Essai de localisation et de quantification des résurgences sous-marines d'un aquifère captif à porosité d'interstices: exemple de la nappe alluviale de la basse vallée du Var (Méditerranée, France)

Abstract

At the mouth of the river Var (Mediterranean Sea, France), the aquifer which is contained within recent alluvial deposits becomes confined as it flows under impermeable sediments of a small coastal delta. To study the water balance of the alluvial aquifer, we used a method of localization and in situ quantification of freshwater discharge into the sea. Vertical profiles of both salinity and temperature and systematic chemical analyses of seawater were made at 40 stations located on two concentric half-circle shaped routes linking the eastern bank of the river mouth to the southern one. Anomalies appear at some stations on the two routes, where, at depths of 20–60 m, profiles show a temperature elevation of 0.1–0.2°C and a salinity decrease of 0.05–0.1 PSU compared with the average value (PSU is an abbreviation for ‘Practical Scale Unit’, defined by SCORE-UNESCO as the new salinity unit; this unit corresponds to ‰ (per mille), which is equivalent to 1 g of salt for 1 kg of seawater). In addition, lower than average values of seawater onic concentrations of Mg 2+, Ca 2+ and K + and higher than average content of silica, a local groundwater tracer, confirm the existence of freshwater discharge areas. These marine outlets are located where the deltaic impermeable sedimentary cover is thin or even nonexistent. An attempt to evaluate the flow rate is made by estimating the dynamic diffusion of the salt flux over the whole discharge area. The results range from 180 to 400 1 s −1, in agreement with values indirectly deduced from water balance calculations. The important uncertainty of the measurement is due to the choice of the dynamic diffusion coefficient (ranging from 10 to 20 m 2 day −1), the approximation of salinity variations (ranging from 0.05 to 0.1 PSU) and the thickness of the discharge area (ranging from 20 to 40 m).