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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.

The age stratification of a freshwater lens on the island of Langeoog, Germany, was reconstructed through depth‐specific sampling and groundwater dating using the tritium‐helium method. The stratification is strongly affected by the land use and resulting differences in recharge rates. Infiltration at the dune tops is significantly lower than in the valleys, due to repellency of the dry sand. Dune valleys contribute up to four times more groundwater recharge per area than other areas. Housing development in dune areas might therefore significantly decrease the available fresh groundwater. The freshwater column shows a distinct increase of stable isotope values with decreasing depths. Hence, the freshwater lens contains a climate archive which reflects changing environmental conditions at the time of recharge. Combined with tritium‐helium dating, this pattern could be matched to climate records which show an increase of the temperature at the time of recharge and rainfall rates during the last 50 years. The spatial and temporal developments of water chemistry during the passage through the lens follow a marked pattern from a sodium and chloride‐dominated rainwater of low conductivity to a more mineralized sodium bicarbonate water type, caused by dissolution of carbonate shells close to the surface and subsequent ion exchange of calcium for sodium in the deeper parts. Age‐dated stable isotope values of groundwater reveal a climate archiveInferred climate development fits observed data wellAge stratification shows strong impact of land use on recharge rates