HS8.2.4

Groundwater age and mean residence times have been invoked as measures of groundwater sustainability, with the idea that old or "fossil" groundwater is non-renewable. This idea appears to come from the link between groundwater age and background recharge rates, which are also of questionable use in assessing the sustainability of groundwater withdrawals. The use of groundwater age to assess renewability is further complicated by its relationship with flow system geometry. Young groundwaters near recharge areas are not inherently more renewable than older groundwaters down gradient. Similarly, there is no reason to preferentially use groundwater from smaller aquifers, which will have smaller mean residence times than larger aquifers for the same recharge rate. In some cases, groundwater ages may provide some information where groundwater recharge rates were much higher in the past and systems are no longer being recharged. However, there are few examples where the relationship between depletion and changes in recharge over long time periods has been rigorously explored. Groundwater age measurements can provide insights into the functioning of groundwater flow systems and calibration targets for numerical models and we advocate for their continued use, but they are not a metric of sustainable development. Simple metrics to assess groundwater sustainability remain elusive and a more holistic approach is warranted to maintain water levels and environmental flows.

How to cite: Ferguson, G., Cuthbert, M., Befus, K., Gleeson, T., Noyes, C., and McIntosh, J.: Renewed thinking on groundwater age, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3479, https://doi.org/10.5194/egusphere-egu21-3479, 2021.

The Krka River in Croatia is a specific groundwater-fed karstic river, characterized by complex hydrology and seasonally variable diffuse subsurface recharge. It represents a unique model system, where tufa is precipitating in a turbulent stream at morphologic discontinuities and in lentic environments. Tufa is especially attracting attention as a potential environmental archive that can provide insight into water-rock interactions, hydraulic connections, recharge, and terrestrial CO₂ cycling in terms of storage, evasion, and transfer to the ocean. In a dynamic karst river system with alternating lentic and turbulent lotic sections, the carbonate precipitation rarely occurs in isotopic equilibrium for either C or O isotopes. Therefore, the use of traditional isotopes in river water (d¹⁸O, d²H, d¹³C_DIC), tufa, surrounding bedrock, soil (d¹³C_CaCO3, d¹⁸O_CaCO3, d¹³C_POC) and geochemical parameters (Ca, Mg, Na, K, HCO₃^-) in river/carbonate system in combination with uranium (U) and thorium (Th) isotopic composition could increase the understanding of this complex karst hydrodynamic system and help with the identification and quantification of authigenic carbonate precipitated in the river.

River water samples, tufa, and surrounding bedrock and soil samples were collected at 11 locations, which were selected based on the spatial distribution of bedrock types and occurrence of tufa. Measurements of U and Th isotope ratios were carried out with multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS) and for assessing U and Th concentrations, triple Quadrupole ICP-MS was used.

The study brought a new perspective to already known data on this highly sensitive karst eco-system. U concentration and the activity ratios of ²³⁴U/²³⁸U in the river show a decrease with the distance from the spring. U isotopic differences reflect the changing bedrock lithology and the mixing of waters from different sources. Therefore, U values show promise as a tracer for studying changes in host rock composition and hydraulic connections in the karst aquifer.

Tufa samples of the studied system demonstrate a much higher activity ratio of ²³⁴U/²³⁸U compared to the bedrock and soil. The ²³⁴U/²³⁸U ratio of carbonate in tufa is almost identical to that of the dissolved U in the river water, indicating that a majority of U present in tufa samples is co-precipitated with the carbonate from the river water. This assumption was confirmed with a much lower ²³⁴U/²³⁸U ratio of the non-carbonate fraction of tufa, which is comparable to that of the soil and bedrock, and the d¹⁸O and d¹³C values of carbonate in tufa, which confirmed its authigenic origin. 

The Th and U concentrations and their isotope ratios in carbonate materials from our study were shown to be reliable indicators of the storage of CO₂ as authigenic carbonate in tufa. Moreover, they were also useful for the determination of tufa with U bond to detrital material and consequently relevant for both the construction of the CO₂ mass balance in a karst aquifer, as well as for dating.

How to cite: Rovan, L., Lojen, S., Zuliani, T., Kanduč, T., Vreča, P., Horvat, B., and Štrok, M.: Novel U and Th isotopic tracers for characterization of karstic freshwater and recent tufa from the Krka River (Croatia), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-455, https://doi.org/10.5194/egusphere-egu21-455, 2021.

In this study, we developed two-dimensional hydrodynamic age expressions for the free-surface aquifer problem required for radiocarbon data interpretation in terms of recharge. Time-varying recharge is accounted for in fully transient or pseudo-steady state alternative analytical expressions applying to weakly or highly hydraulically reactive unconfined aquifers, respectively. These expressions extending a previous model based on a constant recharge represent a convenient alternative approach to the more complex numerical resolution of a particle tracking problem considering climate variability. An application of the developed expressions was carried out to assess past (40ky) and present recharge rates for the North Western Saharan Aquifers System (NWSAS) and the Djeffara plain (Tunisia-Libya). Using the available ¹⁴C data at different outcrops, we obtained local recharge values ranging from 1.27 to 6.86 mm yr^-1. A linear relation between the present recharge and the average annual rainfall was obtained and used to identify the regional distribution of the recharge and domain-averaged values. Global values of 1.32 ± 1.47 mm yr^-1 for the NWSAS and 3.97 ± 1.46 mm yr^-1 for the Djeffara are in excellent agreement with the values obtained by hydrogeological models and derived using Satellite gravity data. The relation between recharge and rainfall obtained here can be used as a first-order estimate for hydrogeological modeling or water management analysis for aquifers in the Saharan area.

How to cite: Chekireb, A., Goncalves, J., Hamelin, B., Deschamps, P., and Seraphin, P.: Analytical expressions for past and present recharge interpretation of radiocarbon data in unconfined aquifers: application to Saharan aquifers, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1130, https://doi.org/10.5194/egusphere-egu21-1130, 2021.

During rainfall, water infiltrates the soil, and percolates through the unsaturated zone until it reaches the water table. Groundwater then flows through the aquifer, and eventually emerges into streams to feed surface runoff. We reproduce this process in a  two-dimensional laboratory aquifer recharged by artificial rainfall. As rainwater infiltrates, it forms a body of groundwater which can exit the aquifer only through one of its sides. The outlet is located high above the base of the aquifer, and drives the flow upwards. The resulting vertical flow component violates the Dupuit-Boussinesq approximation. In this configuration, the velocity potential that drives the flow obeys the Laplace equation, the solution of which crucially depends on the boundary conditions. Noting that the water table barely deviates from the horizontal, we linearize the boundary condition at the free surface, and solve the flow equations in steady state. We derive an expression for the velocity potential, which accounts for the shape of the experimental streamlines and for the propagation rate of tracers through the aquifer. This theory allows us to calculate the travel times of tracers through the experimental aquifer, which are in agreement with the observations. The travel time distribution has an exponential tail, with a characteristic time that depends on the aspect ratio of the aquifer. This distribution depends essentially on the geometry of the groundwater flow, and is weakly sensitive to the hydrodynamic dispersion that occurs at the pore scale.

How to cite: Lajeunesse, E., Jules, V., Devauchelle, O., Guérin, A., Jaupart, C., and Lagrée, P.-Y.: Flow and residence time in a laboratory aquifer recharged by rainfall, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1220, https://doi.org/10.5194/egusphere-egu21-1220, 2021.

   ³⁶Cl and ⁸¹Kr (half-lives of 301 and 229 kyr, respectively) are among a very few age tracers with dating capabilities in the 10⁴–10⁶ yr timescale. Although widely applied since the 1980s in various hydrological studies, the ³⁶Cl/Cl system has been found complex as an effective dating tool. In contrast, ⁸¹Kr has become a practical tool only recently and is considered to be an ideal dating tool due to the inert properties of the noble gas. In the present study, simultaneous measurements of both radioisotopes were used to assess the ³⁶Cl/Cl input ratios and the Cl^- content for paleorecharge into the deep, transboundary Nubian Sandstone Aquifer (NSA) which stretches below the hyperarid deserts of the Sinai Peninsula (Egypt) and the Negev (Israel).

   By means of ⁸¹Kr data, reconstructed Cl^- content of recharge that occurred during the late Pleistocene was found to be 300–400 mg/L with an initial ³⁶Cl/Cl ratio of 50 × 10^-15. This latter value is in agreement with the ³⁶Cl/Cl ratio in recent local rainwater, indicating constancy over prolonged periods with possible variable climatic conditions. This similarity in values suggests a process that is rather insensitive to atmospheric ³⁶Cl fallout rates. Erosion and weathering of near-surface materials in the desert environment could dominate the hydrochemistry of rains, floods, and the consequent groundwater recharge. This near-surface Cl^- reservoir integrates various sources and processes, including marine and terrestrial Cl^-, cosmogenic ³⁶Cl fallout, and cosmogenic ³⁶Cl production in the shallow unsaturated zone, all of which are active over long timescales and accumulate on the land surface and in the epigene zone.  Spatial differences in the reconstructed initial ³⁶Cl/Cl ratio are attributed to differences in the mineral aerosol sources for specific recharge areas of the NSA. The results of this study highlight the potential of integrating ⁸¹Kr age information in evaluating the initial ³⁶Cl/Cl and Cl^- input, which is essential for the calibration of ³⁶Cl radioisotope as a long-term dating tool for a given basin.

How to cite: Ram, R., Purtschert, R., Vockenhuber, C., Yokochi, R., Adar, E. M., Zappala, J. C., Yechieli, Y., Lu, Z.-T., Mueller, P., Bishof, M., Jiang, W., Sy, A., and Burg, A.: Exploring the 36Cl/Cl input in arid environments: New insights gained by 81Kr groundwater dating in the Negev Desert, Israel, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4143, https://doi.org/10.5194/egusphere-egu21-4143, 2021.

While climate change will challenge the future of California’s water resources, groundwater can buffer variability in precipitation and streamflow, if managed sustainably. Enhanced river recharge is an important tool to reach sustainable groundwater management in the California Central Valley (USA). Understanding and predicting recharge rates of river water, either natural river bank infiltration or managed aquifer recharge (MAR) during floods (Flood-MAR) or on agricultural land (Ag-MAR) is essential to evaluate the sustainability of groundwater management plans. Groundwater ages, combined with other isotopic and noble gas evidence, can elucidate surface water-groundwater interactions and support river recharge rates calculations over longer time periods.

Our study is focused on the recharge from the Cosumnes River in the California Central Valley. The Cosumnes River forms the boundary between the Sacramento Valley groundwater basin to the north and the San Joaquin Valley groundwater basin to the south. For this study, 28 new samples were collected for the analysis of 3H/3He age, noble gases, and stable isotopes. 25 additional samples from the California Waterboards Groundwater Ambient Monitoring and Assessment (GAMA) Shallow Aquifer Assessment program were included, which were collected and analyzed by the USGS California Water Science Center in 2017.

We find that 28% of groundwater in the San Joaquin – Cosumnes groundwater subbasin originated as river water recharge, based on the interpolated mean δ¹⁸O (7.7 ‰ ), compared with river water (-9 ‰) and local precipitation recharge (-7 ‰) end-members. River water is a source of modern recharge, resulting in high tritium concentrations close to the Cosumnes River. In contrast, ambient groundwater from local precipitation recharge is predominantly pre-modern or fossil, containing less than 1 pCi/L tritium. Combining groundwater ages with the distance to the river, aquifer thickness, and porosity, estimates of river water recharge rate vary between 0.02 km³/yr and 0.035 km³/yr. These quantitative estimates of river water recharge will constrain the numerical groundwater flow model for this basin and aid groundwater managers in developing sustainability plans to balance groundwater pumping with recharge rates.

How to cite: Visser, A., Foglia, L., Dahlke, H., Vankeuren, A., Kniffin, M., Rodriguez, A., Calderwood, A., Fram, M., Deinhart, A., Oerter, E., and Dozier, J.: Mapping river recharge rates with stable isotopes and tritium-helium groundwater ages, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6887, https://doi.org/10.5194/egusphere-egu21-6887, 2021.

Access to fresh water is a critical humanitarian issue in many regions of the world and on the most important sustainable development goals. This crisis is exacerbated by the effects of climate change, pollutants, increases in demand and overuse. Fractured rock aquifers have been providing potable groundwater for many regions of the world, but there still many unknowns about the storage capacity, transit times and flow paths under changing climate scenarios. In this on-going study we aim to understand the magnitude of groundwater storage and Inter-basin flow for water supply development and sustainable use on the island of Tobago, WI. Samples of springs, surface water and production wells were analyzed for the stable isotopes of water (H₂O) and strontium (Sr), tritium(³H) and sulfur hexafluoride (SF₆). The stable isotopes of water δD_vsmow and δ¹⁸O_vsmow indicates that the groundwater in the northern region of the island is not hydrologically connected to the groundwater to the south because of their distinct D-excess signature.

⁸⁷Sr/⁸⁶Sr and Na-normalized strontium concentrations produces five possible mixing lines which radially increase from the lowest based value of ⁸⁷Sr/⁸⁶Sr ~ 0.70396 and Na/Sr (mg/L) ~0.00652. In detail, the maximum values of each line represent: extremes in Na concentration ⁸⁷Sr/⁸⁶Sr 0.70576 and Sr/Na 0.0008 mg/L, the groundwater to seawater mixing line⁸⁷Sr/⁸⁶Sr 0.70506 and Sr/Na 0.0023 mg/L, the precipitation to rock equilibration mixing line ⁸⁷Sr/⁸⁶Sr 0.70506 and Sr/Na 0.0023 mg/L, water located in silicate rocks to carbonate rocks mixing ⁸⁷Sr/⁸⁶Sr 0.70871 and Sr/Na 0.0085 mg/L, and wells that were once affected by seawater intrusion ⁸⁷Sr/⁸⁶Sr 0.70563 and Sr/Na 0.0692 mg/L.

Tritium results range from 0.02 to 0.60 TU and calculations suggest that wells contain a range of 2 to 44% modern groundwater. When compared to other islands at similar latitudinal locations, Tobago’s groundwater presents the lowest mean and median tritium values even though it is the closest to the equator. Basin flux and effective porosity were calculated for the 10 wells using the apparent age obtained from these results under the assumption of piston flow. It was found that all watershed volumes were magnitudes of orders larger than the sub-basins where the wells were located ranging between 0.09 km³ to 8.23 km³. Basin Flux and effective porosity also contain large range differences 1.40 *10⁵ m³/yr to 9.93 *10⁶ m³/yr, and 0.014 to 0.094, respectively.

This results also suggest that the groundwater in the southern regions of the island contains the oldest water with one well sample >60 years. SF₆ results reflect similar ages except for 3 wells samples which are suspected to be contaminated by excess air. This novel discovery illustrates that small, fractured rock island aquifers can possess structural complexities that lead to older groundwater ages and variances in basin characteristics.

How to cite: Allen, M. and Boutt, D.: Assessment of flow paths and groundwater storage processes in an island fractured rock aquifer system using the stable isotopes of water (H2O) and strontium (Sr), and environmental tracers’ tritium (3H) and sulfur hexafluoride (SF6)., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12262, https://doi.org/10.5194/egusphere-egu21-12262, 2021.

Recharge environmental conditions and residence time, can be studied by the application of different tracers. Several tracers are useful as proxies of the environmental recharge conditions, such as water stable isotopes deuterium and oxygen-18, and the dissolved noble gases. Other tracers are applied in order to know when the recharge occurred. Carbon-14 dating is a widely applied method for dating old groundwater, having an application range up to around 30 ky. Noble gases, as non-reactive and water-soluble substances, constitute useful tracers for studying different processes in hydrologic cycles. One of the applications is dating very old groundwater beyond the range of ¹⁴C. It can be done in a semi-quantitative way by the accumulation of ⁴He, and quantitatively through the radionuclide ⁸¹Kr (t_1/2 = 229,000 y), a more robust method for dating groundwater up to 1.3 million years.

The province of Buenos Aires, Argentina, hosts three deep sedimentary basins, from north to south, Salado, Claromecó and Colorado, with areas of 85,000 km², 3,100 km² and 125,000 km², respectively. In these basins, a thick continental sequence of Neogene sediments contains confined thermal aquifers, at depths from hundreds meters to more than 1 km. The recharge conditions and the water age of the Neogene aquifers are studied through water stable isotopes, ⁴He and ⁸¹Kr tracers.  10 deep wells were sampled for δ²H, δ¹⁸O, ³H, ¹⁴C, for noble gases using clamped copper tubes, and for ⁸¹Kr with a gas extractor. ⁴He analyses were performed at the IAEA laboratory by mass spectrometry, and ⁸¹Kr at the ATTA laboratory of USTC.

³H contents were not detectable in all of the cases, thus no young water components exist. By plotting the isotopic results in a δ²H vs δ¹⁸O diagram, four groups of samples can be recognized. Group 1 includes Colorado basin isotopically depleted samples (δ¹⁸O from -6.5 to -7.5 ‰) along a line parallel to the GMWL and the present LMWL, but with a higher deuterium-excess (d). Samples in G1 have a Ne/He ratio around 0.6. ¹⁴C and ⁸¹Kr ages were from 10 ky to 40 ky. Group 2 includes the samples of the borders of the Salado basin, being isotopically more enriched (δ¹⁸O from -3 to -4.5 ‰) and with a lower d than present precipitation, a Ne/He ratio from 0.2 to 0.8 and one sample with ⁸¹Kr age of 640 ky. Group 3 is formed by brines from the Colorado basin, a Ne/He ratio in the range of 1E-02 to 1E-04, and ⁸¹Kr in ages around 900 ky, and are along a line of slope 1.9, showing a ¹⁸O shift. Finally the Group 4 formed by samples at the axis of Salado basin, are isotopically enriched (δ¹⁸O from -0.5 to -3.7 ‰) along a line of slope 3.9 resembling and evaporation line. However, these samples of ⁸¹Kr ages of 1000 ky and Ne/He ratio of 2E-03, showed a high correlation Cl- vs δ¹⁸O, with increasing values from West to East. This suggest a mixing with a brine or an increasing water-rock interaction.

How to cite: Martinez, D., Albouy, R., Bertolin, L., Jiang, W., Kruse, E., Lexow, C., Lu, Z., Mabry, J., Matsumoto, T., Quiroz-Londoño, O., Ritterbusch, F., Romeo, N., Yang, G., and Zarate, M.: Water stable isotopes, radiocarbon, noble gases and krypton-81 study of thermal groundwater from Upper to Mid-Pleistocene recharge age in deep aquifers of Argentina., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13517, https://doi.org/10.5194/egusphere-egu21-13517, 2021.

The Alpine Foreland Basin (AFB) extends from Geneva to Vienna. It is an highly populated area and also hosts important industrial sites. The competition between different areas of utilization, including groundwater, mineral water, geothermal energy, oil and gas as well as underground gas storage increased within the last decades and is still increasing. Therefore, the understanding of the subsurface, the hydrostratigraphic units and their interactions is essential to develop cross-boundary concepts for sustainable management, economic development and security of supply.

The aims of a three-years project, funded by the ÖAW (Austrian Academy of Science), were to delineate and characterise hydrostratigraphic units, to recognise water composition and to determine possible flow pathways within the Upper Austrian part of the AFB. As main data sources, hydrochemistry, stable isotopes, dissolved noble gases of groundwater and isotopic groundwater ages were used to proof and improve hydrogeological concepts. This presentation focuses on the ⁸¹Kr investigations which were carried out within the project and were partly funded by the government.

Determining the age of deep groundwaters which are free of ¹⁴C has been almost impossible for a long time. Improved analytical methods make it now possible to use ⁸¹Kr for age characterisation. Therefore, nine water samples from deep wells representing different hydrostratigraphic units in the Upper Austrian AFB, were used for ⁸¹Kr investigations. These samples include water from Upper Jurassic geothermal reservoirs.

Results imply a differentiated picture of groundwater residence times. ⁸¹Kr model ages of Malmian water samples are uniform which is in line with hydrochemical analyses and stable isotopes of these samples. However, model ages are exceptional high (390000 – 550000 years) which would suggest low flow velocities. This seems to contradict all existing hydrogeological model concepts of a dynamic thermal water flow in Malmian carbonates. The water sample taken from the Eocene (Gallspach) exhibits a very old groundwater portion (> 900000 years), whereas samples from Oligocene strata show the youngest but strongly varying ⁸¹Kr model ages (<25000 – 240000 years). The water sample of Bad Schallerbach is interpreted as a complex mixed system between young and middle-aged deep groundwaters with elevated mineralisation. A contribution from a deeper aquifer is postulated for the water sample in Andorf (240000 years). 

In summary, results of the hydrochemical and stable isotope investigations together with the krypton analyses have shown that connections between the Upper Jurassic thermal water aquifer and younger groundwater systems (Eocene to Oligocene) are obvious. This confirms the hydrogeological model concepts, which assume a discharge of thermal waters east of the Upper Jurassic carbonate rocks into younger strata. Possible factors which may influence the model ages (e.g. diffusion processes, contact with formation waters containing hydrocarbons) were critically discussed on the gained database. However, the discrepancy between the derived ⁸¹Kr model ages of the Malmian thermal waters and the current hydrogeological models could not be resolved yet. Further investigations should focus on recharge areas and therefore on aligning age data and hydraulic models.

How to cite: Gross, D., Heidinger, M., Götzl, G., Kriegl, C., Kralik, M., and Sachsenhofer, R. F.: Dating (81Kr) deep thermal groundwaters in the Upper Austrian part of the Alpine Foreland Basin, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14217, https://doi.org/10.5194/egusphere-egu21-14217, 2021.

The Upper Austrian Molasse Basin is not limited to the use of drinkable groundwater and balneological use of thermal waters. The extraction of crude oil and natural gas, the storage of natural gas and carbon dioxide, and the increasing use of thermal water as an alternative energy source are of high economic importance. In terms of water management, knowledge of the subsurface conditions, the different groundwater horizons, and their interactivity are indispensable. Already existing hydrodynamical models of the Malmian thermal aquifer, however, exhibit uncertainties due to low data density and lack of information. As part of a multi-year project funded by the ÖAW (Austrian Academy of Sciences), the data sets were expanded, among other things, by determining the strontium isotope ratios of selected water samples in Upper Austria. Therefore, concentrations of the most common cations and the strontium isotopic composition of 48 samples from 12 different aquifer horizons, ranging from shallow to artesian groundwater and geothermal and hydrocarbon wells were analyzed by using (MC) ICP-MS.
⁸⁷Sr/⁸⁶Sr ratios range from 0.70841 to 0.72740 and generally correlate with the respective host rock. The hydrostratigraphic horizons show characteristic strontium isotopic signatures, though they cannot be distinctly assigned to a single formation. Given variations in ⁸⁷Sr/⁸⁶Sr ratios can be traced in most instances. Thus, elevated ⁸⁷Sr/⁸⁶Sr ratios > 0.710 of a few samples of the Innviertel Group (0.70915 – 0.71167) can be explained by their sampling location at the crystalline boundary. Higher variability of results within formations, e.g., of the Linz-Melk Formation (0.70928 – 0.71160) in terms of both, ⁸⁷Sr/⁸⁶Sr ratio and strontium concentration suggest mixing processes with waters depleted in ⁸⁷Sr. Equally large variations are shown in the results of waters from Eocene formations (0.70879 - 0.71011). In this case, supported by results of other hydrochemical data, even a multi-component mixing process is assumed. Samples of the Malmian thermal water (0.70971 - 0.71077) exhibit, with one exception, homogenous ⁸⁷Sr/⁸⁶Sr ratios below 0.710.
The results demonstrate the ability of strontium isotope ratio determination to confirm assumptions based on hydrochemical and physical data as well as to provide additional information about the dynamics of (deep) groundwater systems to identify mixing processes within formations.

How to cite: Foelserl, V., Irrgeher, J., Groß, D., Sachsenhofer, R. F., Götzl, G., Kralik, M., and Heidinger, M.: TRACING GROUNDWATER SYSTEMS IN UPPER AUSTRIA USING 87Sr/86Sr ISOTOPE RATIOS, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14725, https://doi.org/10.5194/egusphere-egu21-14725, 2021.