An isotopic study (H-2 and O-18) of water movements in clayey soils under a semiarid climate

We applied the natural variations of stable isotopes in water to the study of infiltration in clayey soils and to the study of groundwater recharge in the small drainage basin of Barogo in West Africa. The comparison between the variations in isotopic contents of soil water and rainwater in 1988 and 1989 led us to characterize two infiltration processes: (1) an infiltration through the soil matrix of rainwater that mixed with mobile water which was held in the superficial part of the soil and was enriched in heavy isotopes by evaporation and (2) a fast and deeper infiltration by preferential flow along macropores that bypassed the superficial part of the soil. The study of the variations in groundwater isotopic content led us again to separate the recharge process into two components: (1) a slow infiltration through the soil matrix and the weathered basement (distributed recharge) and (2) a fast and direct recharge through conducting fissured zones (localized recharge). A schematic and local representation of groundwater recharge in the Barogo basin allowed us to estimate that direct recharge represented around 70% of the total recharge in 1989.