Abteilung Wasserressourcen und Trinkwasser

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.

Groundwater, enhanced through managed aquifer recharge (MAR), plays a central role in mitigating current and future water stress. Here we evaluate anthropogenic and natural water isotopes as tracers of groundwater flow dynamics within alluvial MAR systems. High-resolution sampling (daily/weekly) of stable isotopes (δ¹⁸O and δ²H) and tritium (³H), influenced by nuclear power plant effluents, is used to trace and quantify the movement of infiltrated river water through an alluvial aquifer along the Rhine River in Switzerland. Time-series deconvolution is applied to quantify the tracer-based travel time distribution and to predict travel times throughout the entire MAR scheme. The results demonstrate the suitability of ³H as a quasi-conservative travel time tracer in systems where the infiltrating river water is marked by nuclear power plant discharges—a situation prevalent along the banks of many large river basins globally. Deuterium excess proved equally effective as a bulk travel time tracer, reflecting distinct seasonal meltwater signals expected in major European rivers. These findings quantify MAR recovery rates and wellhead protection zones, supporting sustainable groundwater management under natural and anthropogenic pressures.

Cyanobacteria produce complex mixtures of secondary metabolites (cyano-metabolites), some of which are toxic and pose a growing concern for water utilities. While physical treatments such as filtration can efficiently remove cells, their lysis can release dissolved cyano-metabolites. This study investigated the efficiency of laboratory-scale sand filtration to abate 19 cyano-metabolites representing various structural classes. Furthermore, abatement of cyano-metabolites in a full-scale sand filtration is presented. Most cyano-metabolites showed abatement similar to or higher than the biodegradable benchmark micropollutants atenolol, paracetamol, and valsartan. Among cyano-metabolites, anabaenopeptins and cyanopeptolins had the highest abatement, while cyclamides and microcystin-LR had the lowest abatement. Abiotic controls and formation trends of 10 identified biotransformation products demonstrated that biodegradation played a major role in their removal. Laboratory-scale sand filters showed a sharp increase in biodegradation efficiency within days due to their adaptation to cyano-metabolites. Increasing the contact time and temperature both enhanced the abatement of most compounds, which could be kinetically modeled. High cyano-metabolite concentrations suppressed their own relative abatement, possibly due to metabolic enzyme inhibition or saturation. These findings suggest that sand filtration can serve as a dual-barrier against cyano-metabolites, including particle removal and biodegradation. However, biodegradation will be affected by the temperature and cyano-metabolite intake dynamics.

5-membered aromatic heterocycles are abundant in natural organic compounds, pesticides and pharmaceuticals. Systematic kinetic data of their aqueous reactions with ozone is scarce, hindering predictions on their fate during ozonation processes. Therefore, second-order rate constants for reactions of ozone (k_O3) with 13 unsubstituted/benzosubstituted aromatic heterocycles were determined and compared to reported values to identify reactivity trends.
The k_O3 of the simplest one-heteroatom compounds, pyrrole, furan, and thiophene decrease in the order N>O>S; pyrrole and furan react rapidly (>10⁶ M⁻¹s⁻¹), whereas thiophene is 35-fold less reactive. In aromatic heterocycles, sulfur seems completely deactivated. For two-heteroatom compounds, both nitrogen in the 2- and 3-position are deactivating, with 1,3-isomers being significantly more reactive (k_O3 = 1.5×10²−2.3×10⁵ M⁻¹s⁻¹) than 1,2-isomers (k_O3 =0.5−56 M⁻¹s⁻¹), maintaining the trend N>O>S. More ring nitrogens further decrease k_O3: 1,2,3-triazole is somewhat reactive (k_O3 = 18 M⁻¹s⁻¹), while 1,2,4-triazole and tetrazole show no measurable reaction, likely due to the absence of an attackable C=C double bond. Benzo-substitution increases k_O3 of one-heteroatom compounds (indole (k_O3 =1.8×10⁶ M⁻¹s⁻¹), benzofuran (k_O3 = 7.2×10⁵ M⁻¹s⁻¹), benzothiophene (k_O3 = 1.8×10⁵ M⁻¹s⁻¹)). In contrast, 1,3-benzazoles (benzimidazole, benzoxazole, benzothiazole) react much slower with ozone (k_O3 = 2.3−90 M⁻¹s⁻¹), consistent with the absence of a C=C double bond.
Quantum-chemical descriptors were evaluated for correlation with k_O3 of 26 aromatic heterocycles. E_HOMO and E_NBO overall correlated weakly with k_O3. Excluding benzosubstituted 1,3-azoles and benzotriazole markedly improved the E_HOMO-k_O3 correlation (R²=0.65, n=22), capturing k_O3 trends across diverse subsets of heterocycles. For substituted 1,3-oxazoles and substituted 1,3-thiazoles, E_HOMO correlated strongly with k_O3 (R²=0.95 and 1.00, n=5, respectively). E_NBO showed weak correlations even within subsets, suggesting that aromaticity of these compounds dominates their ozone reactivity.
Overall, the data presented provides novel insights into the ozone-reactivity of 5-membered aromatic heterocycles to better assess their fate during ozonation.