Submarine Groundwater Discharge Research Articles

Applicability of Landsat 8 thermal infrared sensor for identifying submarine groundwater discharge springs in the Mediterranean Sea basin

Abstract. Submarine groundwater discharge (SGD) has received increasing attention over the past 2 decades as a source of nutrients, trace elements and ocean pollutants that may alter coastal biogeochemical cycles. Assessing SGD flows and their impact on coastal marine environments is a difficult task, since it is not easy to identify and measure these water flows discharging into the sea. The aim of this study is to demonstrate the significant usefulness of the freely available thermal infrared (TIR) imagery of the Landsat 8 thermal infrared sensor (TIRS) as an exploratory tool for identifying SGD springs worldwide, from local to regional scales, for long-term analysis. The use of satellite thermal data as a technique for identifying SGD springs in seawater is based on the identification of thermally anomalous plumes obtained from the thermal contrasts between groundwater and sea surface water. In this study, we use the TIR remote sensing (TIR-RS) imagery provided by Landsat 8 at a regional scale and discuss the principle limiting factors of using this technique in SGD studies. The study was developed in karstic coastal aquifers in the Mediterranean Sea basin during different seasons and under diverse meteorological conditions. Although this study demonstrates that freely available satellite TIR remote sensing is a useful method for identifying coastal springs in karst aquifers both locally and regionally, the limiting factors include technical limitations, geological and hydrogeological characteristics, environmental and marine conditions and coastal geomorphology.

Unveiling the extent of salinization to delineate the potential submarine groundwater discharge zones along the North-western coast of India

The North-West coast of India was notable for the urbanization and semiarid climate, particularly the Gujarat coastal region which was facing water crises and aquifer salinization issue. Under the light of these critical issues, the present study investigates the sources of aquifer salinization and probable location of submarine groundwater discharge (SGD) using an integrated approach of major ion chemistry, statistical techniques, and isotopic signature of groundwater (GW). The evolution of GW reveals that water facies changes from Ca2+-Mg2+-Cl− to Na+-Cl type from the south Gujarat towards the Gulf of Khambhat. Log-normal distribution of Cl− and NO3− divulges that different pollution sources influence the GW quality. Statistical findings supplemented with Isotopic signatures, ionic ratios and cross plots identified four classes of GW, which varies with degree of anthropogenic and seawater influences. Results suggested that seawater intrusion heavily influences 42% of the total GW samples, whereas 58% samples showed the probability of SGD. The study recommends the feasible locations of check dams as a remedial measure for controlling the salinization of coastal aquifer.

The hydrological connection between fresh submarine groundwater discharge and coastal groundwater: an isotopic and a decadal hydrochemistry approach in an alluvial fan, central Japan

The hydrological connection between fresh submarine groundwater discharge and coastal groundwater: an isotopic and a decadal hydrochemistry approach in an alluvial fan, central Japan

Geochemistry of the redox-sensitive trace elements molybdenum, tungsten, and rhenium in the euxinic porewaters and bottom sediments of the Pettaquamscutt River estuary, Rhode Island

Molybdenum (Mo), tungsten (W), and rhenium (Re) concentrations were measured in the porewaters and sediments of the highly stratified Upper Basin of the Pettaquamscutt River estuary (Rhode Island) to investigate the geochemical processes that influence their concentrations in euxinic systems. Total dissolved sulfide (i.e., ΣS-II) concentrations in the porewaters averaged 3.95 mmol kg−1, whereas the porewaters exhibited average Mo, W, and Re concentrations of 830 nmol kg−1, 69 nmol kg−1, and 7.4 nmol kg−1, respectively. The sediments were subjected to chemical extractions to characterize the reactive fraction (i.e., associated with iron (Fe) monosulfides, Fe/manganese (Mn) oxides/oxyhydroxides, and/or Fe/Mn carbonate minerals) and the pyritized fraction of Mo, W, and Re in the sediments. The reactive fraction of each metal decreased with depth from subsurface maxima near the sediment-water interface, whereas the pyritized fractions exhibited a number of subsurface maxima. Rhenium exhibited the greatest degree of trace metal pyritization (DTMP) in the estuary sediments, followed closely by Mo. Specifically, the DTMP for Re ranged from 75 to 88% and exhibited a mean ( ± 1σ) of 80 ± 4% for the entire sediment core, whereas the DTMP for Mo ranged from 34 to 89% with a mean ( ± 1σ) of 73 ± 18%. In contrast, the DTMP for W was lower, ranging from 48 to 73%, but with a mean ( ± 1σ) of 58 ± 8%. Therefore, about 80%, on average, of the Re and 74% of the Mo that enters the northern portion of the Pettaquamscutt River estuary by the combination of stream water inflow (i.e., Gilbert Stuart Stream) and submarine groundwater discharge (SGD) is eventually fixed within the pyrite fraction of the euxinic bottom sediments. However, only about 60% of the W appears to be fixed in the pyrite fraction in these sediments, whereas the other 40% remains in the reactive sediment pool. We suggest that the different behavior of W compared to Re and Mo in the euxinic porewater and sediments of the Pettaquamscutt River estuary's upper basin reflects, in part, differences in their aqueous speciation and particle reactivity that appears to be dictated by differential uptake and fixation of their respective thioanions (i.e., MoS42−, WS42−, ReS4−) with Fe sulfide minerals.

Environmental Surveillance of SARS-CoV-2 RNA in Wastewater and Groundwater in Quintana Roo, Mexico.

The presence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) RNA in wastewater has been reported as a result of fecal shedding of infected individuals. In this study, the occurrence of SARS-CoV-2 RNA was explored in primary-treated wastewater from two municipal wastewater treatment plants in Quintana Roo, Mexico, along with groundwater from sinkholes, a household well, and submarine groundwater discharges. Physicochemical variables were obtained in situ, and coliphage densities were determined. Three virus concentration methods based on adsorption-elution and sequential filtration were used followed by RNA isolation. Quantification of SARS-CoV-2 was done by RT-qPCR using the CDC 2020 assay, 2019-nCoV_N1 and 2019-nCoV_N2. The Pepper mild mottle virus, one of the most abundant RNA viruses in wastewater was quantified by RT-qPCR and compared to SARS-CoV-2 concentrations. The use of three combined virus concentration methods together with two qPCR assays allowed the detection of SARS-CoV-2 RNA in 58% of the wastewater samples analyzed, whereas none of the groundwater samples were positive for SARS-CoV-2 RNA. Concentrations of SARS-CoV-2 in wastewater were from 1.8 × 103 to 7.5 × 103 genome copies per liter (GC l−1), using the N1 RT-qPCR assay, and from 2.4 × 102 to 5.9 × 103 GC l−1 using the N2 RT-qPCR assay. Based on PMMoV prevalence detected in all wastewater and groundwater samples tested, the three viral concentration methods used could be successfully applied for SARS-CoV-2 RNA detection in further studies. This study represents the first detection of SARS-CoV-2 RNA in wastewater in southeast Mexico and provides a baseline for developing a wastewater-based epidemiology approach in the area.

Computation of submarine groundwater discharge from geomorphologically different coastal catchments of SW India using numerical modeling

In this study, we investigate submarine groundwater discharge (SGD) and the influence which coastal geomorphology and geology can have on its behavior. Numerical models have been developed to estimate the magnitude of submarine groundwater discharge (SGD) from catchments with different geomorphology on the Southwest coast of India. Two coastal catchments of Thiruvananthapuram district, Kerala, India namely Varkala (184 km2) and Kadinamkulam (186 km2), have been selected to compute SGD. The results of the study confirmed that the SGD flux from the catchments differs in a great way across its multiple layers as well as in comparison to one another. The fresh SGD flux from Varkala catchment is found to be 0.1 m3/day/m and from Kadinamkulam catchment is 6 m3/m/day. In the present study, it is concluded that SGD is reflected in the heterogeneity of aquifers owing to the differences in geomorphology, land use-land cover, hydrogeology, and lithology. A numerical modeling approach that can yield reliable results with low computational cost and time is a potent method for quantifying SGD. The inferences drawn will be useful for designing aquifer recharge options and optimum pumping quantities for the region.

Hydrogeological and Hydrochemical Characterization of Coastal aquifers with Special reference to Submarine Groundwater Discharge in Uttara kannada, Karnataka, India

Submarine groundwater discharges to the coastal ecosystems have been recognised as a source of dissolved chemical substances that cause chemical and ecological effects on sea waters. Groundwater, in many coastal areas, becomes contaminated or at least enriched with a variety of chemical substances and can have higher concentrations of dissolved solids than river water. As a result SGD makes a larger contribution to the flux of dissolved chemical compounds than river run-off. Therefore, the present investigation has been carried out to understand the process of submarine groundwater discharge based on hydrological, hydrogeological and hydrochemical components. Accordingly water balance components were evaluated based on hydrological and hydrogeological investigations. Hydrochemical parameters were also evaluated to understand the impact of seawater intrusion during both pre-monsoon and post-monsoon seasons of 2019. Study revealed that, there are signatures of considerable quantity of submarine groundwater discharge in parts of Honnavara, Kumta, Ankola and Karwar talukas. The influence of seawater in coastal aquifers is quite rare all along the coast of Uttara kannada district which is attributed to high groundwater recharge(15-20%) occurring in catchment areas.

An Initial Assessment of the Contribution of Fresh Submarine Ground Water Discharge to the Alkalinity Budget of the Mediterranean Sea

AbstractExcess total alkalinity (TA) of Mediterranean Sea (MS) seawater relative to its Atlantic origin has been attributed to high inputs from major rivers and the Black Sea. Submarine groundwater discharge (SGD), which was recently shown to be an important source of nutrients to the MS, may also be an important source of TA to the MS. During 2015–2017, water samples were taken from a shoreline groundwater spring near Tel‐Shikmona, northern Israel, and analyzed for TA, salinity, and dissolved inorganic nutrients. Spring water TA varied seasonally between a low of 3,919 and a high of 7,148 µmol kg−1 in the summer and winter, respectively. Long‐term records from a nearby monitoring site indicate that wintertime coastal water TA is positively correlated with annual rainfall rates. Based on the Tel‐Shikmona groundwater spring measurements and other measurements from the MS region reported in the literature, we estimated that the TA flux from the fresh component of SGD (FSGD) in this region is 22%–37% and 17%–30% of the riverine and Black Sea TA fluxes to the MS, respectively.

Submarine Groundwater Discharge From Non-Tidal Coastal Peatlands Along the Baltic Sea

Submarine groundwater discharge (SGD) is an important pathway for water and materials within the land-ocean transition zone that can impact coastal environments and marine life. Although research from sandy shorelines has rapidly advanced in recent years, there is very little understanding of coastal areas characterized by a low hydraulic conductivity, such as carbon-rich coastal peatlands. The objective of this study was to determine the magnitude and location of terrestrial SGD to be expected from a non-tidal low-lying coastal peatland located along the Baltic Sea and to understand the controlling factors using numerical modeling. We employed the HYDRUS-2D modeling package to simulate water movement under steady-state conditions in a transect that extends from the dune dike-separated rewetted fen to the shallow sea. Soil physical properties, hydraulic gradients, geological stratifications, and topography were varied to depict the range of properties encountered in coastal peatlands. Our results show that terrestrial SGD occurs at the study site at a flux of 0.080 m2 d−1, with seepage rates of 1.05 cm d−1 (upper discharge region) and 0.16 cm d−1 (lower discharge region above submerged peat layer). These calculated seepage rates compare to observations from other wetland environments and SGD sites in the Baltic Sea. The groundwater originates mainly from the dune dike—recharged by precipitation and infiltration from ponded peatland surface water—and to a lesser extent from the sand aquifer. The scenario simulations yielded a range of potential SGD fluxes of 0.008–0.293 m2 d−1. They revealed that the location of terrestrial SGD is determined by the barrier function of the peat layer extending under the sea. However, it has little impact on volume flux as most SGD occurs near the shoreline. Magnitude of SGD is mainly driven by hydraulic gradient and the hydraulic conductivity of peat and beach/dune sands. Anisotropy in the horizontal direction, aquifer and peat thickness, and peatland elevation have little impacts on SGD. We conclude that SGD is most probable from coastal peatlands with high water levels, large Ks and/or a dune dike or belt, which could be an essential source for carbon and other materials via the SGD pathway.

Submarine groundwater discharge as a potential driver of eastern oyster, Crassostrea virginica, populations in Georgia

Reef-building eastern oysters, Crassostrea virginica, provide many ecosystem services, including production of valuable commercial products, formation of complex habitats, improved water quality and shoreline protection. Despite this, oyster populations have experienced dramatic declines throughout their range, spawning massive investment in management and restoration. Restoration efforts typically consider several well-studied metrics that normally influence oyster success; however, one potential factor that has not received much prior attention is submarine groundwater discharge (SGD). We conducted a series of field surveys and field experiments to explore the relationship between SGD and oysters in a marsh-lined tidal creek in Georgia, USA. SGD was mapped across multiple time points using a natural radon tracer (radon-222), and fluxes were paired with discrete measurements of oyster density, condition, size, recruitment and growth at multiple locations along the creek. Variation in oyster metrics was best explained by a combination of SGD, pH, and DO, which displayed a high degree of multicollinearity. We found an overall negative, nonlinear relationship between oyster density and groundwater flux. Interestingly, juvenile and adult condition and growth were not negatively impacted by groundwater. Rather, our results suggest that the likely mechanism for the density-flux relationship was interruption of the larval recruitment, which was also negatively related to flux. We hypothesize that larval interruption is due to the low dissolved oxygen and pH conditions of the groundwater at high flux sites. Overall, the interaction between SGD and oysters appears complex, and may be affected by other variables. This study provides evidence of a potential negative effect of a previously understudied natural phenomena on oyster demographics, and we suggest that SGD be considered in future management efforts.

Storm-Driven Fresh Submarine Groundwater Discharge and Nutrient Fluxes From a Barrier Island

Quantifying and characterizing groundwater flow and discharge from barrier islands to coastal waters is crucial for assessing freshwater resources and contaminant transport to the ocean. In this study, we examined the groundwater hydrological response, discharge, and associated nutrient fluxes in Dauphin Island, a barrier island located in the northeastern Gulf of Mexico. We employed radon (222Rn) and radium (Ra) isotopes as tracers to evaluate the temporal and spatial variability of fresh and recirculated submarine groundwater discharge (SGD) in the nearshore waters. The results from a 40-day continuous 222Rn time series conducted during a rainy season suggest that the coastal area surrounding Dauphin Island was river-dominated in the days after storm events. Groundwater response was detected about 1 week after the precipitation and peak river discharge. During the period when SGD was a factor in the nutrient budget of the coastal area, the total SGD rates were as high as 1.36 m day–1, or almost three times higher than detected fluxes during the river-dominated period. We found from a three-endmember Ra mixing model that most of the SGD from the barrier island was composed of fresh groundwater. SGD was driven by marine and terrestrial forces, and focused on the southeastern part of the island. We observed spatial variability of nutrients in the subterranean estuary across this part of the island. Reduced nitrogen (i.e., NH4+ and dissolved organic nitrogen) fluxes dominated the eastern shore with average rates of 4.88 and 5.20 mmol m–2 day–1, respectively. In contrast, NO3– was prevalent along the south-central shore, which has significant tourism developments. The contrasting nutrient dynamics resulted in N- and P-limited coastal water in the different parts of the island. This study emphasizes the importance of understanding groundwater flow and dynamics in barrier islands, particularly those urbanized, prone to storm events, or located near large estuaries.

Investigation of the Spatio-Temporal Behaviour of Submarine Groundwater Discharge Using a Low-Cost Multi-Sensor-Platform

Submarine groundwater discharge (SGD) is an important pathway of nutrients into coastal areas. During the last decades, interest of researchers in SGDs has grown continuously. However, methods applied for SGD research usually focus on the aquifer or on the mixing processes on larger scales. The distribution of discharged water within the water column is not well investigated. Small remotely operated vehicles (ROV) equipped with environmental sensors can be used to investigate the spatial distribution of environmental parameters in the water column. Herein, a low-cost multi-sensor platform designed to investigate the spatial distribution of water quality properties is presented. The platform is based on an off-the-shelf underwater vehicle carrying various environmental sensors and a short-baseline localisation system. This contribution presents the results of SGD investigations in the area of Woodman Point (Western Australia). Various potential SGD plumes were detected using a skiff equipped with a recreational echo sounder. It was demonstrated that this inexpensive equipment could be used to detect and investigate SGDs in coastal areas. In addition, the low-cost multi-sensor platform was deployed to investigate the spatial distribution of environmental parameters including temperature (T), electric conductivity (EC), dissolved oxygen (DO), oxidation-reduction potential (ORP), pH, and dissolved organic matter fluorescence (FDOM). Three ROV surveys were conducted from different skiff locations. Analyses of the spatial distribution of the environmental parameters allowed the identification of nine potential SGD plumes. At the same locations, plumes were identified during the sonar surveys. In addition, fuzzy logic was used for the fusion of salinity, DO, and FDOM readings in order to enhance SGD detection capability of the designed multi-sensor system. The fuzzy logic approach identified 293 data points as potential within a SGD plume. Average minimum-distance between these points and the identified SGD plumes was 0.5 m and 0.42 m smaller than the minimum-distance average of the remaining data points of survey one and three respectively. It was shown that low-cost ROVs, equipped with environmental sensors, could be an important tool for the investigation of the spatio-temporal behaviour of SGD sites. This method allows continuous mapping of environmental parameters with a high spatial and temporal resolution. However, to obtain deeper insights into the influence of SGDs on the nearshore areas, this method should be combined with other well-established methods for SGD investigation, such as pore water sampling, remote sensing, or groundwater monitoring.

Finite element modelling to assess the submarine groundwater discharge in an over exploited multilayeredcoastal aquifer.

A three-dimensional variable-density finite element model was developed to quantify the impact of groundwaterover use on submarine groundwater discharge (SGD). The model was applied to the Arani-Korttalaiyar river basin, north of Chennai, India. This region has an upper unconfined and lower semi-confined aquifer extending up to 30 km inland from the coast and beyond this distance; the two aquifers merge and become a single unconfined aquifer. The model simulated that during the period from 2000 to 2012, the flux of seawater to the aquifer has increased from 17,000 to 24,500 m3/day due to over-exploitation of groundwater from the semi-confined aquifer. Where as in the unconfined aquifer, SGD has been taking place. Scenarios showing theimpact of newly constructed managed aquifer recharge structures, 10% additionalincrease in rainfall recharge, and termination of pumping from five well-fields on thegroundwater conditions in the area were studied. The model predicteda SGD of 85,243 m3/day from the unconfined aquifer and 22,414 m3/day from the semi-confined aquifer by the end of 2030. By adopting managed aquifer recharge methods, seawater intrusion (rate of 4,408 m3/day) can be reduced and SGD (rate of 22,414 m3/day) increased. The rate of SGD increase and the movement of seawater to aquifer can be completely prevented in the semi-confined aquifer by adopting these management options by 2030. Findings from this study have enhanced the understanding of SGD and water budget, which can be used by decision-makers for the sustainable management of groundwater resources in coastal aquifers.

Delineation of submarine groundwater discharge and seawater intrusion zones using anomalies in the field water quality parameters, groundwater level fluctuation and sea surface temperature along the Gujarat coast of India

Nature mediated processes like seawater intrusion (SWI) and its complementary processes like submarine groundwater discharge (SGD) often cause severe water stress on the coastal water resources worldwide. The present work attempts to delineate the SWI and SGD zones along the North-Western coast of India (constituting the entire coastline of Gujarat) which is currently facing severe water crises due to the freshwater salinization and water loss through the coastal aquifer system. In the present work site-specific water characteristics and groundwater level were used as a proxy for identifying the probable zones of SWI and SGD. For the delineation purpose, we have collected 540 water samples distanced at 5–10 km (seawater, porewater and groundwater; 180 each) from the entire coastline of Gujarat. Further, a three-tier validation system has been adopted for delineating the SWI and SGD zones followed by the physical verification of the locations through the integration of (i) Groundwater fluctuation dynamic, (ii) MODIS derived sea surface temperature (SST) anomaly, and (iii) Electrical conductivity (EC) based gradient mapping. The study has identified 9 out of 14 districts being vulnerable to SWI, whereas the remaining five districts from south Gujarat and the Saurashtra coast are suspectable for SGD. The present work will act as a preliminary basis for formulating a framework for the detailed investigation of the mixing process along the coast.

Estimating submarine groundwater discharge at a subtropical river estuary along the Beibu Gulf, China

Estimating submarine groundwater discharge at a subtropical river estuary along the Beibu Gulf, China

The evolution of stable silicon isotopes in a coastal carbonate aquifer on Rottnest Island, Western Australia

Abstract. Dissolved silicon (dSi) is a key nutrient in the oceans, but data regarding Si isotopes in coastal aquifers are not widely available. Here we investigate the Si isotopic composition of 12 fresh and 16 saline groundwater samples from Rottnest Island, Western Australia, which forms part of the world's most extensive aeolianite deposit (the Tamala Limestone formation). In total, two bedrock samples were also collected from Rottnest Island for Si isotope analysis. The δ30Si values of groundwater samples ranged from −0.4 ‰ to +3.6 ‰ with an average +1.6 ‰, and the rock samples were −0.8 ‰ and −0.1 ‰. The increase in δ30Si values in fresh groundwater is attributed to the removal of the lighter Si isotopes into secondary minerals and potentially also adsorption onto Fe (oxy)hydroxides. The positive correlations between δ30Si values and dSi concentrations (ρ = 0.59; p = 0.02) and δ30Si values and Cl, but not dSi and Cl concentrations, are consistent with vertical mixing between the younger fresh groundwater and the deeper groundwater, which have undergone a greater degree of water–rock interactions. This has produced a spatial pattern in δ30Si across the aquifer due to the local hydrogeology, resulting in a correlation between δ30Si and tritium activities when considering all groundwater types (ρ = −0.68; p = 0.0002). In the deeper aquifer, the inverse correlation between dSi and Cl concentrations (ρ = −0.79; p = 0.04) for the more saline groundwater is attributed to groundwater mixing with local seawater that is depleted in dSi (< 3.6 µM). Our results from this well-constrained island aquifer system demonstrate that stable Si isotopes usefully reflect the degree of water–aquifer interactions, which is related to groundwater residence time and local hydrogeology. Our finding that lithogenic Si dissolution occurs in the freshwater lens and the freshwater–seawater transition zone on Rottnest Island appears to supports the recent inclusion of a marine–submarine groundwater discharge term in the global dSi mass balance. Geologically young carbonate aquifers, such as Rottnest Island, may be an important source of dSi in coastal regions with low riverine input and low oceanic dSi concentrations.

Spatial and temporal variations in dissolved silicate along the Indian coastal groundwaters and their export to adjacent coastal waters

Silicate plays a vital role in the coastal regions biogeochemistry. Submarine Groundwater Discharge (SGD) is one of the significant silicate sources to the coastal regions. The silicate concentrations in groundwater (GW) are considerably higher than in neighbouring coastal waters, as a result, SGD increases the concentrations of silicate in the coastal waters and influences the marine ecosystem. To examine the spatial and temporal variability in silicate concentrations and export of silicate to the adjacent coastal waters through SGD, groundwater samples are collected at 90 locations along the east and west coast regions of India during the dry and wet periods. This study revealed that the concentration of dissolved silicate was lower during the dry period than during the wet period. The soil silicate concentration displayed a significant variability and has a good correlation with silicate concentrations in the coastal groundwater of India. Silicate in groundwater showed significant linear relations with bacterial respiration, bicarbonate, and pCO2 and inverse relation with dissolved oxygen saturation suggesting that remineralization and weathering processes are responsible for the silicate in the coastal groundwater of India. The average SGD of freshwater varied between 1.6 × 104 m3/day and 1.75 × 1011 m3/day from the Indian coastal groundwater into adjacent coastal regions. The fluxes of silicate through SGD to the Bay of Bengal was estimated as 1.97 ± 0.85 Tg (1 Tg = 1012 g) and 3.22 ± 1.56 Tg to the eastern Arabian Sea. The estimated total SGD fluxes of silicate to the northern Indian ocean from east and west coastal regions of India is 3.07 ± 1.15 Tg/y and it is almost three times higher than silicate discharged from monsoonal rivers 1.03 ± 0.25 Tg/y. The silicate through SGD from Indian coastal groundwater contributes 7% of silicate to the total silicate export to the world coastal regions through SGD.

A study of effects of reduction of submarine groundwater discharge on thermal habitats for manatee in a spring-fed estuary using a laterally averaged hydrodynamic model

Effects of the reduction of submarine groundwater discharge (SGD) on thermal habitats for manatee in the Homosassa River, a spring-fed estuary in Florida, were studied using a laterally averaged hydrodynamic model. The estuary receives its hydrologic loading from numerous spring vents, mainly located at its headwaters and two upstream tributaries. The model was calibrated and verified against real-time data collected during November 2014 - August 2017 before it was used to simulate hydrodynamics, salinity transport, and thermal dynamics in the estuary from October 9, 2007 to March 12, 2018 with various SGD reductions. Effects of the SGD reduction on chronic (72-hour timescale) and acute (4-hour timescale) thermal habitats for manatee were analyzed at five critical time points (CTPs), including those of the lowest air temperature in 2010 and 2018, the lowest water temperature, the minimum thermal habitats, and the most sensitive extremely low habitats.It is found that the chronic thermal habitats for manatee decrease linearly with the increase of the SGD reduction at these CTPs. When chronic thermal habitats are extremely low, they are most sensitive to the SGD reduction. Every 1% reduction of the SGD could cause roughly a 1.31 – 1.32% reduction of ≥ 20°C water volume and a 1.28% reduction of ≥ 20°C surface area. The chronic thermal habitats for manatee in the Homosassa River can be predicted using a simple heat flux parameter (Hf), which explains 92% and 93% of the total variances of ≥ 20°C water volume and surface area, respectively using a logarithm relationship. The linearity of the relationships between the acute thermal habitats for manatee and the SGD reduction is weak. In fact, when acute thermal habitats in the estuary are extremely low, their relationships with the SGD reduction are very nonlinear, with the global minimum points varying significantly for various SGD reductions.

Thermal Imaging Shows Submarine Groundwater Discharge Plumes Associated with Ancient Settlements on Rapa Nui (Easter Island, Chile)

Submarine groundwater discharge (SGD) is an important component of many coastal environments and hydrologic processes, providing sources of nutrients to marine ecosystems, and potentially, an important source of fresh water for human populations. Here, we use a combination of unpiloted aerial systems (UAS) thermal infrared (TIR) imaging and salinity measurements to characterize SGD on the remote East Polynesian island of Rapa Nui (Easter Island, Chile). Previous research has shown that coastal freshwater seeps are abundant on Rapa Nui and strongly associated with the locations of ancient settlement sites. We currently lack, however, information on the differential magnitude or quality of these sources of fresh water. Our UAS-based TIR results from four locations on Rapa Nui suggest that locations of variably-sized SGD plumes are associated with many ancient settlement sites on the island and that these water sources are resilient to drought events. These findings support previous work indicating that ancient Rapa Nui communities responded to the inherent and climate-induced hydrological challenges of the island by focusing on these abundant and resilient freshwater sources. Our results highlight the efficacy of using UAS-based TIR for detecting relatively small SGD locations and provide key insights on the potential uses of these water sources for past and current Rapa Nui communities.

Tidal‐Driven Submarine Groundwater Discharge and Its Influences on the Carbonate System of a Coastal Coral Reef in the Northern South China Sea

AbstractThe driving mechanisms of submarine groundwater discharge (SGD) in Sanya Bay in the northern South China Sea were investigated using a combination of time‐series observation and modeling, as well as the influences of SGD on the carbonate system of a coastal coral reef. SGD flux, characterized by high variability on flood‐ebb and spring‐neap tidal cycles, was found to be mainly driven by tidal pumping. SGD posed more significant impacts on coastal water at the ebb phase during the spring tide (higher SGD flux and extended offshore reach of SGD impact), with nearshore water to be more heavily affected. Under the influence of SGD, the diurnal ranges of the carbonate variables observed in the coral reef system were 124 – 313 μmol kg−1 for dissolved inorganic carbon, 29 – 101 μmol kg−1 for total alkalinity, 179 – 717 μatm for partial pressure of CO2, and 0.20 – 0.45 for pH. The variations of the CO2 system were dominated by the enhanced SGD input during the spring tide, while biological metabolism of coral reef played a predominant role during the neap tide. The intensified SGD input resulted in higher diurnal variations of the carbonate variables, enhanced acidification, and oceanic CO2 emission during the spring tide. The SGD‐associated inorganic carbon flux is an additional stressor influencing coastal acidification in the context of rising atmospheric carbon dioxide.