Matthias Beyer

Mineral mediated isotope fractionation of soil water

RATIONALEnThe ability to recover the isotopic signature of water added to soil samples that have previously been oven-dried decreases with the increasing presence of silt and clay. The effects on the isotopic signature of water associated with physicochemical soil properties are not yet fully understood, for either hydration or dehydration of soil samples.nnnMETHODSnThe soil sample chemistry and the crystallinity of minerals were measured by X-ray fluorescence and X-ray diffraction. The organic carbon and the cation-exchange capacity were also determined. Water of known isotopic signature was used to spike an oven-dried substrate and subsequently extracted by cryogenic vacuum extraction at a temperature of 105°C. In addition, the soils were oven-dried at 205°C and water extractions were also performed at 205°C. The isotopic signatures of the water samples were determined by cavity-ring-down spectrometry.nnnRESULTSnThe isotope effects caused by the cryogenic vacuum extraction method applied to soils with elevated clay content were reduced. First, by increasing the extraction temperature to 205°C, we improved the precision of the cryogenic vacuum extraction method and the recovery of the known isotopic signature of the spike water. Secondly, the post-correction of data based on the physicochemical soil properties and a common extraction temperature of 105°C improved the measurement trueness.nnnCONCLUSIONSnThe isotopic signature of soil water is influenced by mineral-water interaction. During the hydration of clay, different minerals deplete free water in heavy isotopes. The extracted soil water (dehydration water) gathered from clay-rich soils is generally more depleted in the heavy isotopes than the spike water, making results obtained for different soil types difficult to compare. Isotope effects observed at the mineral-water interface highlight potential explanations for eco-hydrological separation of water pools. Copyright

Estimation of groundwater recharge via deuterium labelling in the semi-arid Cuvelai-Etosha Basin, Namibia

The stable water isotope deuterium (2H) was applied as an artificial tracer (2H2O) in order to estimate groundwater recharge through the unsaturated zone and describe soil water movement in a semi-arid region of northern central Namibia. A particular focus of this study was to assess the spatiotemporal persistence of the tracer when applied in the field on a small scale under extreme climatic conditions and to propose a method to obtain estimates of recharge in data-scarce regions. At two natural sites that differ in vegetation cover, soil and geology, 500u2005ml of a 70u2009% 2H2O solution was irrigated onto water saturated plots. The displacement of the 2H peak was analyzed 1 and 10 days after an artificial rain event of 20 mm as well as after the rainy season. Results show that it is possible to apply the peak displacement method for the estimation of groundwater recharge rates in semi-arid environments via deuterium labelling. Potential recharge for the rainy season 2013/2014 was calculated as 45 mm a−1 at 5.6 m depth and 40 mm a−1 at 0.9 m depth at the two studied sites, respectively. Under saturated conditions, the artificial rain events moved 2.1 and 0.5 m downwards, respectively. The tracer at the deep sand site (site 1) was found after the rainy season at 5.6 m depth, corresponding to a displacement of 3.2 m. This equals in an average travel velocity of 2.8 cm d−1 during the rainy season at the first site. At the second location, the tracer peak was discovered at 0.9 m depth; displacement was found to be only 0.4 m equalling an average movement of 0.2 cm d−1 through the unsaturated zone due to an underlying calcrete formation. Tracer recovery after one rainy season was found to be as low as 3.6u2009% at site 1 and 1.9u2009% at site 2. With an in situ measuring technique, a three-dimensional distribution of 2H after the rainy season could be measured and visualized. This study comprises the first application of the peak displacement method using a deuterium labelling technique for the estimation of groundwater recharge in semi-arid regions. Deuterium proved to be a suitable tracer for studies within the soil–vegetation–atmosphere interface. The results of this study are relevant for the design of labelling experiments in the unsaturated zone of dry areas using 2H2O as a tracer and obtaining estimations of groundwater recharge on a local scale. The presented methodology is particularly beneficial in data-scarce environments, where recharge pathways and mechanisms are poorly understood.

Hydrogeochemical and isotope study of perched aquifers in the Cuvelai-Etosha Basin, Namibia

ABSTRACT A hydrogeochemical and stable isotope study (2H and 18O) was carried out in the Cuvelai-Etosha Basin in order to characterize available groundwater and to identify possible recharge mechanisms for the perched aquifers. Data were collected during seven field campaigns between 2013 and 2015 from a total of 24 shallow and deep groundwater hand-dug wells. In the investigated groundwaters, hydrogencarbonate is the dominating anion in both well types, whereas cations vary between calcium and magnesium in deep wells, and sodium and potassium in shallow wells. Groundwater chemistry is controlled by dissolution of carbonate minerals, silicate weathering and ion exchange. Stable isotopic composition suggests that deep groundwater is recharged by high-intensity/large rainfall events, whereas the shallow wells can even be recharged by less-intense/small rainfall events. Water in deep wells reflect a mixture of water influenced by evaporation during or before infiltration and water that infiltrated through fast preferential pathways, whereas shallow wells are strongly influenced by evaporation. The findings of this research contribute to improve the understanding of hydrogeochemistry, recharge paths and temporal variations of perched aquifers.

Stable isotope signatures of meteoric water in the Cuvelai-Etosha Basin, Namibia: Seasonal characteristics, trends and relations to southern African patterns

ABSTRACT The study area is the Namibian part of the Cuvelai-Etosha Basin (CEB), located in central northern Namibia. The CEB is home to 40 % of Namibia’s population, and most of the people live in rural areas. These people depend on both surface and groundwater resources which are limited in this dryland (mean annual rainfall ranging from 250 to 550u2005mm/a). The isotopic signatures of δ18O and δ2H from water samples (nu2009=u200961) collected over a course of 9 years from various research projects and existing (but mainly unpublished) data of meteoric water of the CEB (10 sites) were evaluated and local meteoric water lines (LMWLs) developed. Further, the data is discussed in the context of seasonal characteristics and trends and compared to available data from the Global Network of Isotopes in Precipitation (GNIP) for the southern African region. Our results extend the portfolio of previously published LMWLs for southern Africa and provide a more precise baseline for any isotope-based study in that region. The slope of the LMWL from the GNIP stations correlates with latitude. This correlation cannot be found within the CEB. The dominant control on the isotopic signature of the CEB of precipitation is seasonal.

Isotopenhydrologische Methoden ( 2 H, 18 O) zur Bestimmung der Grundwasserneubildung in Trockengebieten: Potenzial und Grenzen

ZusammenfassungDie Quantifizierung der Grundwasserneubildung in Trockengebieten stellt aufgrund klimatischer Gegebenheiten, mächtiger ungesättigter Zonen sowie hochangepasster Vegetation eine besondere Herausforderung dar. In dieser Arbeit wurden Methoden, die auf stabilen Bodenwasserisotopen basieren, detailliert untersucht: (i)xa0die Auswertung natürlicher Isotopenprofile, aus denen Grundwasserneubildung über empirische Beziehungen approximiert wird; und (ii) der Einsatz von Deuterium-angereichertem Wasser (2H2O) als Tracer. Im semiariden Namibia wurden zahlreiche Isotopenprofile aus der ungesättigten Zone ausgewertet. Natürliche Tiefenprofile wurden mit einer neuartigen In-situ-Methode direkt im Feld in gemessen. Für Markierversuche wurde 2H2O gezielt in den Boden injiziert und die Verlagerung über eine Regenzeit ausgewertet. Die ermittelten Grundwasserneubildungsraten liegen im Bereich von 0 bis 29u2009mm/y für drei aufeinander folgende Regenzeiten mit 660u2009mm (2013/14), 313u2009mm (2014/15) und 535u2009mm (2015/16) Jahresniederschlag. Die Untersuchungen zeigen, dass isotopenhydrologische Methoden zur Neubildungsabschätzung in Trockengebieten geeignet sind. Die vorgestellten Ansätze haben enormes Potenzial für die Untersuchung von Wasserdampftransportprozessen sowie ökohydrologischer Aspekte.AbstractThe estimation of groundwater recharge in water-limited environments is challenging due to climatic conditions, the occurrence of deep unsaturated zones, and specialized vegetation. We critically examined two methods based on stable isotopes of soil water: (i)xa0the interpretation of natural isotope depth-profiles and subsequent approximation of recharge using empirical relationships and (ii) the use of deuterium-enriched water (2H2O) as tracer. Numerous depth-profiles were measured directly in the field in semiarid Namibia using axa0novel in-situ technique. Additionally, 2H2O was injected into the soil and its displacement over axa0complete rainy season monitored. Estimated recharge ranges between 0 and 29u2009mm/y for three rainy seasons experiencing seasonal rainfall of 660u2009mm (2013/14), 313u2009mm (2014/15) and 535u2009mm (2015/16). The results of this study fortify the suitability of water stable isotope-based approaches for recharge estimation and highlight enormous potential for future studies of water vapor transport and ecohydrological processes.