Apports des traçages artificiel et naturel à la connaissance des circulations et des zones de recharge en région semi-aride: cas de l'Atlas de Beni Mellal (Maroc) Contribution of artificial and natural tracers to the knowledge of flow and recharge in a semi-arid region: the case of Beni Mellal Atlas (Morocco)

Résumé

Pour mieux comprendre les modalités d'alimentation des sources de l'Atlas, des campagnes de coloration et d'analyses isotopiques ont été effectuées sur l'ensemble des sources. La disparité des circulations, la variabilité des vitesses et des taux de restitution, révèlent un aquifère complexe ou à plusieurs entrées. La majorité des écoulements convergent vers la source de Ain Asserdoune qui constitue une zone d'abaissement d'axe de plis. L'étude isotopique montre la variation des teneurs en 180 avec l'altitude des zones de recharge des différents exutoires et avec la saison. Les gradients définis autorisent la détermination de l'altitude moyenne des impluviums des systèmes jusqu'alors mal connus, en particulier celui de l'exutoire principal du massif, Ain Asserdoune.

Mots-clés:

• Traçage,
• source,
• vitesse,
• aquifère,
• isotope du milieu,
• altitude,
• recharge,
• bassin d'alimentation,
• gradient,
• réserves

Abstract

The chalky massif of Beni-Mellal, which supplies many springs, spreads over a 365 km² area of 500 to 2400 meters elevation (average altitude, 1200 m). Rainfall in this sector varies between 400 mm and 700 mm/year. All springs emerge along the major coastal fault giving the Liassic massif overthrushing the Cenozoic lands of piemont. The most important is Ain Asserdoune spring with an average discharge of about 1.1 m³/s. The mainly chalky and dolomitic formations are very fractured according to preferential directions (N150-180 and N50-70) and show some karstification features at the surface. Some tracing experiments with fluorescein allowed the delimitation of the boundaries of the intake area of Ain Asserdoune Basin and the determination of the groundwater flow velocity. Two types of flow were identified with fast (100 to 231 m/h) and slow circulation (< 100 m/h). Most of circulations converge towards the spring of Ain Asserdoune (fig. 1) with an important intake area. This convergence results from an axis lowering. The variability of the circulation axes, the velocity and the rate of restitution suggest either a complex aquifer or an aquifer with several inputs.

Two water samplings (the first for the Beni Mellal Atlas) were carriet out in March and in September 1993 on the several springs, in order to study the variations of the oxygen-18 isotope. The oxygen-18 contents range between - 5.5 per thousand and - 7.7 per thousand (table 2). The most depleted waters directly exit from the Liassic aquifer or emerge at high altitudes; those which are richer in oxygen-18 emerge at the level of foothills, either from the Tertiary or from Liassic scales. The relationship between the oxygen-18 and the H-2 content of some samples indicates that the points are aligned along the world meteoric line (Craig, 1961). Even if the waters of Dir (piemont) springs are slighly evaporated because they partly originate in seeping Liassic waters, oxygen-18 content is probably mainly controlled by an altitude effect. Oxygen-18 data for local precipitations are not available; however, a local relationship between oxygen-18 content and mean recharge altitude was etablished for eleven (11) springs with small recharge areas whose mean altitude may be estimated from topography and geology. Oxygen 18 ratio is very well correlated to recharge average elevation for spring samples ( [delta]per thousand (O - 18)=- 3 - 0,0026 x altitude, r=0,96, fig. 3), when it is not for autumn samples. This gradient is very close to the regional relationship proposed by Marce (1975) and Kabbaj et al. (1978) (slope: 3 per thousand in oxygen-18 per 1,000 m ). This difference is attributed to the seasonal variations in the origin of water that feeds the springs. In the high water season, the whole basin area contributes to the recharge. In contrast, during low water period, water mainly originates from altitudes higher than the mean altitude of the basin, probably with a longer transit time. The seasonal variations are particularly marked for the Dir (piemont) springs (with a lower flux and intake area range of 800 to 1,200 m). They are weaker for the Lias springs and even non-existent for the main spring of the massif, Ain Asserdoune. In this latter case, the smoothing (- 7.6 per thousand and - 7.5 per thousand) of seasonal variation (which is also observed for temperature: 15.5 to 16°C ) gives evidence of important water reserves. A mean recharge altitude higher than 1,700 m may be proposed for Ain Asseroune.

Based on this analysis, some emergences which in the part were assigned to the same hydrologic system can now be discriminated. On the other hand, with a similar value of oxygen-18 ratio, rather distant springs can be considered as belonging to the same hydrogeological Atlasic unit. So the relationship which was quantified (slope: 2.6 per thousand in oxygen-18 per 1,000 m) is usable for the whole Atlasic range on account of regionaly similar weather. The methodology selected in this work corroborates the results obtained by hydrometric analysis concerning the very important groundwater reserves in the Beni Mellal aquifer.

Keywords:

• Tracer,
• spring,
• velocity,
• aquifer,
• environmental isotope,
• intake area,
• gradient,
• recharge altitude,
• reserves