Utilisation de l'hydrochimie d'un bassin versant de lac collinaire tunisien pour l'étude de la recharge de la nappe alluviale

En climat méditerranéen, les ressources en eau des bassins sédimentaires sont rares. Pour limiter les écoulement de surface et recharger les nappes souterraines, des lacs collinaires ont été construits sous une pluviométrie annuelle comprise entre 250 et 500 mm. Des échantillons d'eau ont été prélevés dans le bassin versant d'El Gouazine, situé en Tunisie centrale, pour mieux expliciter le fonctionnement hydrochimique et la recharge de la nappe phréatique d'un lac collinaire très filtrant. Les eaux ont été échantillonnées à deux périodes distinctes (retenue presque asséchée et retenue pleine), en amont de la retenue dans le bassin versant et en aval dans l'aquifère alluvial.Trois faciès chimiques (bicarbonaté calcique, chloruré sodique et sulfaté calcique) caractérisent les eaux de nappe, l'eau de la retenue étant sulfatée calcique.La roche-mère et la nappe phréatique sont chimiquement très dépendantes. Les isotopes stables de l'eau montre que la nappe amont est peu profonde et d'origine météorique. L'eau de la retenue se mélange avec les eaux souterraines en conservant un caractère météorique en période d'écoulement et en acquérant un caractère évaporé en période d'assèchement. La nappe alluviale aval est alimentée par les eaux mélangées de la retenue. L'altération d'un affleurement gréseux forme un aquifère en rive gauche du lac expliquant en partie les pertes par infiltration. Les principaux processus géochimiques, qui se produisent au cours de l'écoulement de la nappe dans les formations superficielles, permettent une compréhension partielle du fonctionnement hydrochimique de la retenue et de son bassin versant.In a Mediterranean climate, water resources are scarce in sedimentary basins. In Tunisia, as in other semiarid countries, the revival of traditional floodwater harvesting, such as hill reservoirs, can provide water resources for the development of agriculture and agroforestry as well as reduce the use of other valuable water resources such as groundwater. Since the early 1990s, more than 600 hill reservoirs were built within the 250-500 mm range of mean annual rainfall. Most of them can limit water loss by runoff and enhance groundwater recharge. The El Gouazine reservoir in Central Tunisia was chosen within the European Union sponsored project Hydromed (1997-2001) because its groundwater balance is highly negative, ranging annually from -25,408 m3 in 1999-2000 (Tunisian hydrological year, conventionally starting in September and ending in August) to -273,435 m3 in 1995-1996, thus suggesting an important water loss by infiltration. The goal of the present paper consists in studying the hydrochemical behaviour of the watershed to improve the understanding of alluvial groundwater recharge below the hill reservoir.Water sampling was carried out in May 1998 when the reservoir was almost empty (14 samples) and in March 1999 when it was full (21 samples). Surface waters were collected within the open water surface of the reservoir, in a small upstream pond created during road construction, in a temporary affluent river, in a dam seepage and in the downstream riverbed. Underground water samples were collected from three well locations located downstream from the hill reservoir, from 10 upstream well locations and from two downstream pit locations. All the samples were immediately filtered on site.Dissolved oxygen content, temperature, pH, electrical conductivity at 25°C (EC) and alkalinity were measured in the field before filtration. The concentrations of major cations (Ca2+, Mg2+, Na+ and K+) and anions (Cl-, SO42- and HCO3 -) were determined by ion chromatography. The aqueous silica (SiO2) concentration was measured by ICP-AES. Total alkalinity was measured by titration with 0.1N HCl (in the field) and 0.02N H2SO4 (in the laboratory). The ratio of water stable isotopes was measured with a mass spectrometer and expressed in d-values, the deviations in parts per thousands (‰) from the International Standard V-SMOW.The total dissolved solids (TDS) of the surface waters ranged from 0.65 to 6.0 g L-1 in the dry period (DP) and from 0.25 to 5.7 g L-1 in the flow period (FP). Reservoir water was less mineralised in the FP than in the DP with an ion concentration factor of 2.6. The pH was nearly neutral and tended to be higher in reservoir water (10.1 in the DP and 8.6 in the FP). The silica concentration of the reservoir water was much lower in the DP and in the FP as well.The TDS of the ground waters, located within the watershed, varied from 0.6 to 6.2 g L-1 in the DP and from 0.6 to 4.9 g L-1 in the FP. The pH values were mainly neutral. In the DP, total alkalinity ranged from 254 to 529 mg L-1 as well as in the FP. Silica concentrations demonstrated relatively low variation ranging from 22 to 27 mg L-1 in the DP and from 15 to 27 mg L-1 in the FP.From upstream to downstream of the hill reservoir, the mineralisation of groundwater decreased, suggesting that an upstream mineralised groundwater flow is diluted by a weakly mineralised reservoir water.Three groundwater types can be distinguished in relation to the bedrock (limestone, marl, gypsiferous marl, gypsiferous mudstone, sandstone). The first type, weakly mineralised, was represented by three wells from the limestone outcrop. Bicarbonate, which ranged from 45 to 52% mmolc L-1 in the DP, and calcium, which was nearly 50% mmolc L-1, were the major ions and result from limestone weathering. The second type of groundwater included four wells, located in the marly lowlands between the limestone outcrops, and was characterised by a lower concentration of calcium and bicarbonate (30-40% mmolc L-1) coupled with a higher concentration of magnesium (20-30% mmolc L-1). Sodium was the major cation in most wells (33-43% mmolc L-1) whereas chloride was the dominant anion (45-67% mmolc L-1). In the lower part of the basin, the third type of groundwater was draining gypsiferous deposits and was dominated by calcium and sulphate ions. The reservoir water belongs to this type.Most underground waters originated from infiltrating precipitation that was not subject to surface or subsurface alteration of its isotopic composition. Groundwater located in the limestone outcrop was less enriched in stable isotopes.In the DP, reservoir water showed 2H and 18O enrichment, which is typical for water that has been subjected to surface evaporation. However, reservoir water is weakly mineralised, suggesting that the reservoir was an open system with more mineralised groundwater entering the reservoir and a mixed reservoir water downstream outflowing by infiltration. Downstream groundwater was weakly enriched and less mineralised than upstream groundwater. Reservoir water, which was permanently mixed, tends to be meteoric in the FP and evaporated in the DP. Physical and pedological clues indicate that the reservoir was leaking. A sandy layer (over 70% sand), situated on the left side embankment and in the sediment of the reservoir, was nearly 1.5 m thick with a bottom elevation above the reservoir bottom ranging from 3 to 5 m. The layer forms an aquifer resulting from the weathering of the sandstone outcrop and was connected to the downstream alluvial aquifer.The high permeability of the sandy layer partly explained the high water loss of the reservoir. Flowing through clayey materials, which contain variable amounts of easily soluble minerals, such as gypsum, and which are less permeable, the alluvial groundwater was strongly mineralised. The alluvial aquifer was supplied by shallow groundwater stored in limestone aquifers resulting in a strong decrease of the concentration. The limestone aquifers were highly porous and very transmissive. They can accumulate a high water content and rapidly recharge or discharge. The meteoric water collected in the reservoir also decreases the groundwater ion concentration leading to the same effect as the limestone groundwater