Abteilung Wasserressourcen und Trinkwasser

Marine phytoplankton play a key role in biogeochemical selenium (Se) cycling. By transforming inorganic Se into organic species, many of which remain uncharacterized, they influence Se availability for other organisms and may contribute to Se volatilization. Characterizing organic Se species in phytoplankton is challenging and no standard analytical workflow has been established. Here, we used the marine phytoplankton species Gephyrocapsa huxleyi and Effrenium voratum to systematically assess how cultivation (Se concentrations in the medium), cell harvesting, extraction, and chromatographic parameters influence Se speciation measurements quantitatively (extraction yields, column recoveries) and qualitatively (species distribution), when performed by HPLC-ICP-MS/MS. We show that medium Se concentrations and choice of chromatography had the largest effect on measured Se speciation. The two phytoplankton showed marked differences in intracellular Se speciation, likely reflecting a dissimilar selenometabolism. Resolving these speciation differences was only possible with select chromatographies (e.g. anion-exchange but not mixed-mode). Both phytoplankton also showed changes to intracellular Se speciation when grown in Se-enriched media in comparison with near-natural concentrations. The selection of Se concentrations in the medium is therefore crucial for studying Se metabolites, e.g. under environmentally relevant conditions. We recommend extraction–chromatography workflows tailored to specific experimental needs in phytoplankton Se research, providing a framework for future studies to uncover further effects these organisms have on biogeochemical Se cycling.

Arsenic affected alluvial aquifers in South and Southeast Asia commonly exhibit strongly reducing, methanerich conditions, where in-situ gas production can induce substantial degassing of groundwater. Such conditions complicate groundwater dating calculations using the tritium–helium-3 (³H−³He) ingrowth method, which is widely applied to infer groundwater flow velocities and contaminant transport timescales. This study evaluates groundwater from a methane-rich aquifer in Van Phuc, Vietnam, with the aim of applying the ³H−³He method to date groundwater. Dissolved noble gas concentrations were analysed to account for degassing and excess air components required for effective isolation of the tritiogenic 3He fraction. Groundwater from 24 of the 28 sampled wells shows clear evidence of degassing, with up to 98% depletion of dissolved gases relative to expected atmospheric equilibrium concentrations. After correcting for degassing, ³H−³He residence times range from 35 to 65 years and are highly sensitive to both the magnitude and timing of degassing. In addition, a comparison between He, Ar, and Kr concentrations measured on-site using membrane-inlet mass spectrometry (GE–MIMS) and those obtained from conventional copper tube samples revealed substantial discrepancies in He concentrations—of up to 7 × 10⁻⁸ cm³_STP g⁻¹ —whereas comparable differences for Ar and Kr were considerably smaller. Discrepancies were observed in wells with elevated methane concentrations and total dissolved gas pressures exceeding 1 bar. These differences are interpreted as resulting from sampling-induced degassing during copper tube groundwater collection, superimposed on in-situ subsurface degassing, further complicating groundwater age determination. Finally, degassing is shown to be more widespread than originally assumed, contrary to previous assumptions, necessitating revision of earlier groundwater age estimates. The revised understanding of degassing processes and groundwater age distributions is applicable to similar aquifers where in-situ gas production may influence contaminant transport.

Groundwater is a central component of the Earth system. However, our understanding of how it is dynamically interlinked with the atmosphere, hydrosphere, cryosphere, biosphere, geosphere, and anthroposphere remains limited. In the pursuit of understanding groundwater dynamics across diverse global settings, we present GROW (the global-scale integrated GROundWater package). This analysis-ready, quality-controlled dataset combines depth to groundwater and level time series from 55 countries, 91% from North America, India, Europe, and Australia, with associated Earth system variables. The dataset contains >200,000 time series with either daily, monthly, or yearly temporal resolution, accompanied by 36 time series or static attributes of meteorological, hydrological, geophysical, vegetation, and anthropogenic variables (e.g., precipitation, drainage density, rock type, NDVI, land use). 34 data flags regarding well features (e.g., coordinates and country), as well as time series characteristics (e.g., gap fraction or autocorrelation), facilitate quick data filtering. GROW provides a foundation for understanding large-scale groundwater processes in space and time, as well as for calibrating and evaluating models that simulate groundwater dynamics within the Earth system.

The micronutrient selenium (Se) exhibits a narrow range between essentiality and toxicity. In soils, Se speciation influences its mobility and plant availability, with implications for addressing unsafe Se levels in plant-based nutrition. We investigated how Se speciation varies with the molecular composition of organic matter (OM) in 92 Swiss topsoils spanning different land uses (i.e., croplands, grasslands, and forests). OM composition was characterized using pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS), while Se speciation was determined in NaOH extracts using size exclusion chromatography coupled to UV and elemental mass spectrometry (SEC-UV-ICP-MS/MS). We found that Se speciation strongly relates to OM decomposition status and pH, and drastically differs between soil land uses. Cropland soils exhibited higher proportions of Se oxyanions and small hydrophilic organic Se, whereas forest and grassland soils contained more larger, aromatic organic Se compounds. Overall, these larger Se forms correlated with fresh and/or poorly decomposed, plant-derived OM, while oxyanions and small hydrophilic Se were linked to decomposed OM. Additionally, Se extractability by NaOH decreased with increasing soil pH, which may be due to stronger SOM stabilization, microbial processes, or higher Ca contents at higher pH. These results have important implications for Se plant availability considering land use changes and SOM degradation.