Résumé
The Rhone delta, South of France (Camargue, 750km (super 2) ) is a coastal saline wetland located along the Mediterranean Sea. The confined aquifer of this delta shows high values of electrical conductivity rising from the north (4 mS/cm) to the shoreline (58 mS/cm). This work aims to identify the origin of groundwater salinity and the geochemical processes occurring in this coastal confined aquifer according to the degree of salinity. A natural tracing approach is considered using monthly sampling in 8 piezometers for chemical and isotopic analyses ( (super 18) O, (super 2) H, (super 13) C (sub TDIC) ). Ionic and isotopic ratios demonstrate that strong salinities are due to a simple mixing between Mediterranean seawater and freshwater; seawater contribution reaches up to 98% at 8 km from the shoreline. Seawater intrusion induces a particular groundwater chemistry which varies with the degree of seawater contribution: (1) In the less saline part of the aquifer (seawater contribution <20%), the intrusion induces an increase of Na (super +) in groundwater leading to Ca (super 2+) /Na (super +) exchange processes. The delta (super 13) C (sub TDIC) analyses show that matrix exchange processes most likely occur for the less saline samples. (2) In the saline part of the aquifer (seawater contribution >20%), the intrusion induces SO (sub 4) reduction which is confirmed by highly depleted delta (super 13) C (sub TDIC) values (up to -19 ppm). The delta (super 13) C (sub TDIC) also reveals that methanogenesis processes may occur in the most reductive part of the aquifer. Due to SO (sub 4) reduction, the intrusion induces a shift in carbonate equilibrium leading to supersaturation with respect to dolomite and/or magnesian calcite. Thus carbonate precipitation may occur in the area strongly influenced by seawater. Abstract Copyright (2008) Elsevier, B.V.