⁸⁵Kr (t_1/2=10.7 a), ³⁹Ar (t_1/2=268 a) and ⁸¹Kr (t_1/2=229 ka) are valuable isotopes for radiometric dating of groundwater, especially due to their gaseous properties and chemical inertness. Together with ¹⁴C, these radioisotopes cover the age range from present back to 1.5 million years. Due to their extremely low environmental abundances of 10^-17…10^-11, corresponding to only a few thousand atoms per kilogram of water or ice, the detection of these isotopes is very challenging. In the recent two decades, the laser-based method Atom Trap Trace Analysis (ATTA) has succeeded in measuring these radioisotopes in water and ice samples of <10 kg, enabling applications in groundwater, ocean water and glacier ice.

Here, we present dating of groundwater with ⁸⁵Kr, ³⁹Ar and ⁸¹Kr, using ATTA for the radioisotope measurement. Recent progress on high precision ⁸¹Kr analysis has closed the dating gap between ¹⁴C and ⁸¹Kr, allowing for ⁸¹Kr dating of groundwater from the last glacial maximum to the beginning of the Holocene. Crucial advances in the ATTA instruments have moreover enabled a sample size reduction down to 1 kg of water or ice, allowing for dating of groundwater also under special conditions, such as in fractured rock aquifers with very low flow rate. The smaller sample size also facilitates simplified sampling schemes, e.g. sampling water directly instead of degassing it in the field.

How to cite: Ritterbusch, F., Feng, X., Jiang, W., Li, H., Lin, Q.-S., Lu, Z.-T., Wan, Z.-F., Wang, J., and Yang, G.-M.: Dating of groundwater with 85Kr, 39Ar and 81Kr, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7729, https://doi.org/10.5194/egusphere-egu25-7729, 2025.

Saudi Arabia and the Gulf states have been classified as water-scarce countries by the United Nations, which urges the need for groundwater management and protection. Groundwater age is key to the protection and management of non-renewable groundwater resources. Helium diffusion within the fluids of sedimentary basins is a new and little explored groundwater dating tool. The current study constructs a helium diffusion profile from the crystalline basement to the surface for two basins in the Arabian Peninsula: The Jafurah Basin and the Northwestern Basin. These profiles were corrected using ⁴He concentrations (n=56) from fluids of different formations within the sedimentary succession of the two basins. All measured ⁴He concentrations were found to be of crustal origin, with (R/R_a) corrected values ranging between 0.005 and 0.078. The measured ⁴He groundwater concentrations were shown to be within 1σ uncertainty of the diffusion model results, indicating absence of advective groundwater transport and diffusive loss of ⁴He. It was found that major tectonic events at the Oligocene (33-21Ma) and the Miocene (19-8Ma) flushed ⁴He basement flux in the shallow groundwater aquifers, leaving room for in-situ production only. ⁴He produced in-situ was deemed sufficient to be used in groundwater age calculations beyond radiocarbon capabilities. Additionally, analysis of ⁸⁷Sr/⁸⁶Sr, δ¹⁸O, δ²H, and radiocarbon proposed the presence of recent recharge. However, groundwater inheritance of paleo-seawater ⁸⁷Sr/⁸⁶Sr ratios indicated enhanced water-rock interaction (WRI), with little influence from modern-day seawater. This suggests mixing of recent recharge with the old groundwater depicted by ⁴He. Groundwater samples that had measurable ¹⁴C activity were also enriched in ⁴He, and such enrichment is too high to have accumulated over the residence time calculated by radiocarbon (2,817 – 30,100 yrs BP), especially in the absence of deep structural conduits e.g., faults and mega fractures. This challenges the conventional groundwater dating methods that presume one representative groundwater age. We conclude that groundwater age, especially that calculated from radiocarbon, represents a mean residence time of mixture of young and old endmembers, which proves ⁴He as a reliable chronometer for groundwater dating.

How to cite: Souid, F., Hillegonds, D., Mutairi, S., Kazak, E., Shoakar-Stash, O., Cheng, A., and Ballentine, C.: Groundwater Age Dating using Basinal Radiogenic Helium Diffusion Profile , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-534, https://doi.org/10.5194/egusphere-egu25-534, 2025.

DNA-based tracers have recently been used as groundwater tracers, primarily in granular aquifer media. Given the possibility of simultaneously applying and distinguishing multiple tracers with distinct DNA labels, they offer unique opportunities for tracing distinct pathways between injection and arrival points. They are synthesised by adsorbing DNA molecules on a silica or magnetite nanocore and encapsulated with a silica layer to protect the molecule against extreme temperatures, pH, and microbial attack. These nanotracers can be exponentially amplified, pushing the detection sensitivity down to one molecule, thus helping to mitigate the narrower detection range associated with conventional solutes.  However, when co-injected, both tracers are expected to follow the same preferential flow paths in a connected fracture network but with distinct travel times. While solute tracers are attenuated by diffusion into the matrix enhanced by sorption, the nanotracer mobility is dominated by advective transport enhanced by size exclusion. Considering the uncertainty in the nanotracer mobility, especially in bedrock aquifers, pairing these tracers provides complementary insight into the nature and variability of the fracture pathways and rates.

In this study, we co-injected a novel DNA-based nanotracer with magnetite nanocore (acronym: SiDNAMag) and Uranine in a Silurian dolostone aquifer under controlled natural gradient flow conditions to characterise fracture connectivity, groundwater velocities and diffusion process influences. The experiment was conducted at a toluene-contaminated site in Guelph, Canada, where depth-discrete multilevel systems (MLSs) were installed for 3D monitoring, improving insights on spatial variability in tracer transport. The tracer solution was injected at 0.5 L/min over a 1.6 m vertical interval, isolated with straddle packers in an upgradient well 10.5 m from the modestly pumped (0.11 L/min) extraction well and monitored from 15 MLSs comprising 82 ports. Using temporal moment analysis, we compared the transport of SiDNAMag to Uranine and observed the preferential flow geometries through the fractured dolostone aquifer. SiDNAMag showed an earlier breakthrough (2.25 h) compared to Uranine (6.25 h) at the extraction well with higher average velocity. 2.5% of Uranine mass was recovered, while SiDNAMag recovery was unquantifiable due to intermittent detection in the extraction well. SiDNAMag was predominantly detected at depths below the injection interval compared to Uranine, suggesting an influence of density on particle mobility. Preferential pathways also exist in the zone above the injection interval, evidenced by early detection of Uranine in the shallow ports of MLSs between the injection and extraction wells.

These findings enhance our understanding of fracture connectivity and the delineation of dominant flow pathways in the dolostone aquifer. They also provide insights into the variability of discrete fracture pathways within the 3D field domain, supporting the generation of fracture networks that accurately represent field conditions. Using HydroGeosphere, a discrete fracture matrix (DFM) numerical flow and transport model, these networks can be used to evaluate remediation strategies effectively. Although this study reveals SiDNAMag as a promising tool for groundwater tracing in fractured dolostone aquifers, a critical aspect of understanding its transport behaviour lies in examining the effects of groundwater chemistry and aquifer mineralogy on SiDNAMag. 

How to cite: Nyarko, F., Manna, F., Foppen, J. W., and Parker, B. L.: Testing Silica-Encapsulated DNA Molecules with Iron Nanocore as a Groundwater Tracer in Fractured Silurian Dolostone Bedrock, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1934, https://doi.org/10.5194/egusphere-egu25-1934, 2025.

The Milk River Aquifer (MRA) is a regional transboundary aquifer covering over 26,000 km² across northern Montana (USA) and southern Alberta (Canada). Extensive groundwater extraction since 1960s has led to a decline in groundwater levels, thereby emphasizing the need for informed water management strategies. The objective of this study was to improve the understanding of spatial variations in major ion concentrations with respect to groundwater age and flow paths, and to  identify key geochemical processes that influence groundwater quality within the aquifer. A comprehensive digital database was developed using hydrogeological and geochemical data from 1,429 water samples collected from 549 wells. Additionally, 20 new  groundwater samples and associated gases were collected during a 2022 field campaign, and these samples were analyzed for concentrations of major and minor ions, gas composition, stable isotope ratios (²H/¹H and ¹⁸O/¹⁶O of water, ¹³C/¹²C of DIC and ³⁴S/³²S of sulfate, ¹³C/¹²C and ²H/¹H of methane), and radioactive isotopes (⁸¹Kr, ³⁶Cl and ¹⁴C_DIC).

Utilizing a newly updated groundwater numerical flow model (FEFLOW software) in combination with recent ¹⁴C and ⁸¹Kr-based groundwater age dates, distinct patterns in chloride (Cl) concentrations dependent on groundwater age and flow path were identified. Groundwater less than 34,000 years old exhibited Cl concentrations < 25 mg/L near the recharge zone, while groundwater exceeding 200,000 years in age had Cl concentrations > 100mg/L at distances of 125 km from the recharge zone. Increasing δ²H and δ¹⁸O values in older groundwater with elevated Cl concentrations indicate possible mixing of fresh recharge water with formation water from northern regions of the aquifer (Taber and Bow Island formations) or associated aquitards (Pakowki and Colorado formations). Ongoing analysis explores variations in other major ions with a specific interest in redox-sensitive species as a function of flow distance and groundwater age. Preliminary results reveal that elevated sulfate concentrations (> 1200 mg/L) in recharging groundwater are due to pyrite oxidation, but at groundwater flow distances between 50 and 75 km bacterial sulphate reduction becomes dominant resulting in sulfate concentrations < 1 mg/L. At flow distances >80 km, redox conditions become favourable for methanogenesis resulting in occurrence of biogenic methane in groundwater. A particle tracking algorithm within the updated numerical flow model was employed to compare residence times with groundwater ages determined from ⁸¹Kr measurements. The tracer ages (¹⁴C and ⁸¹Kr) were confirmed using a numerical particle tracking model based on an existing numerical steady-state groundwater flow model (FEFLOW). The outcomes of this study that utilizes innovative groundwater age dating tools (⁸¹Kr) are new insights into how geochemical processes evolve with respect to flow distance and groundwater age thereby modifying spatial variability of key water quality parameters within the Milk River Aquifer.

How to cite: Date, A., Mayer, B., Humez, P., Nightingale, M., Mueller, P., Bishof, M., Lantis, J., Vockenhuber, C., Corcho, J., Purtschert, R., Yokochi, R., Sturchio, N., Nagare, R., and Wheatcraft, S.: The impact of groundwater age and flow patterns on water quality in the Milk River Aquifer, Canada, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3967, https://doi.org/10.5194/egusphere-egu25-3967, 2025.

Reconstructing flow directions and velocities in highly heterogeneous contaminated aquifers, such as fractured and karstified systems, poses significant challenges. These arise from both the inherent complexity of the hydrogeological context and the technical and physicochemical interferences that contaminated sites can impose on the logistics and outcomes of tracer tests.

A short-term (14-day) tracer test was conducted under perturbed conditions in a fractured and karstified aquifer in Southern Italy, at a site contaminated by petroleum hydrocarbons and other pollutants. The objective was to gather critical insights into the dynamics of subsurface flow to inform subsequent contamination management and remediation strategies. Multiple tracers were used, including fluorescent dyes and silica-encapsulated DNA-labeled nanoparticles. Silica-encapsulated DNA nanoparticles hold significant potential as hydrogeological tracers due to their non-toxic nature, physicochemical stability, and exceptional detectability at extremely low concentrations via qPCR. However, their performance in real-world applications remains under investigation.

Three synthetic DNA nanotracers were injected into three wells, alongside two conservative dye tracers, Uranine and Tinopal. All three DNA tracers were successfully detected in groundwater samples collected from pumping wells at distances ranging from 30 to 160 meters from the injection point, indicating flow velocities between 7 and 130 m/day. While the fluorescent dyes traveled at comparable velocities, they covered shorter distances and exhibited delayed peak arrivals relative to the DNA tracers. Additionally, the detection of fluorescent dyes was sometimes hindered by the presence of hydrocarbons, a limitation that did not affect the DNA nanotracers. The tracer recovery results ultimately facilitated the identification of primary and secondary groundwater flow directions, some of which deviated from expectations based on the site’s preliminary conceptual groundwater flow model. Overall, the tracer test results underscored notable differences between the two tracer types, suggesting that DNA nanotracers and fluorescent dyes may navigate distinct porosity systems, offering complementary insights into the aquifer's structure and dynamics.

While the results are promising and demonstrate the potential of combining fluorescent dyes and DNA nanotracers for applications in highly heterogeneous contaminated aquifers, further research is needed to evaluate the versatility of DNA tracers in field applications. This includes addressing both field and analytical challenges, as well as better assessing their comparative utility relative to conventional dye tracers.

How to cite: Filippini, M., Neuert, L., Pugliese, E., Giuliani, C., Bolognesi, G., Tamagnini, E., Cavallini, M. E., Filippini, S., Mozzi, R., and Gargini, A.: Tracing groundwater flow paths in a contaminated fractured and karst aquifer using fluorescent dyes and silica-encapsulated DNA nanoparticles: challenges and insights, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7985, https://doi.org/10.5194/egusphere-egu25-7985, 2025.

Due to the high heterogeneity of karst aquifers, understanding the transport of contaminants within karst underground river systems remains challenging. Moreover, there is limited knowledge regarding the risks posed by pollutants in karst aquifers and their attenuation potential. To characterize the differential transport and release of various contaminants through different recharge pathways in a karst underground river system. This study integrates intermittent inputs of acid mine drainage (AMD) and conservative dye tracers. High-frequency monitoring of discharge and water quality, along with breakthrough curve (BTC) analysis of the tracers, was conducted to perform both qualitative and quantitative assessments. The research was carried out in a typical karst underground river system (Qingxisi) located in western Hubei, China. The results revealed three distinct, non-intersecting karst conduits that converged at the underground river outlet. Among them, Conduit 1 was unaffected by AMD pollution, while Conduits 2 and 3 exhibited contaminant transport distances exceeding 10 km. Notably, the arrival of AMD pollutants was significantly delayed compared to the arrival of Conduit  1, with Conduit 2 demonstrating a faster transport velocity than Conduit 3. The pollution pattern in the underground river system suggests intermittent leakage of AMD pollutants, leading to periodic water quality responses upon contaminant release. In contrast to the rapid attenuation of the conservative dye tracers, AMD pollutants exhibited slower and more persistent attenuation processes. The rates and extents of contaminant attenuation varied among the conduits, depending on the degree of conduit development. Sulfate (SO₄²⁻), a characteristic pollutant of AMD, showed the fastest attenuation rate, while several heavy metal elements displayed negative attenuation rates, indicating secondary pollution during transport , potentially related to adsorption-desorption processes with sediments. The storage and release of contaminants, driven by hydraulic gradient changes between karst conduits and fracture media, were found to delay the natural attenuation of pollutants, suggesting potential limitations in the long-term natural attenuation capacity of the underground river system. This study enhances the understanding of contamination processes and the mechanisms of water quality change and natural attenuation in vulnerable karst groundwater systems, contributing to the management and prevention of groundwater pollution in karst environments.

How to cite: Ji, H., Huang, K., Luo, M., Chiogna, G., and Richieri, B.: Identification of hydrologic response and contaminant transport processes in karst underground river systems using acid mine drainage and artificial dye tracer , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9094, https://doi.org/10.5194/egusphere-egu25-9094, 2025.

The Potiguar Basin is a passive margin sedimentary basin formed during the breakup of Africa and South America. It encompasses important onshore oil reservoirs which have been exploited since the 1970s. It is located in the Atlantic Septentrional margin in the semi-arid region of Northeastern Brazil. The hydrogeological framework of the Potiguar Basin includes two main reservoirs: the karst Jandaíra Aquifer and the porous Açu Aquifer. The latter is the most significant aquifer in the region, responsible for supplying water to the population and for fruit production irrigation. Nowadays exhaustive drilling and groundwater extraction have led to a decrease in the water table posing problems for water resources sustainability in this semi-arid region. Despite the increasing demand of groundwater in the last decades, the Açu Aquifer has not been properly studied regarding groundwater flow, isotopic, and hydrochemical groundwater evolution. The semi-confined Açu Aquifer is bounded by the Potiguar basin, at the contact with the overlying carbonate platform, the Jandaíra Aquifer, which confines the Açu Aquifer across the entire platform toward the offshore areas. From the outcrop area, groundwater flows preferentially in the SW to NE direction influenced by the geological and structural framework of the basin. Aiming at a better understanding of groundwater flow and paleorecharge temperatures in the Açu Aquifer, environmental isotopes (H, O, and C) and noble gases were measured. Groundwater isotopic signatures values were found to be more depleted than those of precipitation (<-3.5‰ for δ¹⁸O and -10‰ for δ²H in groundwater, and up to -2.0‰ for δ¹⁸O and close to 0 ‰ for δ²H in precipitation). The age tracer C-14 indicates that young groundwater (close to 100 pMC) is present in the outcrop area and that very old groundwater is present at the deepest zones of the basin, where the C-14 concentration was near the detection limit of the method (0.3 pMC). In addition, the noble gas concentrations also suggest colder climate conditions during recharge. Our first results yield temperatures up to 8ºC below the modern mean annual temperature, in agreement with previously paleoclimate temperature studies in the region. The high excess air values ​​from older deep waters indicate a larger fluctuation of the water table level. These preliminary results suggest that the recharge of the Açu Aquifer took place under colder climate conditions, in possible association with the Last Glacial Maximum (LGM). Even though the groundwater residence time was not precisely determined, it is expected to be over 40k years. Our results are consistent with previous studies that point to changes in temperature since the LGM for the Northeastern Brazil climate. Although further sampling and more precise data analysis are needed, these initial findings highlight the complexity of groundwater flow in the Açu Aquifer, and the importance of effective groundwater management to ensure the sustainability of the resource.

How to cite: Arruda, N., Gastmans, D., Eckert, H., von Reventlow, B. G., Engelhardt, E., Aeschbach, W., Filgueira, J. G., Ferreira, G., Bernardo, N. Q., and Teixeira, Z. A.: Assessments of Groundwater Circulation and Recharge Paleotemperature in the Açu Aquifer, Northeastern Brazil, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12145, https://doi.org/10.5194/egusphere-egu25-12145, 2025.

Research on groundwater residence times is crucial for assessing groundwater infiltration rates and aquifer vulnerabilities, both playing a vital role in sustainable water resource management. This study aimed at advancing the use of the short-lived cosmogenic radionuclide ³⁵S (“radio-sulfur”, t_1/2 = 87.4 d) for determining groundwater residence times of less than one year. The results show that ³⁵S provides a valuable tool for evaluating groundwater residence times, infiltration rates, and aquifer vulnerabilities. The preconcentration of ³⁵SO₄^2- using an anion exchange resin prior to Liquid Scintillation Counting (LSC) is a technique designed to improve the detection of ³⁵S in groundwater and precipitation samples. Our optimized method involves a custom setup where up to 500 mg of SO₄^2- can be extracted from a large-volume water sample in less than an hour by passing the sample through a column pre-packed with an ion exchange resin. The retained sulfate ions are then eluted and the eluate is subsequently concentrated by evaporating excess water, while ensuring the elimination of organic compounds resulting in the formation of a clear, colorless sample. Once all colored compounds are removed, the sample is mixed with a scintillation cocktail and analyzed using LSC. The SO₄² preconcentration procedure has been adapted for application directly in the field and eliminates therefore the need to transport large volumes of water sample to the laboratory and addresses logistical challenges associated with shipping and storage. Furthermore, we explored various LSC optimization methods for the detection and quantification of ³⁵S in natural water samples resulting in improvements of both background and efficiency during LSC measurement. This work represents advancements in the utilization of radio-sulfur analysis, thereby expanding the suite of natural radionuclides to constrain water residence time distributions in terrestrial waters.

How to cite: Wangari, S., Harjung, A., Machado, D., McGuire, B., Schubert, M., Kopitz, J., Lin, M., Copia, L., and Bibby, R.: Field sampling, sample preparation and measurement of radio-sulfur in natural water samples, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17720, https://doi.org/10.5194/egusphere-egu25-17720, 2025.

Tritium radioisotope (H-3 or T) with a half-life of 12.32 years was released to the atmosphere in the 2011 Fukushima Daiichi Nuclear Power Plant (FDNPP) accident prompting tritium monitoring efforts in Fukushima waters. Tritium in precipitation has been measured monthly from 2012 accumulating a decade-long record in Namie Town [1] and the Fukushima Prefectural Government sampled river, lakes, dam reservoir, and coastal sites twice per year for direct tritium measurements [2]. Since the FDNPP anthropogenic tritium was measured at several coastal sites influencing natural background tritium, which is a cosmogenic radionuclide traced as a water molecule (HTO), a combined time-series of both anthropogenic and natural tritium in precipitation was required for the transit times interpretation using the atmospheric FDNPP release tritium simulation [3]. In October 2023, tritium measurements at several Fukushima city headwater catchments indicated natural background levels and tritium-tracer was useful for estimating water transit times and volume in the subsurface [4]. While tritium is a useful tracer to estimate water transit times in Fukushima, the continuation of tritium monitoring is needed to disentangle natural levels with the ongoing tritium-related FDNPP activities such as the tritiated water discharge from the FDNPP site. 

References: 
[1] Yamada R., Hasegawa, H., Akata, N., et al. (2024) Temporal variation of tritium concentration in monthly precipitation collected at a Difficult-to-Return Zone in Namie Town, Fukushima Prefecture, Japan. Environmental Science and Pollution Research (5): 7818–7827. https://doi.org/10.1007/s11356-023-31652-9
[2] Fukushima Revitalization Portal Site (2024). https://www.pref.fukushima.lg.jp/site/portal/
[3] Gusyev, M., Cauquoin, A., Igarashi, Y., et al. (2024) Anthropogenic and natural tritium radioisotope in terrestrial water cycle of Fukushima, Japan, EGU General Assembly 2024, Vienna, Austria, EGU24-17332, https://doi.org/10.5194/egusphere-egu24-17332.
[4] Cauquoin, A., Gusyev, M., et al. (2025) Modeling tritium release to the atmosphere during the Fukushima Daiichi Nuclear Power Plant accident and application to estimating post-accident water system transit times, Japan, Environmental Science and Pollution Research, https://doi.org/10.1007/s11356-025-35919-1

How to cite: Gusyev, M., Cauquoin, A., Hirao, S., and Akata, N.: A review of tritium radioisotope in Fukushima waters, Japan, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20793, https://doi.org/10.5194/egusphere-egu25-20793, 2025.