Groundwater flow patterns, hydrogeochemistry and metals background levels of shallow hard rock aquifer in a humid tropical urban area in sub-Saharan Africa- A case study from Olézoa watershed (Yaoundé-Cameroon)
André Firmin Bon, Therese Anny Michelle Ngo Ngoss, Guillaume Ewodo Mboudou, Lucian Asone Banakeng, Jules Rémy Ndam Ngoupayou, Georges Emmanuel Ekodeck
Abstract
Olézoa watershed, Yaoundé-Cameroon, Central Africa This study aimed to determine the hydrogeological characteristics of shallow hard rock aquifer and the environmental levels of heavy metals in an urbanized equatorial region. Physico-chemical parameters and heavy metals were determined on 25 groundwater samples and analyzed using conventional hydrochemical methods, multivariate analysis, heavy metal pollution index (HPI) and natural background levels (NBL). Hydrogeological functioning is not marked by a mass of basal groundwater with a low hydraulic gradient Hydrochemical facies were predominantly Ca-Mg-Cl, HCO3-Ca-Mg and Ca-Cl type, which were controlled water-rock interaction, cation exchange, dissolution of silicates and anthropogenic activities. Multivariate analysis did not show very significant hydrochemical spatial zonation but the first major component can be used as an indication of the groundwater flow direction. Vertically, a three-part structure around depths of 1 m and 3−7 m were identified and associated with local and intermediate flow systems at the Olézoa watershed-scale. HPI showed short range variations similar to that of EC, suggesting compartmentalization of the shallow aquifer on a small scale. Thus, a preliminary conceptual model describing groundwater dynamics has been proposed. NBL and median HPI values indicate a serious concern in terms of groundwater qualitative potential and the urgency of regular monitoring of heavy metals in groundwater in this urbanized part of sub-Saharan Africa.