Assessment of the water resource in Tambarga (Burkina Faso) based on stable isotope analysis

Assessment of available water is the basis that allows for sustainable management of the water resource. Special need concerns the assessment of groundwater resources, about which, often little is known. The present study aims to explore the water resource, with a focus on groundwater, of a small catchment (3.5km2), situated in a semi-arid zone in Burkina Faso (West Africa). A 5-year data set of stable isotopes of water, groundwater level and other hydrologic data is used to gain insight into the water resources in this small catchment. The main effort is devoted to the analysis of isotope data, with the secondary goal to asses the potential of the use of stable isotopes of water as an innovative tool in hydrology. Analyzed are temporal and spacial variations of the isotopic signature in groundwater, surface water and rain. Isotope data is compared to groundwater level data. Isotope data allowed to distinguish two different categories of wells. First, wells where evaporation dominated compared to recharge at the end of the wet season through the dry season, and wells that have constant isotope ratios throughout the year. Evolution of the isotope signature of the former suggests that these wells get disconnected from the aquifer at some point of the year. Comparison of isotope ratios of different sampling sites (springs, wells, pumps, surface water and rain) revealed that the upstream springs diverge from the other sites in their isotopic signature, and thus either have a different origin or were subject to different processes. Negligible diurnal variations observed in isotopic signatures of wells prompt that diurnal fluctuations observed in groundwater levels are due to non isotope fractionation processes. Periods deduced from seasonal changes in isotopic signatures match periods of rising and decreasing groundwater levels. Though groundwater levels show inter-annual variability, especially in their peak levels, this was not reflected in the isotope data. Isotope data confirmed conclusions gained from other data (recharge periods), added additional information (diurnal fluctuations of wells are non isotope fractionation processes), and pointed out the fact that upstream spring water distinguishes in some form or another from other groundwater.