Apports des outils chiniques et isotopiques à l'identification des origines de la salinisation des eaux : Cas de la nappe de La Chaouia côtière (Maroc)

L'étude hydrochimique des eaux de la nappe libre de la Chaouia côtière montre une teneur excessive en sels dans ces eaux, spécialement des chlorures (jusqu'à 3 g.l-1).Cette salinité élevée peut, à terme, nuire gravement à l'économie de la région qui se consacre essentiellement à la culture maraîchère sous irrigation.Des mesures isotopiques (18O/16O, 2H/1H) couplées aux éléments chimiques, notamment Br-/Cl-, excluent l'hypothèse d'une intrusion marine généralisée. L'existence d'une paléosalinité (SHIVANNA andal.,1993) n'est pas soutenue par les mesures des teneurs en isotopes radioactifs (3H, 14C). Il en est de même pour le lessivage de dépôts chlorurés préexistants dans les sédiments. L'apport essentiel en chlorures provient de l'altération de la roche mère, des fertilisants et des embruns marins lessivés par suite de l'irrigation et entraînés vers la nappe par percolation. Le recyclage de l'eau souterraine depuis au moins trente ans, par l'irrigation, n'a fait qu'augmenter cette salinité.Toutefois en bordure de l'océan quelques puits semblent montrer la présence d'eau marine, en effet les débits d'exhaure des puits sont à la limite d'exploitation de la nappe et toute augmentation de débits dans ces puits engendrera fatalement une invasion marine généralisée.In West of Morocco, the coastal plain Chaouia is located between Casablanca and Azemmour (south-west of Casablanca) over a distance of 65km (Figure 1). This plain aquifer spreads over 1100 km2 in semi-arid climatic conditions with about 370 mm.year-1 of mean precipitation and 17°C in mean air temperature.Irrigated agricultural farming is the main economic resource of the region. The only source of water is provided by highly mineralized groundwaters, which are harmful for the rural population and agricultural irrigation. The chloride content represents the main contribution of the salinity of ground waters. Most of the previous authors and authorities have attributed the high mineralization to seawater intrusion (YOUNSI, 1994). The objective of the present study is to determine the cause of salinity, considering three hypothesis: seawater intrusion, leaching of salts and evaporation of water. The environmental isotopes (2H,18O, 3H, 14C) have been used with hydrogeology together with major and minor ion chemistry to identify the source of salinity and to estimate the residence time of ground water. Moreover an environmental isotope study was carried out in order to identify the origin of the salinization (MERLIVAT et al., 1970; COTECCHIA et al., 1974; GASPARINI, 1989; CABRAL et al., 1991; GOMEZ-MARTOS et al., 1993; HASHASH et al., 1995). Both isotopic studies and Br-/Cl- ratios have been used to identify the salinity origin. The potential sources for this salinization are: seawater encroachment as a consequence of intense exploitation of the aquifer, dissolution of the halite from the several diapiric structures intruded in the aquifers. The isotopic waters sampling were carried out in August 1993 on several wells (Figure 2), in order to study the variations of the oxygen-18, deuterium, tritium and carbon-14 isotopes. Table 1 shows the results of the isotope and chemical analyses carried out. As oxygen-18 data for local precipitation are not available in Morocco, the oxygen-18 and deuterium mean contents in precipitation were taken from stations of the International Atomic Energy Agency (IAEA) network near the studied area (Gibraltar, Faro, Ponta Delgada) and/or deduced from published isotopic composition of shallow groundwaters in Morocco (KABBAJ et al., 1978; LOUVA and BICHARA, 1990; BOUCHAOU et al., 1995); the mean values for the precipitation are -4.0 ± 0.5‰ and -23.0 ± 2 ‰ (vs. V-SMOW) respectively. The oxygen-18 and deuterium mean contents in ground waters are -3.7 ± 0.3 ‰ and -20.1 ± 1.8 ‰ (vs.V-SMOW) respectively. The relationship between the oxygen-18 and deuterium contents of some groundwater samples indicates a good fitting along the world meteoric line (CRAIG, 1961) (Figure 3), suggesting that the ground waters are mainly of meteoric origin. There is no indication of possible mixing with seawater because there is no indication of a relation between the oxygen-18 content of the wells and their distance from the sea (Figure 4). On the other hand, no isotopic enrichment is noted during the infiltration of water from precipitation and/or irrigation.The diagram d18O-Cl- (Figure 5) shows that all the data do not lie along a theoretical mixing line with seawater and/or an evaporation line. Every 18O data point is very close to the isotopic values of precipitation [-4.0± 0.5 ‰ (vs. V-SMOW)] but with a large scatter in Cl- content. In this zone, an increase in the salt concentration is not accompanied by an isotopic effect. The sources of the chloride must be found among the products of rock alteration, fertilizers and marine airborne salts. The recycling of salt water by irrigation increases the salinity of soil and ground waters. Halite is characterized by a very low Br-/Cl- ratio(0.183 x 10-3). Water in some wells presents Br-/Cl- ratios similar to those of halite, but most of the groundwater samples have a greater ratio. These results argue against the hypothesis of dissolution of halite in the aquifer formation. But the Br-/Cl- ratio alone is not sufficient for detecting a possible seawater intrusion.During August 1994, 20 drilled wells were sampled over the study area for determining tritium concentrations. Carbon-14 activities were measured only on some samples. The relatively high tritium content in the majority of the sampled waters [between 2 and 9 tritium units (TU)] indicates a recent groundwater recharge (Table 1). The use of natural radioactive isotopes (3H, 14C) in the ground waters gives an estimate of mean residence time of 50 ± 20 years (mixing model). Some waters show a mean residence time greater than 100 years because they are isolated from the general circulation and limited to local spots. The recent 14C activity of the groundwater (_100 pcm) allows us to reject the hypothesis of a mixing with paleoconnate waters as demonstrated elsewhere (SHIVANNA et al., 1993).In the case of the coastal Chaouia (semi-arid environment), the use of both chemical tracers (Cl-, Br-) and isotopes (18O, 2H,3H, 14C) has shown that rainfall constitutes the essential origin of ground water in the area. The high salinity caused by seawater intrusion is probably limited in space to some coastal wells; neither evaporation of water before and during percolation, nor dissolution of evaporitic deposits (halite), has been found. The past marine transgression salt origin has also been discarded. The main source of the salinity is the washout of the rock alteration salts, fertilizers, marine airborne salts, and the unusable salts rejected by plants in the soil. Thus the increase of salinity is well explained by the recycling of salty ground waters by irrigation during at least thirty years