Isotopic Characterization of Rainwater for the Development of a Local Meteoric Water Line in an Arid Climate: The Case of the Wadi Ziz Watershed (South-Eastern Morocco)

Abstract

For any hydrological or hydrogeological system, the arrival of new rains is the input signal to the system. This isotopic signature of precipitation is of major interest in understanding the recharge processes of the aquifer system. On the scale of a given basin, staged stations at different altitudes and spread out in space allow this input signal to be well characterized and to draw the local meteoric water line. In south-eastern Morocco, specifically, in the Errachidia region, several chemical and isotopic studies of the waters of the various aquifers have been carried out. In the absence of a local meteoric water line, these studies were based on the use of the global meteoric water line (GMWL). Thus, the objective of this work is the isotopic characterization and the elaboration of the local meteoric water line of the rainwater of the Ziz watershed. This characterization of the input signal in the study area is based on 41 measurements of stable isotopes (δ18O and δ2H) relating to the precipitations collected during the period from December 2019 to November 2020 in four staged stations at different altitudes and spread over the space from upstream to downstream of the watershed. The linear relationship of δ2H as a function of δ18O describes the local meteoric water line (LMWL) by equation δ2H = 7.5 ± 0.3 δ18O + 4.6 ± 1.7; R2 = 0.93. This equation displays evaporation confirmed by the arrival of continental currents in an arid environment. The variation in precipitation δ18O as a function of the sampling altitudes for the rains highlighted the relationship δ18O = −0.0026 ∗ Z − 1.67, with R2 = 0.93, which means an altitudinal gradient of −0.26‰ per 100 m of altitude. In this regard, the development of the local meteoric water line and the determination of the altitudinal gradient for the first time in this arid to semi-arid region of the watershed will be of great use to researchers and water resource managers; for example, to help determine the groundwater recharge areas, determine the exchanges between surface water and groundwater, and analyze many other hydrological problems.

Groundwater Hydrochemical and Isotopic Evolution from High Atlas Jurassic Limestones to Errachidia Cretaceous Basin (Southeastern Morocco)

“…This group is depleted in 18 O, with 2 H contents between −61.2‰ and −55.6‰. The isotopic concentrations of this group (1,2,4,12,15,16,22,23) are close to the average values of the High Atlas rainwater [31]. The values are characteristic of a protected aquifer free of any contribution from other aquifers.…”

“…In the south, rainfall is about 80 mm, with an effective rainfall varying from 1.52 to 4.55 mm. The low rainfall in the south is due to the fact that the Atlas mountains prevent any oceanic influence and the presence of warm air masses coming from the Sahara [31].…”

“…This causes the dissolution of dolomite and thus an increase in the concentration of Mg 2+ according to: . δ 18 O‰ vs. δ 2 H‰ plot of groundwater samples from the study area compared to GMWL [30] and LMWL [31].…”

“…The isotopic gradient was determined from the relationship between δ 18 O (‰) and the estimated average elevation of the recharge area of four stations in the study area (Figure 9), which was established by El Ouali in 2022 [31]. δ 18 O (‰) ratios are related to the elevation of the recharge area and can be expressed by the following equation: We projected all samples to estimate the elevation of their recharge area.…”

“…Figure8. δ 18 O‰ vs. δ 2 H‰ plot of groundwater samples from the study area compared to GMWL[30] and LMWL[31].…”