Groundwater monitoring in challenging environments: an argument for the construction of observation wells based on data from Niamey, Niger

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Groundwater sampling in challenging environments often leads to compromises in following best practices to obtain representative samples from aquifers. This includes collecting samples from existing production or domestic wells instead of using properly constructed monitoring wells or using a bailer instead of a submersible pump for sampling. To address unusual patterns and trends in groundwater chemistry data collected in Niamey, Niger from 2012–2021, a state-of-the-art monthly sampling routine was established for eight wells tapping the basement aquifer. This was based on the hypothesis that the observed changes in groundwater composition were mainly due to differences in sampling technique, and the aim of the study was to gain insights into possible seasonal variations in water composition, to examine if the previously observed trends could be validated and to provide baseline data for future studies. The results indicate that in most cases the long well response zones in the stratified aquifer system led to the collection of water from different strata/aquifers or of strongly mixed samples. Therefore, any sample from those wells is only of limited value for the interpretation of hydrogeological processes. To obtain sound data for the development of groundwater management strategies, the monitoring has to be shifted from existing production wells to properly constructed monitoring wells. In the complex hydrogeological setting of Niamey, with hydraulically interacting aquifers and occurrences of density layering, it is fundamental to ensure that a monitoring well taps one specific depth of one target aquifer and that well-volume purging is applied properly.