Geochemical processes controlling groundwater quality under semi arid environment: a case study in central Morocco

Abstract

Bahira plain is an important area for Morocco due to its agriculture and mining activities. Situated in a sub-arid to arid climate, this plain hosts an aquifer system that represents sequences of carbonates, phosphates, evaporates and alluvial deposits. Groundwater flows from Ganntour plateau (recharge area) to the basin-fill deposits and Zima Lake and Sed Elmejnoun where water evaporates. The objective of this study was to characterize the chemical properties of the groundwater and to assess the processes controlling the groundwater's chemistry. We can divide water samples into three hydrochemical water groups: recharge waters (Ca/Mg-HCO₃), transition zone waters (Ca-HCO₃-SO₄/Cl) and discharge waters (Na-Cl/SO₄). Accordingly, compositions of waters are determined by the availability of easily soluble minerals like calcite (Ca-HCO₃ dominant), halite (Na-Cl dominant) and gypsum (Ca-SO₄ dominant). Cl/Br ratios show that Cl concentration increases from dissolution of natural halite. When groundwater is affected by extreme evaporation Cl/Br ratios may increase up to 1900. High fluoride concentrations are associated with low Ca²⁺ concentrations (<100mg/L). That means when recharge waters enter the aquifer, it starts dissolving fluorite since the Ca²⁺ concentration is low. Once groundwater becomes saturated with Ca²⁺, the immobilization of fluoride is occurring by precipitation of fluoride-rich minerals like fluoro-apatite. According to the environmental isotope (¹⁸O and ²H) analyses, they are three potential processes affecting groundwater: 1. Evaporation as verified by low slope value, 2. Water-rock interaction, 3. admixture of waters showed different stable isotope compositions and salinities.