Effet auto-épurateur de la lithologie des affleurements géologiques dans un climat semi-aride: cas du bassin versant de l'Oued Mellègue (Nord-Ouest de la Tunisie)

Résumé

Le Nord-Ouest de la Tunisie renferme plus que 81% des eaux de surface et 95% des mines et gîtes miniers. Le bassin versant de l'Oued Mellègue, situé dans cette zone, englobe 13 mines de plomb, zinc, fer, strontium, fluor, barium et phosphates. L'impact des rejets de ces mines sur les eaux de surface a été apprécié grâce à plusieurs compagnes d’échantillonnage. Les résultats d'analyse par ICP/MS de ces échantillons montrent qu'ils renferment des quantités non négligeables d'Al, As, B, Ba, Br, Cd, Co, Cr, Cu, Hg, Mo, Ni, Sr, U et de Zn. L'analyse des ces éléments dans les eaux du Mellègue, de ses affluents et dans la retenue de Nebeur montre que leurs concentrations sont inférieures aux normes d'eau potable. Ce comportement pourrait être dû à la nature des affleurements géologiques formés par des carbonates des argiles et des évaporites, à l'absence de cours d'eau drainant directement les rejets, à la formation de minéraux secondaires caractérisés par une importante rétention des métaux et à l’étendue des bassins versants non pollués.     

Use of a time-domain electromagnetic method with geochemical tracers to explore the salinity anomalies in a small coastal aquifer in north-eastern Tunisia

The study area is a small coastal plain in north-eastern Tunisia. It is drained by an ephemeral stream network and is subject to several pollutant discharges such as oilfield brine coming from a neighboring oil company and wastewater from Somâa city, located in the upstream of the plain. Furthermore, a hydraulic head near the coastal part of the aquifer is below sea level, suggesting that seawater intrusion may occur. A time-domain electromagnetic (TDEM) survey, based on 28 soundings, was conducted in Wadi Al Ayn and Daroufa plains to delineate the saline groundwater. Based on longitudinal and transversal resistivity two-dimensional pseudosections calibrated with boring data, the extent of saline water was identified. Geochemical tracers were combined with the resistivity dataset to differentiate the origin of groundwater salinization. In the upstream part of the plain, the infiltration of oilfield brine through the sandy bed of Wadi Al Ayn seems to have a considerable effect on groundwater salinization. However, in the coastal part of the aquifer, groundwater salinization is due to seawater intrusion and the saltwater is reaching an inland extent around 1.3 km from the shoreline. The contribution ratios of saline water bodies derived from the inverted chloride data vary for the oilfield brine from 1 to 13 % and for the seawater from 2 to 21 %.     

Seismic, gravity, and wire line logging characterization of the Zéramdine fault corridor and its influence in the deep Miocene aquifers distribution (east-central Tunisia)

In the Jemmel?CZéramdine region, the combined analysis of petroleum well data, seismic reflection profiles, and gravity data show the presence of the Zéramdine fault corridor, which was activated in the Neogene period. Both tectonic activity and fluvio-deltaic sedimentation are the origin of the lithologic and hydrodynamic heterogeneities. Thus, the Zéramdine fault corridor constitutes a limit between two hydraulic systems formed by Miocene sandy reservoir layers. These reservoirs have not been exploited yet and could be new water resources which can extremely benefit the region; only the upper one is exploited in the Jemmel?CBembla and Zéramdine?CBéni Hassen deep aquifers. The wire line logging, seismic reflection, and gravity interpretations show the distribution of the Miocene layers which formed the Sahel Miocene deep aquifers. The Zéramdine fault corridor leads to a spatial variability in number, distribution, depths, and thicknesses of these reservoir layers. In the northern part of the Zéramdine fault corridor, seven layers were highlighted; their thicknesses range between 15 and 105?m. However, in the southern part, only four strata were deposited: their thicknesses vary between 45 and 53?m. The total porosity of the studied aquifers is about 30%.