Rechare of the Tafilalet plain aquifer, an arid zone of Morocco

I. General features of the area I. 1. Localisation of the study site The area is located to the south east of Morocco (Fig. 1) between latitudes 31° and 31°42’ N and longitudes 4°11 and 4°24 W. The study area with a maximum surface of 630 Km2, is limited by the Errachidia Cretaceous basin in the north and the Saharian platform in the south. I. 2. Hydrography Major rivers in the area are Ziz and Rheris. The two non perennial rivers start from the high Atlas Mountains and passes through the Tafilalet plain in north-south direction. The upstream dam built on Ziz river in 1970 is located at 80 Km in the north of Tafilalet. I. 3. Socio-economical aspects The population can be estimated to be more than 100, 000 inhabitants. The population activities

comprise traditional agriculture, exploitation of more than 15, 000 ha of Palm plantation and herding. During the last 30 years the region known a severe period of drought which lead to an over exploitation of the groundwater reservoir. So, in order to maintain the population and their agricultural activities, and also to prevent the threat of desertification process it becomes urgent to define the optimum conditions for a sustainable management of water resources in the region. For this purpose we need to: - identify the origin of the recharge to the Tafilalet aquifer and the infiltration process - estimate the mean residence times of water in the aquifer which could help to optimize management of the water resources.

I. 4. Geologic context The Tafilalet plain is a depression resulting from intense erosion that underwent the Anti Atlasic Chain, and is full filled by quaternary deposits overlying an impermeable Palaeozoic substratum. I. 5. Climate and Hydrogeology The climate is arid with no oceanic influence. The average annual rainfall is 50-100 mm while the number of the pluviometric days by years is 10 to 15. The aridity indice is about 0.05 . The reference evapotranspiration rises 1600 mm per year.

The aquifer is composed of Quaternary alluvial deposits (Fig. 2) The aquifer is composed of Quaternary alluvial deposits (Fig. 2). The upper sedimentary deposits consist mainly of fine sand and silt, sandy fine silt and gravely formations. The lower unit of sediments, up to 20 m thick, consists of conglomerates and lacustrine limestones which constitute the main reservoir of water resource in the aquifer. The thickness of the unsaturated soil zone varies from 8 to 22 m whereas the thickness of the saturated zone is 1-19 m. The transmissivity ranges from 10-2 to 2.10-2 m2 /s and the hydraulic conductivity varies from 10-3 to 2.10-3 m/s.

The piezometric map (Fig The piezometric map (Fig. 3) indicates a general groundwater flow from north to south. II. Sampling and analysis

Sampling is performed from the existing active supply wells over the period 1995-1997 along the flow path (Fig. 3 and 4). In relation to the general situation, 4 further water points are sampled from the Cretaceous (Infracenomanian) formation in the north border limit of the study site (Fig. 4). Major anions analysis are done by ion chromatography on filtred sample (0.45 μm filter). Cations are analysed by AAS on filtred (0.45 μm) and acidified sample (HNO3, pH ≈ 2). The pH, conductivity and temperature are determined in situ (Table I). Stable isotopes (18O, 2 H), 3H, 14C and 13C measurements are performed in the AIEA laboratory (Table II and III).

II. Sampling and analysis The piezometric map (Fig. 3) indicates a general groundwater flow from north to south. II. Sampling and analysis

Sampling is performed from the existing active supply wells over the period 1995-1997 along the flow path (Fig. 3 and 4). In relation to the general situation, 4 further water points are sampled from the Cretaceous (Infracenomanian) formation in the north border limit of the study site (Fig. 4). Major anions analysis are done by ion chromatography on filtred sample (0.45 μm filter). Cations are analysed by AAS on filtred (0.45 μm) and acidified sample (HNO3, pH ≈ 2). The pH, conductivity and temperature are determined in situ (Table I). Stable isotopes (18O, 2 H), 3H, 14C and 13C measurements are performed in the AIEA laboratory (Table II and III).

III. Results and discussion III. 1. Hydrochemistry The major ion compositions in Piper diagram indicate that the Tafilalet water belongs to Na-Cl type. This water is saline to brackish. Table I

The conductivity is up to 12, 000 μs/cm (Table I).

Fig. 6

Fig. 7

Fig 5, 6 and 7 indicate clearly that the mineralization of water would be mostly originated from chloride and gypsum evaporates. Further, as the Ca/Mg ratio for all the water samples is higher than 0.5 the dedolomitization mechanism is favoured. So the origin of the high concentrations of Mg2+ could be explained by the dedolomitization process.

III. 2. Isotope hydrology

(-) means no measurements

Fig. 8

- In the δ2H-δ18O plot (Fig. 8), the position of the whole groundwater samples as measured in 1995-1997 below the Global Water Meteoric Line (GWML: δ2H = δ18O + 10) constitutes a fingerprinting of an evaporated water. - The isotope composition of this water show an evaporation trend with a slope ranging from 3.5 to 5.2. The original composition of this water as deduced from the δ2H- δ18O plot (Fig. 8) would be -8.16 < 18O < -7.8 ‰ and -52.5 < 2H < -55.3 ‰, whereas the local precipitation has an isotope signature around δ18O: -6.39 ‰ and δ2H: -41.2 ‰ (Table II). - The Tafilalet aquifer is likely recharged by water originated from high Atlas mountains or the dam who regulate the water coming from high Atlas mountains, through the Ziz river.

- Isotopic compositions of the I - Isotopic compositions of the I.C formation at the north border limit of Tafilalet is highly depleted and lie on the GWML in one cluster (Fig. 8): -9.8 < δ18O < -9.4 ‰ -64.9 < δ2H < -68.6 ‰. This aquifer seems to be confined: no tritium content, 14C around 0 to 1 pcm and δ13C ≤ 0 ‰ PDB. All these data exclude any connection between this aquifer and Tafilalet Quaternary aquifer. -Except two water points 3903/57 and 3465/57 with tritium content close to the tritium concentration in precipitation, the others groundwater tritium concentrations are in the range of 12-20 TU. -The carbon-14 values are in the range of 95-108 pmC These isotopic data confirm the modern water of the Tafilalet aquifer.

III. 2.1. Mean residence times The mean residence time of water is calculated based on a lumped parameter model: C(t)=Cin (t-)exp (-)f()d (1) where Cin (t) is the tritium concentration in the infiltrated precipitation and C (t) the tritium content in the groundwater sampling,  is the radioactive decay constant for the input tracer,  is the residence time, and is the mean residence times of water defined as: (2) where f () is the residence time distribution function. The mean residence times is estimated by a one-dimensional dispersion model (justified by shape of the aquifer) using environmental tritium as a tracer:

The f () for the dispersion model characteristic of sampling performed from the existing production wells is given by: f()=(4tD/vx)-1/2exp-(1-t/)²vx/4Dtt-1 (3) where D is the dispersion coefficient (m2/s), x is the distance from the recharge area to the measuring point, v is the mean transit velocity, v = x/, and D/vx is the dimensionless dispersion parameter. Due to the lack of regular sampling in the Tafilalet recharge area, the evolution of the tritium concentration in precipitation over the past 30 years, is estimated from: -Bamako input data for the period 1964-1979, -Fes-Sais data (Morocco) for the period 1993-1996, -Values for the period 1980-1992 without measurements in those stations have been constructed by correlation with data from the Tunis station.

Model calculations were performed for environmental tritium (3H) using the computer code FlowPC. Each water sample required several trials before a good fit was obtained. The range of possible mean residence times of water in the aquifer and peclet number values is given in Table III.

Table III

Some examples of observed and computed output function are given in Fig.9.

The short mean residence times obtained (14-32 yrs) by the tritium transport modelling indicate: 1) The efficiency of artificial recharge by the upstream dam, 2) The need of the continuous recharge through Ziz river, 3) Give an idea of the limit of exploitation in the actual conditions.

IV. Conclusions 1- The Tafilalet aquifer is recharged from high Atlas precipitations, regulated by the dam on ziz river. 2- There no interconnection between the Tafilalet aquifer and the Infracenomanian aquifer in the north border of the plain. 3- The artificial recharge by the “Hassan Addakhil” dam compensate actually the deficit due to the conjugated effects of droughts and over exploitation. Actually the major problem is the high salinisation of the unsaturated soil zone due mainly to the chloride evaporates by evaporation effect and lowness of drain-down.