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.
Highlights
A large part of the world’s population lives in coastal areas and as such depends on the water supply of coastal aquifers [1]
The salinity data displays two minimum values at the same period of the year (April–May) in 2017 and 2018. This begs one of the driving questions of this study: is the salinity variation of the karstic spring over time the result of (1) a brackish karst coastal aquifer being periodically diluted by rainfall, or (2) a fresh karst coastal aquifer subject to temporally variable seawater intrusion, or (3) a combination of (1) and (2)? There is no obvious relationship between precipitation and the salinity changes of the karstic spring (Figure 2)
Water temperature of the spring water is fairly constant around 18.5 ◦ C and has no relationship with the air temperature
Summary
A large part of the world’s population lives in coastal areas and as such depends on the water supply of coastal aquifers [1]. The population density of the French Mediterranean coast is five times larger than the average population density of the metropolitan French territory (the nationwide population density was 113 inhabitants per square kilometer in 2009; www.CartesFrance.fr). This coast is impacted by seasonal anthropogenic pressures due to tourism (i.e., increased water demand, impact on the quality of the coastal seas and coastal aquifers, etc.). Major threats to water resources inofthese densely populated be accurately characterized, especially regimes in light of challenges faced in be a context of rapid climate areas [2,3,4,5]. The hydrological ofthe coastal aquifers must accurately characterized, change [2].
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