[en] The Swiss National Cooperative for the Disposal of Radioactive Waste (Nagra) carried out an underground investigation program for a comparative evaluation of possible sites for a deep geological repository (DGR) for nuclear waste. Between 2019 and 2022, one inclined and eight vertical multi-purpose exploratory boreholes were drilled in three siting regions in northern Switzerland. In the frame of these works, accurate groundwater sampling and analytical data interpretation has been fundamental to derive solid information on groundwater composition, evolution, and residence times in aquifers limiting the low permeability sequence of the host rock (Opalinus Clay) and the confining units. Eighteen deep groundwater samples were successfully collected from the Malm, Keuper and Muschelkalk aquifers at depths between approx. 350 m and 1150 m below ground level. The following selected highlights and lessons learned regarding hydrochemical groundwater characterisation will be presented: Procedures to correct analytical results for the contamination of groundwater with different types of drilling fluids, using artificial and environmental tracer data and geochemical modelling approaches; Krypton-81 (T1/2 229 ka) in the groundwaters was successfully analysed and, combined with hydrochemical and isotopic data, helped to constrain groundwater mixtures and model residence times far beyond the classical carbon-14 method; Tools to cope with technical challenges at a drill site resp. in deep exploratory boreholes (i.e. time constraints, low groundwater flow rates). We demonstrate that a high level of quality for hydrochemical and isotope data of groundwaters can be reached even under challenging operational conditions. The hydrochemical data of groundwater and the profiles of natural tracers in pore water are relevant datasets to constrain the site model used in the site selection process and in the long-term performance assessment for a DGR.

[en] Full text: Chlorinated hydrocarbons are one of the most common pollutants found in groundwater. Due to complex contamination situations with overlapping contamination plumes the assessment of the organic contaminants requires the installation of expensive observation wells and high analytical effort. Here the determination of the stable isotope ratio ¹³C/¹²C of the organic compounds offers a promising and efficient tool to investigate the origin and the biodegradation characteristics of the chlorinated hydrocarbons in groundwater. The application of the method is based on characteristic isotope fingerprints, differing in chlorinated solvents. This isotope fingerprint is derived from different production pathways and is not influenced by transport or by retardation processes in the underground. Due to the fact, that two different contaminations can easily be distinguished by isotope ratios, an improved distinction of spatially and temporally different contamination plumes might be possible. In course of biologically mediated degradation processes a shift of the isotope ratios between the precursor and the product can frequently be observed, such as with denitrification or sulfate reduction processes. The isotope fractionation is due to a preferential reaction of the bonds formed by the lighter isotopes and leads to a progressive enrichment of the heavy isotopes in the precursor while the product becomes depleted in the heavy isotopes. Biological degradation of the highly chlorinated hydrocarbons is due to a co-metabolic dechlorinisation. Tetrachloroethene (PCE) for example degrades under anoxic conditions via trichloroethene (TCE) to cis-1,2-dichloroethene (cDCE). Subsequent degradation to vinyl chloride (VC) and ethene may appear under aerobic as well as reducing environments depending on the site specific conditions. In several laboratory studies it has been shown, that biodegradation of the chlorinated hydrocarbons is accompanied by an isotope fractionation of the stable carbon isotopes. The isotope ratios may be used to assess and quantify the degradation of the organic compounds at the field sites. This application has gained great interest for remediation strategies including monitored natural attenuation of contaminations. In contrast to the laboratory studies, many of the field investigation show no evidence for isotope fractionation although biodegradation of the chlorinated hydrocarbons in the groundwater is significant. Here, we present the results of 21 field studies, where compound specific ¹³C isotope ratios have been applied. Only in some cases isotope fractionation processes of chlorinated hydrocarbons due to biodegradation have been observed. The measured δ¹³C values agree reasonably with a Rayleigh type isotope fractionation model, where the fractionation factors are used as fitting parameters. The occurrence and the degree of significant isotope fractionation of chlorinated hydrocarbons is still an open question. Major factors that control the extend of measurable ¹³C isotope fractionation of chlorinated hydrocarbons in groundwater most likely include parameters as activity and type of the microbiological species, availability of cosubstrates as well as hydrochemical and hydrogeological conditions. (author)

[en] This study demonstrates the use of isotope tracers as a tool for monitoring groundwater resources, their depletion and recovery due to a phase of over-exploitation. It is based on spatial hydro-geological data of a Quaternary basin, a three dimensional numerical groundwater flow model and detailed hydraulic, hydro-chemical and isotope records over a period of more than 30 years at selected major production wells. Hydro-chemical data and tritium values have been used to comprehend the complex flow system throughout this period of exploitation. Tritium values in groundwater samples are influenced by the temporal tritium input function in precipitation, radioactive decay or residence time as well as by the mixing portion of various groundwater components due to exploitation. The mixing portions of groundwater components were dramatically changed by groundwater abstraction, especially during over-exploitation in the late 1960s and early 1970s. The long-term tritium record (1969-2001) demonstrates this change for the central part of the aquifer system where main discharge took and takes place. Various snapshots of calculated mean residence times prove increased flow dynamics in the system during the period of over-exploitation and the recovery of the system due to a more sustainable groundwater management. The combination of various investigation methods applied here resulted in a profound understanding of the flow and transport conditions within the studied groundwater reservoir under transient flow conditions over a time scale of more than three decades. The experience of a shallow, fast responding aquifer system, is an encouragement for groundwater managers and experts dealing with (strongly) exploited aquifer systems to use isotope tracer techniques for monitoring purposes and to find adjustment of exploitation to sustainability. (author)

[en] A two in one well in southwest Germany, separately tapping aquifers in the formations of Muschelkalk and Keuper, was tested in a one year pumping test. The waters were continuously analysed for chemical and isotopic composition (major ions, ³H, ¹⁸O, ²H, ¹³C, ¹⁴C, ⁸⁵Kr) and trace gases (CFC, SF₆). The analytical results of ³H and ⁸⁵Kr showed a shift in the composition of 25% young, ³H-bearing water to a proportion of 50% young water after half a year of pumping. The residence time of the young water of about 10 to 20 years remained the same. The shift is also visible in the increasing contents of nitrate and chloride. Though the analytical results of SF₆ showed the same shift, SF₆ - most probably influenced by crystalline gravel - indicates a residence time of the young water of less than one year. The CFCs, on the other hand, point to lower proportions of young water as they are influenced by degradation processes and/or adsorption. Though both aquifers are effectively separated from each other, the same shifting of age structure can be observed. (author)

[en] This study demonstrates the use of isotope tracers as a tool for monitoring groundwater resources, their depletion and recovery due to a phase of over-exploitation. It is based on spatial hydro-geological data of a Quaternary basin, a three dimensional numerical groundwater flow model and detailed hydraulic, hydrochemical and isotope records over a period of more than 30 years at selected major production wells. Hydro-chemical data and tritium values have been used to comprehend the complex flow system throughout this period of exploitation. Tritium values in groundwater samples are influenced by the temporal tritium input function in precipitation, radioactive decay or residence time as well as by the mixing portion of various groundwater components due to exploitation. The mixing portions of groundwater components were dramatically changed by groundwater abstraction, especially during over-exploitation in the late 1960s and early 1970s. The long-term tritium record (1969-2001) demonstrates this change for the central part of the aquifer system where main discharge took and takes place. Various snapshots of calculated mean residence times prove increased flow dynamics in the system during the period of over-exploitation and the recovery of the system due to a more sustainable groundwater management. The combination of various investigation methods applied here resulted in a profound understanding of the flow and transport conditions within the studied groundwater reservoir under transient flow conditions over a time scale of more than three decades. The experience of a shallow, fast responding aquifer system is an encouragement for groundwater managers and experts dealing with (strongly) exploited aquifer systems to use isotope tracer techniques for monitoring purposes and to find adjustment of exploitation to sustainability. (author)

[en] This study demonstrates the use of isotope tracers as a tool for monitoring groundwater resources, their depletion and recovery due to a phase of over-exploitation. It is based on spatial hydro-geological data of a Quaternary basin, a three dimensional numerical groundwater flow model and detailed hydraulic, hydrochemical and isotope records over a period of more than 30 years at selected major production wells. Hydrochemical data and tritium values have been used to comprehend the complex flow system throughout this period of exploitation. Tritium values in groundwater samples are influenced by the temporal tritium input function in precipitation, radioactive decay or residence time as well as by the mixing portion of various groundwater components due to exploitation. The mixing portions of groundwater components were dramatically changed by groundwater abstraction, especially during over-exploitation in the late 1960s and early 1970s. The long term tritium record (1969-2001) demonstrates this change for the central part of the aquifer system where main discharge took and takes place. Various snapshots of calculated mean residence times prove increased flow dynamics in the system during the period of over-exploitation and the recovery of the system due to a more sustainable groundwater management. Groundwater hydro-chemistry shows admixtures of NaCl-rich groundwater, that is derived from a waste disposal site. The significant increase in NaCl-concentrations during the 1970s can be attributed to the strongly reduced dilution processes with fresh groundwater from the already over-exploited system. Due to the reduction of groundwater withdrawal, freshwater dilution has improved and the groundwater dynamics have been reduced. The pollutant plume has affected all wells along the flow path into the central cone of depression and is still a problem to groundwater quality. The combination of various investigation methods applied here resulted in a profound understanding of the flow and transport conditions within the studied groundwater reservoir under transient flow conditions over a time scale of more than three decades. The experience of a shallow, fast responding aquifer system, is an encouragement for groundwater managers and experts dealing with (strongly) exploited aquifer systems to use isotope tracer techniques for monitoring purposes and to find adjustment of exploitation to sustainability. (author)

No abstract available

[en] The objectives of our study were to identify the anaerobic/aerobic dechlorination mechanisms at two chloroethene-contaminated sites, to assess the isotopic enrichment factors in groundwater microcosms, and to evaluate microbial degradation in the field based on the isotopic signatures. In anaerobic microcosms, a transformation of PCE and TCE to cisDCE was observed. Halo respiration was accompanied by isotopic fractionation effects similar to previous publications. In aerobic microcosms, cisDCE and VC were degraded indicating a co-metabolic degradation of cisDCE in the course of VC degradation. In other experiments, only VC was degraded. Enrichment factors were determined during aerobic degradation of cisDCE and VC. These factors are significantly higher than factors published previously. This study demonstrates that isotope fractionation is suitable for assessing anaerobic/aerobic chloroethene degradation. However, the factors vary for different degradation pathways and micro organisms. Therefore, site-specific enrichment factors have to be determined in order to quantify natural attenuation processes in the field. (author)

[en] The groundwater flow regime in the jurassic karst and tertiary terrain near the Danube-river in the area of Ingolstadt/South Germany has been well discussed and investigated for years. However, a stringent explanation of the complex deep groundwater system at the meeting-point of young, karstic groundwater from the north (open karst) and old deep groundwater in the south (covered karst) is still lacking. Today, because of the increasing water use for drinking water supply in the high industrialized area of Ingolstadt, reliable hydrogeological answers and a future sustainable groundwater management system are needed. First symptoms of overexploitation are visible by hydrochemical and isotopic measurements. Coming from the actual state of hydrogeological knowledge, the use of isotope techniques provide distinct explanation for the complex genesis of the occurring groundwaters

[en] In the frame of an EU Interregional IIIa project we investigated to what extent the interregional Upper Jurassic karst aquifer, which underlies parts of southern Germany and the area around Schaffhausen in Switzerland, contributes to the water budget of shallow Quaternary basins on both sides of the border. Because the differentiation between mixing end members based on chemical parameters is ambiguous, isotope tracers were emphasized. Proportion and spatial occurrence of deep karst water were determined based on ³H, ⁸⁵Kr, ³⁹Ar and ¹⁴C.The data were interpreted based on a 3D-hydrogeological setup which was completely re-evaluated using reprocessed seismic profiles. The reviewed scientific results provide the basis for sustainable groundwater protection and resource management overcoming national borders as the groundwater does. (author)