干旱区生态环境知识资源中心

Under the arid climate in the southern sub-Saharan catchments of Morocco, groundwater is the only permanent water supply. Usually, the groundwater is scarce because of the low recharge. However, the mining activities in Oumjrane area (Eastern Anti-Atlas Mountains) reveal that water resources are relatively abundant in the fissured Ordovician aquifer. To understand this phenomenon, geochemical and isotopic investigations were carried out in the region. The results show that the quality of water is variable in superficial and deep aquifers. The specific conductivities range from 0.44 to 10 mS/cm. Most groundwater samples from the deep aquifer indicate high salinity, low pH, and CO2 anomalies. Geochemical analyses show that groundwater concentrations are mainly controlled by (i) evaporite dissolution (halite and gypsum), (ii) cation exchange with clay minerals, (iii) silicate hydrolysis, and (iv) dedolomitisation. Carbonate precipitation takes place in the groundwater issuing from the mine sector. The discharge is estimated at 386 T/year downstream of the Bounhass mine. Moreover, stable isotope (delta H-2 and delta O-18) and chemical analyses reveal a clear difference between the alluvial and deep aquifers. They also show a local recharge for the alluvial aquifer from Saghro inliers. Low isotope ratios are found in the Ordovician formation aquifer, indicating a deep water circulation. These waters could originate from the High Atlas and/or Saghro Mountains and flow through fissures and fault networks. Connections to regional faults and veins may play a role in it, as well as the proximity of the major contact with the High Atlas.