LOCATING AND ESTIMATING SUBMARINE FRESH-W ATER DISCHARGE FROM AN INTERSTITIAL CONFINED COASTAL AQUIFER BY MEASUREMENTS AT SEA - EXAMPLE FROM THE LOWER VAR VALLEY, FRANCE

At the mouth of the river Var (Mediterranean Sea, France), the aquifer which is contained within recent alluvial deposits becomes confined a s it flows under impermeable sediments of a small coastal delta. To st udy the water balance of the alluvial aquifer, we used a method of loc alization 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 temperatu re elevation of 0.1-0.2 degrees C and a salinity decrease of 0.05-0.1 PSU compared with the average value (PSU is an abbreviation for 'Pract ical Scale Unit', defined by SCORE-UNESCO as the new salinity unit; th is unit corresponds to parts per thousand (per mille), which is equiva lent to 1 g of salt for 1 kg of seawater). In addition, lower than ave rage values of seawater ionic concentrations of Mg2+, Ca2+ and K+ and higher than average content of silica, a local groundwater tracer, con firm the existence of freshwater discharge areas. These marine outlets are located where the deltaic impermeable sedimentary cover is thin o r even nonexistent. An attempt to evaluate the flow rate is made by es timating the dynamic diffusion of the salt flux over the whole dischar ge area, The results range from 180 to 4001 s(-1), in agreement with v alues indirectly deduced from water balance calculations. The importan t uncertainty of the measurement is due to the choice of the dynamic d iffusion coefficient (ranging from 10 to 20 m(2) day(-1)), the approxi mation of salinity variations (ranging from 0.05 to 0.1 PSU) and the t hickness of the discharge area (ranging from 20 to 40 m).