Combining airborne thermal infrared images and radium isotopes to study submarine groundwater discharge along the French Mediterranean coastline

The French Mediterranean coastline, which includes karstic springs discharging into coastal seas and coastal lagoons. Study focus We investigated submarine groundwater discharge (SGD), an important vector for many chemical elements that may impact the quality of the coastal environment. First, we acquired airborne thermal infrared (TIR) images to detect terrestrial groundwater inputs. Then we report in situ data (salinity; temperature; radium isotopes). We use these data i) to confirm the presence of groundwater discharge and to characterize the different systems, and ii) to quantify SGD fluxes and estimate the residence time of the water bodies. New hydrological insights for the region Few studies have been conducted on SGD along the French Mediterranean coastline. The terrestrial groundwater spring inputs in La Palme and Salses-Leucate coastal lagoons are in the range (0.04–0.11) m 3 s −1 , ≤ 2% of the local river inputs. In comparison, total SGD estimates to La Palme lagoon (0.56–1.7 m 3 s −1 ) suggest that the recirculation of lagoon water through the sediment is two orders of magnitude greater than the terrestrial groundwater inputs. At the Calanque of Port-Miou, the terrestrial groundwater flux to the coastal seas was between 0.6 and 1.2 m 3 s −1 in July 2009. This study demonstrates the application of airborne TIR remote sensing for detecting surficial groundwater springs, and the inability of the method to detect deeper, submerged springs

Temporal variability of lagoon–sea water exchange and seawater circulation through a Mediterranean barrier beach

The subterranean flow of water through sand barriers between coastal lagoons and the sea, driven by a positive hydraulic gradient, is a net new pathway for solute transfer to the sea. On the sea side of sand barriers, seawater circulation in the swash-zone generates a flux of recycled and new solutes. The significance and temporal variability of these vectors to the French Mediterranean Sea is unknown, despite lagoons constituting ~ 50% of the coastline. A one-dimensional 224Raex/223Ra reactive-transport model was used to quantify water flow between a coastal lagoon (La Palme) and the sea over a 6-month period. Horizontal flow between the lagoon and sea decreased from ~ 85 cm d−1 during May 2017 (0.3 m3 d−1 m−1 of shoreline) to ~ 20 cm d−1 in July and was negligible in the summer months thereafter due to a decreasing hydraulic gradient. Seawater circulation in the swash-zone varied from 10 to 52 cm d−1 (0.4–2.1 m3 d−1 m−1), driven by short-term changes in the prevailing wind and wave regimes. Both flow paths supply minor dissolved silica fluxes on the order of ~ 3–10 mmol Si d−1 m−1. Lagoon–sea water exchange supplies a net dissolved inorganic carbon (DIC) flux (320–1100 mmol C d−1 m−1) two orders of magnitude greater than seawater circulation and may impact coastal ocean acidification. The subterranean flow of water through sand barriers represents a significant source of new DIC, and potentially other solutes, to the Mediterranean Sea during high lagoon water-level periods and should be considered in seasonal element budgets

Submarine groundwater discharge and circulation of seawater through the sediment as vector of chemical elements along the French Mediterranean Sea

L’importance hydrogéologique et biogéochimique des apports d’eau souterraine en mer a récemment été mise en évidence. De nombreuses études ont été réalisées à travers le monde, mais très peu ont été menées sur le littoral méditerranéen, malgré la présence de nombreuses résurgences karstiques (Port-Miou, La Palme, Font Estramar…). L’utilisation de l’imagerie infrarouge thermique a permis la détection des décharges d’eau souterraine le long de ce littoral. Les flux d’eau et de nutriments ont ensuite été estimés à l’aide des isotopes du radium (223Ra, 224Ra et 228Ra). Nous montrons que les flux de nutriments associés aux décharges d’eau souterraine et aux flux de circulation d’eau dans les sédiments sont similaires voire supérieurs (jusqu’à 20 fois) aux apports de nutriments associés aux fleuves, à l’exception du Rhône. Nous montrons finalement que les flux associés aux décharges d’eau souterraine en mer et à la circulation de l’eau de mer dans les sédiments ont lieu tout le long du littoral méditerranéen français, à l’inverse des fleuves qui se déchargent ponctuellement. Ces flux sont donc à prendre en compte dans les bilans d’éléments chimiques, nottament à l’échelle régionale (Golfe du Lion).The hydrogeological and biogeochemical importance of submarine groundwater inputs has recently been highlighted. Numerous studies around the world have shown that these contributions can challenge fluvial inputs. However, very few studies have been conducted along the French Mediterranean coastline despite the presence of several springs discharging directly into the sea. Submarine groundwater discharge locations were first detected using thermal infrared images. Water and nutrient flux were then estimated using radium isotopes (223Ra, 224Ra and 228Ra). Nutrient inputs are similar (and even greater, up to 20 times higher) to the local river inputs, except the Rhône River. We show that the nutrient fluxes driven by either submarine groundwater discharge (direct inputs of groundwater) or the circulation of seawater through the sediments are significant. These fluxes exist all along the French Mediterranean coastline. This is in contrast with the river inputs which are local inputs. These fluxes need to be considered in the chemical oceanic budgets (coastal seas and likely also in the sea/ocean budgets). These nutrient fluxes may have an impact on the quality of the coastal waters and on the coastal ecosystems

Décharge d’eau souterraine en mer et circulation d’eau de mer dans les sédiments : flux d’eau et d’éléments chimiques le long du littoral méditerranéen français

The hydrogeological and biogeochemical importance of submarine groundwater inputs has recently been highlighted. Numerous studies around the world have shown that these contributions can challenge fluvial inputs. However, very few studies have been conducted along the French Mediterranean coastline despite the presence of several springs discharging directly into the sea. Submarine groundwater discharge locations were first detected using thermal infrared images. Water and nutrient flux were then estimated using radium isotopes (223Ra, 224Ra and 228Ra). Nutrient inputs are similar (and even greater, up to 20 times higher) to the local river inputs, except the Rhône River. We show that the nutrient fluxes driven by either submarine groundwater discharge (direct inputs of groundwater) or the circulation of seawater through the sediments are significant. These fluxes exist all along the French Mediterranean coastline. This is in contrast with the river inputs which are local inputs. These fluxes need to be considered in the chemical oceanic budgets (coastal seas and likely also in the sea/ocean budgets). These nutrient fluxes may have an impact on the quality of the coastal waters and on the coastal ecosystemsRésuméL’importance hydrogéologique et biogéochimique des apports d’eau souterraine en mer a récemment été mise en évidence. De nombreuses études ont été réalisées à travers le monde, mais très peu ont été menées sur le littoral méditerranéen, malgré la présence de nombreuses résurgences karstiques (Port-Miou, La Palme, Font Estramar…). L’utilisation de l’imagerie infrarouge thermique a permis la détection des décharges d’eau souterraine le long de ce littoral. Les flux d’eau et de nutriments ont ensuite été estimés à l’aide des isotopes du radium (223Ra, 224Ra et 228Ra). Nous montrons que les flux de nutriments associés aux décharges d’eau souterraine et aux flux de circulation d’eau dans les sédiments sont similaires voire supérieurs (jusqu’à 20 fois) aux apports de nutriments associés aux fleuves, à l’exception du Rhône. Nous montrons finalement que les flux associés aux décharges d’eau souterraine en mer et à la circulation de l’eau de mer dans les sédiments ont lieu tout le long du littoral méditerranéen français, à l’inverse des fleuves qui se déchargent ponctuellement. Ces flux sont donc à prendre en compte dans les bilans d’éléments chimiques, nottament à l’échelle régionale (Golfe du Lion)

Combining airborne thermal infrared images and radium isotopes to study submarine groundwater discharge along the French Mediterranean coastline

The French Mediterranean coastline, which includes karstic springs discharging into coastal seas and coastal lagoons. Study focus We investigated submarine groundwater discharge (SGD), an important vector for many chemical elements that may impact the quality of the coastal environment. First, we acquired airborne thermal infrared (TIR) images to detect terrestrial groundwater inputs. Then we report in situ data (salinity; temperature; radium isotopes). We use these data i) to confirm the presence of groundwater discharge and to characterize the different systems, and ii) to quantify SGD fluxes and estimate the residence time of the water bodies. New hydrological insights for the region Few studies have been conducted on SGD along the French Mediterranean coastline. The terrestrial groundwater spring inputs in La Palme and Salses-Leucate coastal lagoons are in the range (0.04–0.11) m 3 s −1 , ≤ 2% of the local river inputs. In comparison, total SGD estimates to La Palme lagoon (0.56–1.7 m 3 s −1 ) suggest that the recirculation of lagoon water through the sediment is two orders of magnitude greater than the terrestrial groundwater inputs. At the Calanque of Port-Miou, the terrestrial groundwater flux to the coastal seas was between 0.6 and 1.2 m 3 s −1 in July 2009. This study demonstrates the application of airborne TIR remote sensing for detecting surficial groundwater springs, and the inability of the method to detect deeper, submerged springs

Freshening of a Coastal Karst Aquifer Revealed by the Temporal Changes in a Spring Water Composition (La Palme, Southern France)

International audienceCoastal karst aquifers are vulnerable to sea level rise and seawater intrusion. Knowledge of aquifer hydrological characteristics is therefore essential to managing this water resource. Long-term aquifer monitoring may not always be possible, especially in areas that humans frequent for recreational purposes. However, hydrological information can be deduced from the chemical composition of periodically sampled groundwaters. We characterized the complete chemical composition (temperature, pH, salinity, ORP, O 2 , Na-K-Ca-Mg-Ba-Sr-Si-Cl-SO 4-DIC, and DOC) of a brackish karstic spring located along the French Mediterranean coast (La Palme). The salinity of the spring water varied between 4 and 9 during the 2.5 year period of observation. Chemical analyses revealed that the spring is modified from modern seawater, as shown by Na-normalized dissolved element concentrations. Thermodynamic calculations of mineral saturation states (PHREEQC) point to aragonite and barite saturation and elevated equilibrium CO 2 partial pressure. The simultaneous salinity minima and oxygen maxima coincide with extreme values of dissolved element concentrations. This indicates that variation in salinity and water chemistry in La Palme coastal aquifer is primarily driven by infiltration of fresh rainwater. This study shows that geochemical investigations can provide an alternative approach to characterizing the hydrological properties of coastal karst aquifers when wells or boreholes are not readily available

Nutrient fluxes associated wth submarine groundwater discharge from karstic coastal aquifers (Côte Bleue, French Mediterranean Coastline)

International audienceDetermination of submarine groundwater discharge (SGD) from karstic coastal aquifers is important to constrain hydrological and biogeochemical cycles. However, SGD quantification using commonly employed geochemical methods can be difficult to constrain under the presence of large riverine inputs, and is further complicated by the determination of the karstic groundwater endmember. Here, we investigated a coastal region where groundwater discharges from a karstic aquifer system using airborne thermal infrared mapping and geochemical sampling conducted along offshore transects. We report radium data (223 Ra, 224 Ra, 228 Ra) that we used to derive fluxes (water, nutrients) associated with terrestrial groundwater discharge and/or seawater circulation through the nearshore permeable sediments and coastal aquifer. Field work was conducted at different periods of the year to study the temporal variability of the chemical fluxes. Offshore transects of 223 Ra and 224 Ra were used to derive horizontal eddy diffusivity coefficients that were subsequently combined with surface water nutrient gradients (NO 2 − + NO 3 − , DSi) to determine the net nutrient fluxes from SGD. The estimated DSi and (NO 2 − + NO 3 −) fluxes are 6.2 ± 5.0 * 10 3 and 4.0 ± 2.0 * 10 3 mol d −1 per km of coastline, respectively. We attempted to further constrain these SGD fluxes by combining horizontal eddy diffusivity and 228 Ra gradients. However, SGD endmember selection in this area (terrestrial groundwater discharge vs. porewater) adds further uncertainty to the flux calculation and thus prevented us from calculating a reliable flux using this latter method. Additionally, the relatively long half-life of 228 Ra (5.75 y) makes it sensitive to specific circulation patterns in this coastal region, including sporadic intrusions of Rhône river waters that impact both the 228 Ra and nutrient surface water distributions. We show that SGD nutrient fluxes locally reach up to 20 times the nutrient fluxes from a small river (Huveaune River). On a regiona

A comparison between water circulation and terrestrially-driven dissolved silica fluxes to the Mediterranean Sea traced using radium isotopes

International audienceThe circulation of seawater through permeable coastal sediments is increasingly recognized as an important source of nutrients, including dissolved silica (DSi), to the coastal ocean. Here, we utilized a Ra isotope (223 Ra, 224 Ra ex , 228 Ra) mass balance to quantify DSi fluxes driven by water circulation to a small shallow coastal lagoon (La Palme; French Mediter-ranean) during June 2016, as compared to karstic groundwater spring inputs. The DSi flux driven by lagoon water circulation (derived from 224 Ra ex) was approximately one order of magnitude greater (1900 ± 1700 mol d À1) than the DSi load of the karstic groundwater spring (250 ± 50 mol d À1) and greater than molecular diffusion (970 ± 750 mol d À1). Lagoon water circulation was a negligible source of 228 Ra, indicating that circulation-driven DSi inputs occur over a timescale of days. Offshore transects were studied to quantify fluxes of marine-derived submarine groundwater discharge (SGD) from the permeable sandy coastline adjacent to the lagoon, into the Mediterranean Sea. Surface water transects revealed near-shore enrichments of Ra and DSi, attributed to wave-setup and water exchange through the permeable beach between the lagoon and the sea. Upscaling over the 9.5 km stretch of sandy beaches results in a marine SGD-driven DSi flux of 2.3 ± 1.3 Â 10 4 mol d À1 , similar in magnitude to the Têt river during November 2016 (3.3 ± 2.4 Â 10 4 mol d À1), the largest river in the region. A positive relationship between DSi and 224 Ra ex in lagoon water and seawater, but not 228 Ra, suggests that 224 Ra ex and DSi enrichments are derived from a similar source, the sediment (i.e. lithogenic particle dissolution), operating on short timescales. A marine SGD-driven DSi flux to the Gulf of Lions (3.8 ± 2.2 Â 10 5 mol d À1) is likely continuous over time. The relatively constant DSi inputs from water circulation for the shallow lagoons and beaches along the French Mediterranean Se

Estimating groundwater discharge in the coastal lagoons of La Palme and Salses-Leucate along the French Mediterranean coastline by using radium isotopes

International audienceAlthough submarine groundwater discharge (SGD) has been investigated in many places of the world, very fewstudies were conducted along the French Mediterranean coastline, despite presence of several well-known karsticsprings. Almost no information is available on the fluxes of water and chemical elements associated with thesesystems and on their potential impact on the geochemical cycling and ecosystems of the coastal zones. In this study,we report airborne thermal infrared (TIR) images that allowed us to locate fresh groundwater inputs in the coastallagoons of La Palme and Salses-Leucate. The four radium isotopes (223Ra, 224Ra, 226Ra, 228Ra) were analyzedin these coastal lagoons and along transects conducted on board RV NEREIS II between the coast and offshore(between 200 m and 8 km). We also analyzed major elements, nutrients (nitrates, phosphates and silicates), DIC,DOC, DON, DOP, as well as several trace elements in these water samples. A mass balance of radium isotopeswas used to quantify SGD fluxes and to determine the fluxes of nutrients and other chemical species associatedwith SGD. Ra isotopes were also used to estimate the residence time of the waters in the lagoons