Contribution of Major Ions in Identifying of Groundwater Flow in Dense Vegetation Cover Area: Case of Sassandra Watershed (South-Western Côte d'Ivoire)

Abstract

In West Africa, particularly in Côte d'Ivoire, the groundwater is contained in hard-rock aquifers and serves as main source of drinking water supplies to the population. To improve access, several studies were conducted in various parts of the country. Most of them use mapping of lineaments related to tectonic fractures to represent corridors of groundwater. In this article, chemistry of major ions was used to highlight quantitatively the axes of groundwater movement, and mixing between different aquifers in Sassandra watershed which is located in the Southwest of the Ivory Coast. The sampling campaigns were accomplished respectively during the dry and wet seasons in the department of Soubré (8 590 km2), located in Sassandra watershed, area where the effects of climate change are observed. The processing of satellite images (optical and active) has produced a map of major lineaments. Geographic positions and technical data of boreholes were integrated into a geographic information system (GIS) to identify point near major lineaments for groundwater sampling and chemical analysis. The waters were collected, and then analyzed by using atomic absorption spectrometer (AAS) and a Varian Vista ICP. The results indicate that groundwater samplings are primarily Ca-HCO3 type or NaK-HCO3 and NaK-SO4 types. Calcium and low pH were encountered in the highlands where infiltration of meteoric water occurs relatively quickly through preferential pathways. Chadha diagram has highlighted differences in the chemistry of groundwater between aquifers on one hand, and between systems of surface runoff and deep runoff on the other hand. Most groundwater seems to move relatively quickly. In addition, some groundwaters show a denitrification coupled with pyrite oxidation. These groundwaters have been longer circulating along opened fractures with gentle slopes. The observations and hydrochemical characterization, especially SO42-/Cl- ratio, permitted to identify axes of groundwater movements in the study area. However, the major lineaments which are similar to major fractures are not primarily responsible for the groundwater motions. Rather, there are small fractures and topography which control the flow of groundwater in the crystalline hard-rock. Also, the groundwater levels are not always guided by the major lineaments observed. Some small lineaments and topography control fairly the groundwater flow.