TRAÇAGE NATUREL APPLIQUE A UN AQUIFERE CÔTIER CAPTIF EN CONTEXTE DELTAÏQUE

L’étude hydrochimique et isotopique à court ou long terme permet de déterminer l’origine de la minéralisation d’un aquifère. Dans le cas des aquifères côtiers, l’étude de l’origine de la salinité des eaux est essentielle pour gérer et protéger la ressource en eau. Deux approches sont successivement abordées. La première approche correspond à une étude spatiale de l’origine et de l’évolution géochimique de la minéralisation de l’eau, alors que la seconde s’intéresse à l’étude temporelle de cette minéralisation. Ce chapitre a fait l’objet d’une publication (de Montety et al., 2008), présentée ici dans sa version originale. Cependant, par souci de lisibilité, la description du site d’étude et des méthodes, déjà présentée dans les parties 1 et 2, a été enlevée. Une discussion a été ajoutée pour compléter cet article. L’intégralité de la publication est fournie en Annexe 4. Enfin, les principes de base de l’hydrochimie, des interactions eau-roche et leur « adaptation » en milieu salé ne sont pas détaillés. Le lecteur pourra se référer à e.g. Appelo et Postma (2005), Post (2002) ainsi qu’à Clark et Fritz (1997) et Mook et Vries (2000) pour ce qui concerne les isotopes stables. Groundwater salinization in coastal areas occurs in many aquifers around the world (Barlow, 2003) and in numerous Mediterranean countries (Benavente et al., 2004; Cost Environment Action 621, 2005). Understanding the origin and mechanisms of the salinization process is an important point for preventing further deterioration of groundwater resources. Many investigations have dealt with the origin of saltwater in coastal aquifer (Custodio, 1997; Bear et al., 1999; Araguas Araguas, 2003; Ghabayen et al., 2006) and several sources have been identified: evaporite dissolution (Pulido-Leboeuf et al., 2003), downward leakage from surficial saline water through failed or improperly constructed wells (Aunay et al., 2006), deep brines or upward flow from deep saline water (Vengosh et al., 1999), fossil seawater (Yamanaka et Kumagai, 2006) or present seawater intrusion often due to excessive pumpings (Kim et al., 2003). Seawater intrusion is frequently observed in the case of unconfined aquifer connected to the sea where a strong demand in water resources induced a decrease of piezometric level. In the case of confined aquifers like the deep aquifer of the Rhône delta, the process groundwater compositions. Major ions and δ13CTDIC are also used to highlight hydrogeochemical processes occurring in the aquifer according to the degree of salinity, and to evaluate the consequences of the water-rock interaction.

Average data from monthly samples of the groundwater (May 2005 to May 2006 and November 2006) are given in Table 12. Seawater values and average data of the Rhône River and the Crau aquifer are also added as possible sources of recharge of the Camargue aquifer. The piezometers are relatively far from the coast (8 km at least), but the water is strongly mineralized. All the groundwater samples are of Na-Cl type (Fig. 64). The Cl- content of the groundwater increases towards the south-west direction, in relation to the proximity of the coast, from 35 meq/l (S7) to 600 meq/l (S5), a value close to that of the Mediterranean  reproducibility). The δ13C shows an opposite trend with a strong depletion near the coast (from -6.7‰ near the Rhône River to -19.36‰ close to the coastline) where reducing environments have been identified (-20 mV to -120 mV). The Rhône River and the Crau aquifer present typical features of fresh water. Both are from calcium bicarbonate water type (Fig. 64) and have very low chloride contents compared to the Camargue groundwater. Nevertheless, the Crau aquifer is slightly more mineralized than the Rhône River reproducibility). The δ13C shows an opposite trend with a strong depletion near the coast (from -6.7‰ near the Rhône River to -19.36‰ close to the coastline) where reducing environments have been identified (-20 mV to -120 mV). The Rhône River and the Crau aquifer present typical features of fresh water. Both are from calcium bicarbonate water type (Fig. 64) and have very low chloride contents compared to the Camargue groundwater. Nevertheless, the Crau aquifer is slightly more mineralized than the Rhône River  of recharge of the confined aquifer are however negligible compared to seawater composition.