Vertical hydrochemical profiles in the unsaturated zone of louga (senegal) aquifer controlled by geochemical processes

Abstract

Solutions chemistry of the rainwater and the unsaturated zone interstitial water of Louga (Northern Senegal) local aquifer provide valuable information related to water-rock interaction occurring during natural recharge. Non reactive chloride ion and rainwater ion/ chloride molar ratios are used as a baseline against which sequential uptake and release of solutes from/to the unsaturated zone solution may be studied. Cation exchange capacity (CEC) and exchangeable cation (EC) experiments, together with chemical analysis of the interstitial water carried out through the entire unsaturated zone profile have revealed that base exchange reactions and dissolution of silicate minerals are the dominant processes controlling the water chemistry. In the uppermost 7 m of the profile, silicate weathering dominates, this process occurs at low pH (3.5 – 4). Major cations and silicium are released to the interstitial water above the calibrated rainwater reference. Below this depth, exchange reactions dominate, they are accompanied by a gradual increase in pH (up to 5). However, ion selectivity reaction seems to control the concentration of the cations. Preference of bivalent cations (Ca2+, Mg2+) over monovalent cations (K+, Na+) by the clay minerals may explain the chemistry of both the unsaturated zone solutions and at the immediate water table.