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

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