Reliability of geochemical and hydrodynamic sampling in a semi-arid water table

This paper evaluates the reliability of the geochemical and hydrodynamic sampling in a phreatic aquifer of semi-arid Africa. These remarks are based on a dense network and on detailed data collected for about ten years in the Continental Terminal water table near Niamey, Niger. The natural potentiometric fluctuations are seasonal (up to 6 m during the rainy season) and interannual (up to 10 m since the 1960s). Hydrodynamic measurements can be disturbed by artificial flooding into the wells (20% of the network) and by domestic drawings (up to 2 m of lowering). Geochemical characteristics can vary seasonally: isotopic modifications and salinity increase near recharge areas, with more stability further away. They can also vary artificially in wells because of their large opening towards the open air. Artifical increases of salinity of one order of magnitude and contaminations of C-14 activities of up to 20 pmC occur in some wells by atmospheric dust inputs or organic matter which has fallen into the well. In conclusion, some recommendations are given for a representative sampling in semi-arid phreatic aquifers. (C) 2001 Elsevier Science Limited. All rights reserved.