Sustainability of acquifers in Mali: Sr and Pb isotopic signatures and tritium age.

An exploratory assessment of the sustainability and vulnerability of water wells in aquifers in northern Mali can be attempted on the basis of two criteria: the connectivity of aquifers and their recharge age. The former can be addressed using Sr-Pb isotopic signatures, as isotope ratios are dependent on the isotope geochemistry of the bedrock but not on saturation/precipitation reactions. The latter can be addressed using tritium activity measurements, which are most sensitive to recharges that occurred between 1950 and 1965. A total of 11 water samples from wells already in operation and from wells being constructed were collected over 700 km E-W distance between the Kayes and Mopty regions. Typical sample sizes were 100 mL. Sr and Pb were separated from the other dissolved ions by cation-exchange resin chromatography. Isotopic compositions of Pb and Sr were measured my multicollector ICP mass spectrometry. Results show that each well has a different Sr-Pb isotopic signature. A first-order conclusion appears to be that aquifers have very limited areal extents and each well taps a separate aquifer. In principle it would be possible to model water interactions with the bedrock in such a way that neighbouring wells can be matched pairwise, by assuming ad-hoc Sr and Pb concentrations and isotopic compositions; however, even in this highly artificial scenario it is impossible to fit more than two wells at the time. The robust conclusion is that no aquifer extends for the entire 700 km in E-W direction. A likely, and more strict, conclusion is that the E-W extensions of aquifers are smaller than 50 km or even less in some cases. Tritium measurements were performed on small (10 mL) sample aliquots. Detection limits and uncertainties are therefore very high. Five wells (irregularly scattered in the sampling area) have detectable tritium anomalies; calculated recharge and residence ages range between 30 and 60 a. Sustainability requires achieving a steady state between withdrawal and recharge. Considering the very long recharge times, this puts strict limits on the pumping speed. The vulnerability is a function both of total water mass and of connectivity: pollution of a small aquifer can occur as an effect of even a small pollutant mass, but its effects are likely to be restricted to the area covered by the aquifer if it is indeed unconnected to neighbouring ones.