-the-art concept of a STE under tidal influence and is caused by a runnel and ridge system at the study site, which is a rather wide-spread type of beach morphology with topographical highs and interfering lows in the intertidal zone. Residence times within the two saline plumes were between weeks and several months. Freshwater from the island’s freshwater lens resided up to 18 years in the brackish beach aquifer before exfiltrating at the discharge points. Field measurements confirmed the discharge of decade-old water at the discharge points. Model results were highly dependent on the tested model parameters. A change between a one- and two-plume system, as well as overlapping conditions at the study site dependent on the change in beach topography are likely.

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.

Expanding Freshwater Lenses Adjacent to Gaining Rivers Through Vertical Low‐Hydraulic‐Conductivity Barriers: Analytical and Experimental Validation

AbstractFreshwater lenses within riparian zones of some arid and semiarid settings assist in maintaining the health of riparian ecosystems. We propose an approach for expanding freshwater lenses in saline aquifers adjacent to gaining rivers through the addition of a vertical barrier of low‐hydraulic‐conductivity (low‐K) parallel to the river bank. Sharp‐interface analytical solutions for the lens shape and water table distribution are developed to examine the effectiveness of the proposed method and are verified using sand tank experiments and numerical simulations. The sensitivity analysis is used to apply the method to parameters typical of the Lower River Murray (South Australia) and its floodplain aquifers. The results show that the barrier can create significant freshwater lenses in head‐controlled systems, whereas the barrier may lead to lens shrinkage in flux‐controlled systems due to saline water table rise. That is, the effectiveness of the barrier is highly dependent on the inland boundary condition. The analytical solution presented herein can be used to efficiently predict the riparian freshwater lens extent in response to engineered barriers, adding to existing techniques for studying and modifying riparian freshwater lenses.

High-resolution mapping of the freshwater–brine interface using deterministic and Bayesian inversion of airborne electromagnetic data at Paradox Valley, USA

Salt loads in the Colorado River Basin are a primary water quality concern. Natural groundwater brine discharge to the Dolores River where it passes through the collapsed salt anticline of the Paradox Valley in western Colorado (USA) is a significant source of salt to the Colorado River. An airborne electromagnetic survey of Paradox Valley has provided insights into the three-dimensional distribution of brine in the surficial aquifer. A combination of stochastic and deterministic resistivity inversions was used to interpret the top of the freshwater–brine interface and to qualitatively describe the vertical salinity gradients across the interface. Low-resistivity regions indicative of brine occur near the land surface where brine discharges to the Dolores River and increase in depth several kilometers up-gradient along the axis of the valley. The most conductive parts of the brine plume are found in the areas below and adjacent to the river, suggesting that the brine becomes shallower and more concentrated as it reaches its natural discharge location. A significant freshwater lens overlying the brine west of the Dolores River is spatially correlated to the intermittent West Paradox Creek and agricultural irrigation. Below this lens, the transition from freshwater to brine appears to occur abruptly over a few meters and correlates to available well information. However, away from these regions and particularly with distance from the river, the freshwater–brine interface appears to be more diffuse.

Using hydrogeochemical indicators to interpret groundwater flow and geochemical evolution of a freshwater lens on Majuro Atoll, Republic of the Marshall Islands

Groundwater flow and the geochemical evolution of a freshwater lens in an aquifer on a Pacific atoll were investigated by hydrogeochemical surveys. Sulfur hexafluoride measurements showed that deeper groundwater and groundwater at the periphery of the lens are older, consistent with a downward and outward groundwater flow scheme. This is the typical flow scheme on Pacific atolls where a Holocene–Pleistocene unconformity restricts the shape of the freshwater lens. Enrichment of Mg/Ca in the groundwater is another indicator of a longer residence time, because contact between the groundwater and the carbonate sediments composing the aquifer leads to the release of Mg from high-Mg calcite and the precipitation of Ca as low-Mg calcite. Groundwater quality was also affected by anthropogenic nitrogen loading and aboveground organic matter, which were altered by denitrification and sulfate reduction in the aquifer, especially in the older groundwater. The chemical composition of the groundwater in the center of the island, where saline water is up-coning, implies that freshwater recharge dilutes the older saline water, which as time passes will eventually be replaced by newly recharged freshwater.

Light attenuation as a control for microbiogeomorphic features: Implications for coastal cave speleogenesis

San Salvador (Bahamas) is a carbonate island with dozens of flank margin caves formed in the phreatic zone by fresh seawater mixing within the freshwater lens. These caves have no direct connection with the sea, and form at or close to the tidally influenced fluctuating water table. After sea-level fall, in their subaerial parts caves are enlarged mainly by rock dissolution and by erosion close to the water level, condensation-corrosion and breakdown processes. For understanding the geomorphological features observed in these caves and how they are related to light attenuation, we investigated three sampling sites in the tidally influenced zone of Lighthouse Cave, which has been re-invaded by seawater during the Holocene sea-level highstand. A freshwater lens no longer exists within or adjacent to the cave. Rock samples were collected above and below the internal lake shores close to the entrance, and in the twilight and dark zones of this cave. Light and electron microscopy examinations were conducted for detecting microbial cells, as well as bioconstruction and bioweathering features. In addition, a high precision laser scanner was used for characterising sample microtopography. Our data showed that the microtopography and geomorphology of the lake shore samples (cave entrance) are dominated by bioweathering, whereas the samples of the twilight and dark zones are controlled by a combination of both bioweathering and bioconstructive processes depending on light availability. Bioconstructive structures, such as semi-planar lamination, at the fluctuating water level of the Lighthouse Cave show that dissolution due to water mixing of sea and freshwater in the Holocene is no longer the most important speleogenetic process. We propose that the geomorphological evolution is strongly influenced by the degree of rock diagenesis more than the initial mechanism of speleogenesis.

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.

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

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

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.

How water isotopes (18O, 2H, 3H) within an island freshwater lens respond to changes in rainfall

Coastal aquifers provide an important source of water globally. Understanding how groundwater responds to changes in rainfall recharge is important for sustainable development. To this end, we investigate how water isotopes (18O, 2H, 3H) and chloride (Cl) concentrations within an island freshwater lens respond under varying rainfall conditions in a region experiencing climate change. Uniquely, this study presents a three year dataset of groundwater collected seasonally between May 2013 and August 2016 from ten wells. Variation in all tracers was observed. The Cl and tritium (3H) show opposing seasonal variation in some sections of the lens, with higher Cl observed in the austral summer when less rainfall occurs and evapotranspiration is highest. The opposite occurs in the austral winter months when 3H increases from atmospheric input via rainfall recharge, and Cl is diluted. An overall decline in 3H values and enrichment in stable water isotopes over the study period was also observed. This study shows that understanding groundwater of freshwater lenses should not rely on a single sampling campaign because seasonal variability is large. The identification of a dual recharge regime, with contributions from both winter rainfall and episodic events, has important implications for understanding the future fate of the freshwater lens on Rottnest Island. The finding that episodic rainfall is a major contributor to groundwater recharge is important and can only be assessed with a multi-year isotope dataset for groundwater and rainfall.

SEA-POL Goes to Sea

AbstractA new, advanced radar has been developed at Colorado State University (CSU). The Sea-Going Polarimetric (SEA-POL) radar is a C-band, polarimetric Doppler radar specifically designed to deploy on research ships. SEA-POL is the first such weather radar developed in the United States. Ship-based weather radars have a long history, dating back to GATE in 1974. The GATE radars measured only reflectivity. After GATE, ship radars also provided Doppler measurements. SEA-POL represents the next advancement by adding dual-polarization technology, the ability to transmit and receive both horizontal and vertical polarizations. This configuration provides information about hydrometeor size, shape, and phase. As a result, superior rain-rate estimates are afforded by the dual-polarization technology, along with hydrometeor identification and overall improved data quality. SEA-POL made its first deployment as part of the Salinity Processes in the Upper Ocean Regional Study, second field phase (SPURS-2) fall 2017 cruise to the eastern tropical Pacific, sailing on the R/V Roger Revelle. SPURS-2 was a field project to investigate the fate of freshwater deposited on the ocean’s surface. Oceanographers are keenly interested in how fast these freshwater patches mix out by wind and upper-ocean turbulence, as the less dense rainfall sitting atop the salty ocean inhibits mixing through increased stability. To this end, during SPURS-2, SEA-POL produced rain maps identifying the location of freshwater lenses on the ocean’s surface thereby providing context for measurements of SST and salinity. Examples of SEA-POL polarization measurements are also discussed to assess microphysical processes within oceanic convection. Future ocean-based field campaigns will now benefit from SEA-POL’s advanced dual-polarization technology.

Behaviour of Tritium and Tritiogenic Helium in Freshwater Lens Groundwater Systems: Insights from Langeoog Island, Germany

Tritium (3H) and its daughter product 3He have been widely used as tracers in hydrological studies, but quantitative analyses of their behaviour in freshwater lenses and the transition zone in coastal aquifers are presently lacking. In this paper, the fate of 3H and 3He in the freshwater lens and the transition zone as well as the saltwater wedge is studied using numerical variable-density flow and transport models of different degrees of complexity. The models are based on the conditions on the German island of Langeoog, which is uniquely suited for this purpose because of the high 3H concentration of the North Sea. It is found that most bomb-related tritiogenic 3He still resides in the freshwater lens, making it a useful tracer for young (<60 years) groundwater. Differences in dispersive transport between 3H and 3He can cause an apparent age bias on the order of 10 years. Under favourable conditions, 3H from seawater can penetrate deep into the offshore part of the aquifer and has potential to be used as a tracer to study saltwater circulation patterns. Our modelling suggests that the field-observed 3H in the transition zone does not originate from seawater but from freshwater affected by the bomb peak. Yet in models with a low (αL=0.5 m) dispersivity, no 3H was sequestered into the transition zone and the transition zone width was underestimated. Better results were obtained with αL=5 m, a value that is higher than in comparable modelling studies, which suggests that further work is needed to better understand the controls (tides, lithological heterogeneity, or transience of recharge and pumping) on transition zone mixing processes.

Shifting baselines in coastal forests: Rising seas transform plant communities from the ‘ground’ up

Increases in the rate of sea level rise are likely to result in changes to disturbance-adapted coastal forests and associated freshwater resources over the next several decades. In this study, we investigated how the press disturbance of two decades of sea level rise altered island hydrology and interacted with pulse disturbances (frequent hurricane impacts) to alter coastal forest composition and structure. The research was conducted on two low-elevation islands located within the lower Florida Keys: Big Pine and Upper Sugarloaf Keys. Groundwater salinity and vegetation structure and composition were sampled in the early 1990s prior to impact from two hurricanes – Hurricanes Georges (1998) and Wilma (2005) – and again in 2012/2013 in permanent plots inside and outside the boundaries of the islands’ freshwater lenses. Using linear mixed effects modeling, we examined whether groundwater salinity varied over time among locations, and investigated whether this variation was reflected in changes in forest structure and composition in three height strata. The results of this analysis revealed that groundwater salinity underlying plots outside the freshwater lens increased over the two decades, while salinity of groundwater at plots inside the freshwater lens remained stable. The greatest shift in composition occurred in the understory strata along a gradient of increasing salinity, and plots located outside the freshwater lens gained species typical of tidally-influenced buttonwood forest and lost glycophytic species. Viewed against the background of recurring hurricane impacts, these findings suggest that sea level rise is currently altering both groundwater resources and the composition of coastal forests in the Florida Keys. Similar dynamics should be observed in low-lying coastal forests within ocean basins subject to increased tropical cyclone activity. Management of these island coastal forests must now consider the continually shifting nature of the resource in light of acceleration in sea level rise.