Abstract
Determination 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 (223Ra, 224Ra, 228Ra) 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 223Ra and 224Ra were used to derive horizontal eddy diffusivity coefficients that were subsequently combined with surface water nutrient gradients (NO2- + NO3-, DSi) to determine the net nutrient flux from SGD. The estimated DSi and (NO2- + NO3-) fluxes are 6.2 ± 5.0 *103 and 4.0 ± 2.0 *103 mol d-1 per kilometer of coastline, respectively. We attempted to further constrain these SGD fluxes by combining horizontal eddy diffusivity and 228Ra gradients. SGD endmember selection in this area (terrestrial groundwater discharge versus porewater) adds further uncertainty to the flux calculation and thus prevented us from calculating a reliable flux. Additionally, the relatively long half-life of 228Ra (5.75 y) makes it sensitive to specific circulation patterns in this coastal region, including sporadic intrusions of Rhone river waters that impact both the 228Ra 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 regional scale, DSi fluxes driven by SGD vary between 0.1 and 1.4 % of the DSi inputs of the Rhone River, while the (NO2- + NO3-) fluxes driven by SGD on this 22 km long coastline are between 0.1 – 0.3 % of the Rhone River inputs, the largest river that discharges into the Mediterranean Sea.
Highlights
Submarine Groundwater Discharge (SGD) is recognized as a vector for many chemical elements that impact the biogeochemistry and ecology of the coastal seas (Moore, 1996; Charette and Buesseler, 2004; Slomp and Van Cappellen, 2004; Burnett et al, 2006)
It is located 15 km east of the Rhône River, the largest river discharging into the French Mediterranean Sea, and is 15 km west of the Huveaune River, a small river discharging in Marseille (Figure 1)
For Carryle-Rouet (Figure 2c), the plumes are located throughout the coastline. These two locations were further investigated with salinity and radium isotope measurements to validate that the surface water temperature signatures were impacted by SGD
Summary
Submarine Groundwater Discharge (SGD) is recognized as a vector for many chemical elements that impact the biogeochemistry and ecology of the coastal seas (Moore, 1996; Charette and Buesseler, 2004; Slomp and Van Cappellen, 2004; Burnett et al, 2006). SGD has been shown to supply essential nutrients (Charette et al, 2001; Slomp and Van Cappellen, 2004; Hwang et al, 2005; Knee et al, 2010; Beusen et al, 2013; Tamborski et al, 2017) and trace elements to the coastal sea (Jeong et al, 2012; Trezzi et al, 2017), relatively few studies have been conducted on nutrient fluxes driven by SGD in the Mediterranean Sea (Garcia-Solsona et al, 2010; Weinstein et al, 2011; Tovar-Sánchez et al, 2014; Rodellas et al, 2015, 2017; Tamborski et al, 2018). Little information is available on the nutrient fluxes associated with SGD along the French Mediterranean coastline (Rodellas et al, 2017; Tamborski et al, 2018), despite the presence of several well-known karstic springs (Arfib et al, 2006; Fleury et al, 2007; Stieglitz et al, 2013; Bejannin et al, 2017)
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