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Abstract

Hydrogeological data are scarce in large parts of sub-Sahara Africa, but of increasing importance due to growing pressure on water resources. Groundwater sampling is particularly challenging and samples are often collected in the “best way possible” from existing production wells. To assess whether observed changes in groundwater chemistry were due to seasonal or long-term temporal variations or caused by different sampling procedures, data from eight wells in Niamey, collected 2012 - 2021, were evaluated. Sampling techniques included sample collection from permanently installed manual pumps, sample collection using a bailer, and sample collection using submersibles pumps with no purging, limited purging, or proper purging. Three patterns were observed: (i) Non-purged and purged samples are similar and show little variations. (ii) Non-purged and purged samples differ substantially, with either the non-purged or the purged samples showing great variations in ion concentrations (while ion concentrations in the respective other group remain stable). (iii) Non-purged and purged samples both vary in ion concentrations depending on sampling depth, with the non-purged samples showing bigger differences. The results show that in most cases the long well response zones in the stratified aquifer system lead 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. Repeated sampling with different methods can give an idea about well design and capture horizon(s), but the time and money would be better invested in the installation of a proper monitoring network.