Current State of Knowledge on Groundwater Quality in Moundou, Southwestern Chad: Hydrochemical Processes, Contamination Pathways and Implications for Public Health

Groundwater is the main source of domestic water supply in Moundou, the second-largest city in Chad, located in the Western Logone basin and within the Chari-Logone hydrological system. The literature published specifically on groundwater quality in Moundou remains sparse, originating from a small number of research teams and often based on one-off sampling campaigns. This body of work is too narrow to support a systematic review in the PRISMA sense, but it lends itself to a narrative review that brings together, contextualises and critically interprets the available knowledge on Moundou and relates it to the regional and comparative literature on urban aquifers of the Sahel and West Africa. Groundwater in Moundou is generally low to moderately mineralised, with predominantly bicarbonate facies ranging from calcium-magnesium to sodium-potassium types, shaped by silicate weathering, carbonate dissolution and ion exchange and, to a lesser extent, by the dissolution of gypsum and CaCl 2 salts. This relatively reassuring hydrochemical signature contrasts with a worrying bacteriological situation: the available studies report frequent contamination by total coliforms, faecal coliforms, Escherichia coli and Salmonella spp., with marked seasonal variability and the recent emergence of strains resistant to certain antibiotics. Comparison with other Sahelian and West African urban aquifers (N’Djamena, Cotonou, Ibadan, Dakar) shows that Moundou fits a regional pattern of vulnerability of shallow aquifers to on-site sanitation, while differing from them in its less arid climate, which for now limits the risks of salinisation and saline intrusion observed elsewhere. The review highlights major gaps: the lack of systematic coupling between hydrochemistry and bacteriology, limited temporal monitoring, and the still incomplete attribution of contamination sources. Sustainable management of the resource requires integrated, risk-based surveillance combining hydrochemistry, microbiology, sanitary inspection and geospatial tools.