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

Abstract

Coastal 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, O2, Na-K-Ca-Mg-Ba-Sr-Si-Cl-SO4-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 CO2 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.

The Chemistry of Hyperalkaline Springs in Serpentinizing Environments: 1. The Composition of Free Gases in New Caledonia Compared to Other Springs Worldwide

“…Small amounts of oxygen would not drastically change the correction (Equation ). Oxygen is very rapidly consumed when oxic waters flow into geological formations (see the examples of oxygen dynamics in karstic environments; [Monnin et al., 2019; Young et al., 2018]). This lends support to the conclusion that the hydrothermal systems responsible for the discharge of high‐pH waters at the surface are anoxic.…”

Conceptual uncertainties in groundwater and porewater fluxes estimated by radon and radium mass balances

“…La Palme Lagoon receives continuous brackish groundwater inputs from a regional karst aquifer (3× 10 3 to 25 × 10 3 m 3 d −1 ; Stieglitz et al 2013; Bejannin et al 2017; Rodellas et al 2018), chiefly from a karstic spring connected to the lagoon via a small stream in the northwestern part of the lagoon (Wilke and Boutière 2000; Monnin et al 2019). For the purpose of this study, the input of low‐salinity groundwater through this spring‐fed stream is treated as a surficial input and thus it is not included within porewater fluxes.…”