Study area Hardwald

Urban groundwater management requires a thorough and robust scientific understanding of flow and transport processes. ³H/³He apparent ages have been shown to efficiently help provide important groundwater-related information. However, this type of analysis is expensive as well as labor- and time-intensive, and hence limits the number of potential sampling locations.
To overcome this limitation, we established an inter-relationship between ³H/³He apparent groundwater ages and ⁴He concentrations analyzed in the field with a newly developed portable gas equilibrium membrane inlet mass spectrometer (GE-MIMS) system, and demonstrated that the results of the simpler GE-MIMS system are an accurate and reliable alternative to sophisticated laboratory based analyses. The combined use of ³H/³He lab-based ages and predicted ages from the ³H/³He–⁴He age relationship opens new opportunities for site characterization, and reveals insights into the conceptual understanding of groundwater systems.
For our study site, we combined groundwater ages with hydrochemical data, water isotopes (¹⁸O and ²H), and perchloroethylene (PCE) concentrations (1) to identify spatial inter-aquifer mixing between artificially infiltrated groundwater and water originating from regional flow paths and (2) to explain the spatial differences in PCE contamination within the observed groundwater system. Overall, low PCE concentrations and young ages occur when the fraction of artificially infiltrated water is high. The results obtained from the age distribution analysis are strongly supported by the information gained from the isotopic and hydrochemical data. Moreover, for some wells, fault-induced aquifer connectivity is identified as a preferential flow path for the transport of older groundwater, leading to elevated PCE concentrations.

This study aimed to compare three approaches for predicting the service life of full-scale GAC adsorbers for the removal of micropollutants. The approaches included (i) rapid small-scale column tests (RSSCTs), (ii) two pilot-scale sampling approaches, and (iii) predictive correlations that consider micropollutant properties and background water matrix characteristics. The RSSCT could predict full-scale performance only if a micropollutant-specific fouling index was applied. At the pilot-scale, water samples were collected (1) over time at the top sampling point only (empty bed contact time (EBCT) of 1 minute) to minimize time to breakthrough (method 1) and (2) at different column depths at a single time point (method 2). Breakthrough curves obtained with method 2 more closely matched those obtained at the full-scale. In addition, method 2 is more convenient since it requires only one sampling campaign. Method 2 was used as a prognostic tool to predict breakthrough curves for micropollutants without full-scale data and a comparison with an existing prediction model gave satisfactory results for 6 out of 13 compounds.

A null‐space Monte‐Carlo (NSMC) approach was applied to account for uncertainty in the calibration of the hydraulic conductivity (K) field for a three‐dimensional groundwater flow model of a major water supply system in Switzerland. The approach generates different parameter realizations of the K field using the pilot point methodology. Subsequently, particle tracking (PT) was applied to each calibrated model, and the resulting particles are interpreted as the spatial pathline density distribution of multiple sources. The adopted approach offers advantages over classical PT which does not provide a means for treating uncertainty originating from the incomplete description of subsurface heterogeneity. Uncertainty in the K field is shown to strongly influence the spatial pathline distribution. Pathline spreading is particularly evident in locations where the information content of the head observations does not sufficiently constrain the estimated parameters. Despite the predictive uncertainty, the pumped drinking water at the study site is most likely dominated by artificially‐infiltrated groundwater originating from the local infiltration canals. The model suggests that within the well field, the central pumping wells could be extracting regional groundwater, although the probability is relatively low. Nevertheless, a rigorous uncertainty assessment is still required since only a few realizations resulted in flow paths that support the field observations. Model results should therefore not be based on only one model realization; rather, an uncertainty analysis should be carried out to provide a sufficiently large suite of equally probable simulations that include all potential sources and pathways.

Analyzing groundwater mixing ratios is crucial for many groundwater management tasks such as assessing sources of groundwater recharge and flow paths. However, estimating groundwater mixing ratios is affected by various uncertainties, which are related to analytical and measurement errors of tracers, the selection of end‐members, and finding the most suitable set of tracers. Although these uncertainties are well recognized, it is still not common practice to account for them. We address this issue by using a new set of tracers in combination with a Bayesian modeling approach, which explicitly considers the possibility of unknown end‐members while fully accounting for tracer uncertainties. We apply the Bayesian model we developed to a tracer set, which includes helium (⁴He) analyzed on site to determine mixing ratios in groundwater. Thereby, we identify an unknown end‐member that contributes up to 84 ± 9% to the water mixture observed at our study site. For the ⁴He analysis, we use a newly developed Gas Equilibrium Membrane Inlet Mass Spectrometer (GE‐MIMS), operated in the field. To test the reliability of on‐site ⁴He analysis, we compare results obtained with the GE‐MIMS to the conventional lab‐based method, which is comparatively expensive and labor intensive. Our work demonstrates that (i) tracer‐aided Bayesian mixing modeling can detect unknown water sources, thereby revealing valuable insights into the conceptual understanding of the groundwater system studied, and (ii) on‐site ⁴He analysis with the GE‐MIMS system is an accurate and reliable alternative to the lab‐based analysis.

A three-dimensional groundwater model was used to improve water resource management for a study area in north-west Switzerland, where drinking-water production is close to former landfills and industrial areas. To avoid drinking-water contamination, artificial groundwater recharge with surface water is used to create a hydraulic barrier between the contaminated sites and drinking-water extraction wells. The model was used for simulating existing and proposed water management strategies as a tool to ensure the utmost security for drinking water. A systematic evaluation of the flow direction between existing observation points using a developed three-point estimation method for a large number of scenarios was carried out. It is demonstrated that systematically applying the developed methodology helps to identify vulnerable locations which are sensitive to changing boundary conditions such as those arising from changes to artificial groundwater recharge rates. At these locations, additional investigations and protection are required. The presented integrated approach, using the groundwater flow direction between observation points, can be easily transferred to a variety of hydrological settings to systematically evaluate groundwater modelling scenarios.

Im Wassergewinnungsgebiet Hardwald werden rund 15 Mio. m³/a Trinkwasser produziert. Es finden sich jedoch Spuren von chlorierten organischen Verbindungen im Grundwasser. Als Fallstudie werden hier die Ergebnisse von Feld- und Laborarbeiten zur Bestimmung der räumlichen Verteilung der chlorierten organischen Verbindungen, der stabilen Wasserisotope (δ¹⁸O und δ²D), der Hauptkationen- und -anionen und ausgewählter Spurenstoffe, welche über ein Rheininfiltrat eingetragen werden, vorgestellt. Als Ergebnis der Untersuchungen zeigte sich, dass die künstliche Rheinwasserinfiltration ganz entscheidend zur Trinkwassersicherheit beiträgt und das entnommene Grundwasser vorwiegend der chemischen Signatur des infiltrierten Rheinwassers entspricht. Jedoch zeigt sich auch, dass durch die über die Fläche ungleichmäßig verteilte Infiltration vor allem eine Beimischung von Muschelkalkwasser in süd-westlichen Bereichen des Untersuchungsgebiets wahrscheinlich ist. Diese Interpretation wird durch die Verteilung der chlorierten organischen Verbindungen, Hauptkationen- und -anionen, stabilen Wasserisotopen und Spurenstoffen gestützt. Trotz der hier vorhandenen komplexen Randbedingungen wird durch das Zusammenspiel von künstlicher Infiltration und Entnahme eine sichere Trinkwasserversorgung ermöglicht.

At the Hardwald study site, Switzerland, 15 million cubic metres per year of drinking water is being pumped. Chlorinated compounds, however, have been detected in the groundwater. We present results from field sampling and lab analyses to determine the spatial distribution of chlorinated organic compounds, stable water isotopes (δ¹⁸O und δ²D), major ions as well as selected micropollutants, which enter the groundwater by artificial recharge. We demonstrate that artificial groundwater recharge is essential for water security and that the pumped groundwater has a close chemical signature to that of the recharged river water. However, due to the heterogeneous infiltration, Muschelkalk water from the regional flow system is mixed with the recently infiltrated water in the south-west.This interpretation is based on the spatial distribution of chlorinated organic compounds, stable water isotopes, major ions as well as selected micropollutants. Despite the complex boundary conditions, the interaction between artificial recharge and pumping provides a secure drinking water supply.

Knowledge about the residence times of artificially infiltrated water into an aquifer and the resulting flow paths is essential to developing groundwater-management schemes. To obtain this knowledge, a variety of tracers can be used to study residence times and gain information about subsurface processes. Although a variety of tracers exists, their interpretation can differ considerably due to subsurface heterogeneity, underlying assumptions, and sampling and analysis limitations. The current study systematically assesses information gained from seven different tracers during a pumping experiment at a site where drinking water is extracted from an aquifer close to contaminated areas and where groundwater is artificially recharged by infiltrating surface water.
We demonstrate that the groundwater residence times estimated using dye and heat tracers are comparable when the thermal retardation for the heat tracer is considered. Furthermore, major ions, acesulfame, and stable isotopes (δ²H and δ¹⁸O) show that mixing of infiltrated water and groundwater coming from the regional flow path occurred and a vertical stratification of the flow system exist. Based on the concentration patterns of dissolved gases (He, Ar, Kr, N₂, and O₂) and chlorinated solvents (e.g., tetrachloroethene), three temporal phases are observed in the ratio between infiltrated water and regional groundwater during the pumping experiment. Variability in this ratio is significantly related to changes in the pumping and infiltration rates. During constant pumping rates, more infiltrated water was extracted, which led to a higher dilution of the regional groundwater. An infiltration interruption caused however, the ratio to change and more regional groundwater is extracted, which led to an increase in all concentrations. The obtained results are discussed for each tracer considered and its strengths and limitations are illustrated. Overall, it is demonstrated that aquifer heterogeneity and various subsurface processes necessitate application of multiple tracers to quantify uncertainty when identifying flow processes.

Ozonation is a water treatment process for disinfection and/or micropollutant abatement. However, ozonation of bromide-containing water leads to bromate (BrO₃^–) formation, a potential human carcinogen. A solution for mitigating BrO₃^– formation during abatement of micropollutants is to minimize the ozone (O₃) concentration. This can be achieved by dosing ozone in numerous small portions throughout a reactor in the presence of H₂O₂. Under these conditions, O₃ is rapidly consumed to form hydroxyl radical (^·OH), which will oxidize micropollutants. To achieve this goal, a novel process (“MEMBRO₃X”) was developed in which ozone is transferred to the water through the pores of polytetrafluoroethylene (PTFE) hollow fiber membranes. When compared to the conventional peroxone process (O₃/H₂O₂), the MEMBRO₃X process shows better performance in terms of micropollutant abatement and bromate minimization for groundwater and surface water treatment. For a groundwater containing 180 μg/L bromide, a 95% abatement of the ozone-resistant probe compound p-chlorobenzoic acid yielded <0.5 μg/L BrO₃^–, whereas in the conventional peroxone process, 8 μg/L BrO₃^– was formed. In addition, the efficacy of the MEMBRO₃X process was demonstrated with river water and lake water.

Stable isotopes of water, organic micropollutants and hydrochemistry data are powerful tools for identifying different water types in areas where knowledge of the spatial distribution of different groundwater is critical for water resource management. An important question is how the assessments change if only one or a subset of these tracers is used. In this study, we estimate spatial artificial infiltration along an infiltration system with stage–discharge relationships and classify different water types based on the mentioned hydrochemistry data for a drinking water production area in Switzerland. Managed aquifer recharge via surface water that feeds into the aquifer creates a hydraulic barrier between contaminated groundwater and drinking water wells. We systematically compare the information from the aforementioned tracers and illustrate differences in distribution and mixing ratios. Despite uncertainties in the mixing ratios, we found that the overall spatial distribution of artificial infiltration is very similar for all the tracers. The highest infiltration occurred in the eastern part of the infiltration system, whereas infiltration in the western part was the lowest. More balanced infiltration within the infiltration system could cause the elevated groundwater mound to be distributed more evenly, preventing the natural inflow of contaminated groundwater.

Im Hardwald bei Muttenz wird zur Trinkwasseraufbereitung Rheinwasser versickert. Während der Bodenpassage wird rund die Hälfte der vorhandenen Spurenstoffe im Rheinwasser entfernt. Ein Grossteil der restlichen Substanzen wird durch die Aktivkohlefiltration des angereicherten Grundwassers entfernt. Wie gut die Spurenstoffe im Aktivkohlefilter zurückgehalten werden und ob eine vor- oder nachgeschaltete Oxidation zu einer besseren Entfernung führen kann, wird in der nachfolgend vorgestellten Studie aufgezeigt. Die Studie wurde im Rahmen des Projekts "Regionale Wasserversorgung Basel-Landschaft 21" realisiert.

Das Trinkwassergewinnungsgebiet Hardwald ist geologisch und hydrogeologisch komplex, zudem existieren zahlreiche Randeinflüsse sowie diverse historische Belastungen. Um eine gute Trinkwasserqualität zu gewährleisten, ist es deshalb von grosser Bedeutung, mögliche Mischungsprozesse verschiedener Wässer zu identifizieren. Hierfür wurde im Rahmen des Projekts «Regionale Wasserversorgung Basel-Landschaft 21» die zur Verfügung stehende Datenmenge zur Hydrochemie ausgewertet.

A combined approach of multivariate statistical analysis, namely factor analysis (FA) and hierarchical cluster analysis (HCA), interpretation of geochemical processes, stable water isotope data and organic micropollutants enabling to assess spatial patterns of water types was performed for a study area in Switzerland, where drinking water production is close to different potential input pathways for contamination. To avoid drinking water contamination, artificial groundwater recharge with surface water into an aquifer is used to create a hydraulic barrier between potential intake pathways for contamination and drinking water extraction wells. Inter-aquifer mixing in the subsurface is identified, where a high amount of artificial infiltrated surface water is mixed with a lesser amount of water originating from the regional flow pathway in the vicinity of drinking water extraction wells. The spatial distribution of different water types can be estimated and a conceptual system understanding is developed. Results of the multivariate statistical analysis are comparable with gained information from isotopic data and organic micropollutants analyses. The integrated approach using different kinds of observations can be easily transferred to a variety of hydrological settings to synthesise and evaluate large hydrochemical datasets. The combination of additional data with different information content is conceivable and enabled effective interpretation of hydrological processes. Using the applied approach leads to more sound conceptual system understanding acting as the very basis to develop improved water resources management practices in a sustainable way.