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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., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Labelling experiments on a 1 m2 plot with deuterium depleted (25 ppm deuterium, approx. -800 ‰, Preventa Inc.) on 19/10/2020 and deuterium enriched water (50 ml of 75% deuterium oxide) on 9/11/2020 were conducted in an 174-year-old spruce forest in the Solling highland, Northern Germany. A Picarro L2120-i laser absorption spectrometer connected to a 16-port multivalve system and a vacuum pump (Picarro Inc.) were used for in situ analyses of deuterium concentrations in soil water. Continuous measurements were possible using a self-built soil probe capillary sampling device at 20 cm (atmosphere) above ground and depths of 0 cm, 5 cm, 10 cm, 15 cm, 30 cm, 50 cm, 75 cm and four standards in glass bottles for corrections and standardization. Capillaries were sampled for ten-minute intervals in predefined order interspersed with standards every six to seven hours. Additionally, soil samples were collected manually for vacuum extraction in the laboratory before the labelling and frequently during the experiments in 10 cm intervals up to 70 cm depths. The observed tracer breakthrough curves allow for an interpretation of travel times as well as tracer and soil water behavior. Deuterium depleted water could be detected to a depth of 10 cm (-250 ‰) and the enriched water to a depth of 30 cm with fast signals in deeper layers indicating preferential flow pathways. Merits and limitations of in situ stable isotope instrumentation and measurements in forest soils will be discussed., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Stable isotopes (deuterium, 2H, and oxygen-18, 18O) of soil water were measured in the field using a liquid water isotope analyzer (tunable off-axis integrated cavity output spectroscope, OA-ICOS, LGR) and commercially available soil gas probes (BGL-30, UMS, Munich) in the semi-arid Cuvelai–Etosha Basin (CEB), Namibia. Results support the applicability of an in situ measurement system for the determination of stable isotopes in soil pore water. High spatial and temporal resolution was achieved in the study area with reasonable accuracy and measurements were in agreement with laboratory-based cryogenic vacuum extraction and subsequent cavity ring-down laser spectroscopic isotope analysis (CRDS, L2120-i, Picarro Inc.). After drift and span correction of the in situ isotope data, precision for over 140 measurements taken during two consecutive field campaigns (June and November 2014) was 1.8 and 0.48 ‰ for δ2H and δ18O, respectively. Mean measurement trueness is determined using quality check standards and was 5 and 0.3 ‰ for δ2H and δ18O, respectively. The isotope depth profiles are used quantitatively to calculate a soil water balance. The contribution of transpiration to total evapotranspiration ranged between 72 and 92 %. Shortly after a rain event, the contribution of transpiration was much lower, at 35 to 50 %. Potential limitations of such an in situ system are related to environmental conditions which could be minimized by using a temperature-controlled chamber for the laser spectrometer. Further, the applicability of the system using previously oven-dried soil material might be limited by physicochemical soil properties (i.e., clay minerals). Uncertainty in the in situ system is suggested to be reduced by improving the calibration procedure and further studying fractionation effects influencing the isotope ratios in the soil water, especially at low water contents. Furthermore, the influence of soil-respired CO2 on isotope values within the root zone could not be deduced from the data.