Young Groundwater Research Articles

A method for groundwater 14C age correction accounting for mixing with young and old groundwaters: application to an alluvial aquifer in the Jianghan Plain, China

A method for groundwater 14C age correction accounting for mixing with young and old groundwaters: application to an alluvial aquifer in the Jianghan Plain, China

Short high-accuracy tritium data time series for assessing groundwater mean transit times in the vadose and saturated zones of the Luxembourg Sandstone aquifer

Abstract. Among the manifold of environmental tracers at hand, tritium is the only one that can give information on groundwater age within the timescale of 100 years for the entire flow system, i.e., unsaturated and saturated. However, while in the Southern Hemisphere, a single water sample is sufficient for tritium-based young groundwater dating, several tritium measurements spanning multiple years are still needed in the Northern Hemisphere to disentangle the natural cosmogenic tritium input from that caused by the atmospheric thermonuclear weapons tests mainly carried out in the early 1960s. Although it is advised to focus tritium dating on sites where long chronicles of tritium data are available, in this study we tested the potential for short high-accuracy tritium data series (∼4 years) to date groundwater from 35 springs draining the Luxembourg Sandstone aquifer (central western Europe). We determined groundwater mean transit times using the lumped-parameter model approach in a Monte Carlo uncertainty estimation framework to provide uncertainty ranges inherent to the low number of tritium data at hand and their related analytical errors. Our results show that unambiguous groundwater mean transit time assessments cannot be determined solely based on such recent short tritium time series, given that several ranges of mean transit times appeared theoretically possible. Nonetheless we succeeded in discriminating groundwater mean transit times in the vadose and saturated zones of the aquifer through a stepwise decision process guided with several supplementary data. The mean transit time required for water to cross the vadose zone was estimated to be between 0.5±0.5 and 8.1±1.2 years depending on the spring, while for water to flow through the saturated zone, it varied from 5.7±2.4 to 18.9±4.6 years (median ± half of the 5–95 percentile range). Our findings are consistent with both the tritium measurements of individual springs and the hydrogeological context of the study area. We specifically corroborated the dating results using the known hydrogeological properties of the Luxembourg Sandstone aquifer, the hydrochemistry of the studied springs, and their discharge dynamics. When translated into water velocities (which average ∼12 and ∼170 m yr−1 for the vadose and the saturated zones, respectively), the tritium dating results mirrored the horizontal–vertical anisotropy of the aquifer's hydraulic properties caused by the bedded character of the Luxembourg Sandstone. In addition to improving our understanding of water transit times in the Luxembourg Sandstone aquifer, this study demonstrates how it is currently possible to use short tritium time series to date young groundwater bodies at new sites in central Europe.

New insights into the flow dynamics of a deep freshwater aquifer in the semi-arid and saline Cuvelai-Etosha Basin, Northern Namibia: Results of a multi-environmental tracer study

Study regionA paleo-megafan system of the Cubango River in the northern parts of the semi-arid Cuvelai-Etosha Basin, shared by Angola and Namibia. It hosts a deep freshwater aquifer, the so-called Kalahari-Ohangwena 2 (KOH-2), with the potential to resolve the imminent regional water supply shortages. Study focusHydrogeochemical and multi-environmental tracer studies incorporating the use of age tracers 14C, 36Cl, 81Kr and 4He to determine the age of groundwater and provide insights into the flow dynamics of the KOH-2. New hydrological insights for the regionStable water isotopes and noble gas thermometry show that in a period with higher rainfall and recharge, temperatures were at least 3 – 4 °C lower than today. Several arguments led to the conclusion that younger groundwater, possibly of an age of 35,000 years, is mixed with ancient saline pore water. These include: 1) the correlation of measured 36Cl and 81Kr ratios, as well as 4He concentrations, using a binary mixing model, and 2) the substantial variation in 81Kr ages, ranging from 40,000 to 170,000 years, over relatively short distances—a phenomenon challenging to explain by advective groundwater flow equations. Consequently, the ages derived from 81Kr measurements serve as indicators of the extent of freshening and therefore describe mixing ages rather than absolute travel times.

ENVIRONMENTAL ASSESSMENT OF POLLUTION IN AN INDUSTRIAL ENVIRONMENT USING HEAVY METAL POLLUTION INDICES AND STABLE ISOTOPES, CASE STUDY OF THE INDUSTRIAL ZONE OF SKIKDA, NORTH-EAST ALGERIA

An environmental assessment study was conducted in the industrial area of Skikda in the Northeast of Algeria which involved physicochemical parameters, heavy metal concentrations, and stable isotopes of surface water, groundwater, and soil. The results revealed high concentrations of Cd, Ni, Fe, Cu, As, Cr, Pb, Co, and Mn in groundwater. The southern part exhibited the lowest heavy metal concentrations, while the industrial zone in the North contained the highest levels. Therefore, the industrial area showed significant pollution in groundwater by Cr, Pb, and As with respectively 0.5, 0.69, and 0.44 mg/l, surpassing the respective WHO(World Health Organization) standards of 0.05, 0.01, and 0.01 mg/l. Spatial analysis showed that the industrial area presents higher concentrations of As, Cr, Cd, Pb, Cu, Zn, Ni, and Fe which demonstrated similar trends in spatial distribution, their concentrations increasing within the industrial zone. However, the western part had no significant pollution sources except for a public landfill and SONATRACH industrial complex, high concentrations of Co and Mn in the south and the west parts were observed. Mean concentration in soils indicated that Cd is ranged between35.32 to 95.12 ppm, for Fe between 145.1 and 702 ppm, As varies between 1.03 and 81.85 ppm, and Zn between 50 and 520.32 ppm and considered as dominant metals. However, Mn, Co and Cr were lower respectively with 1.97, 12.11 and 7.25 ppm. Cd and As were most influenced by the polluting companies, while Cr was mainly influenced by natural factors, such as pH and soil type. Based on the contamination degree index, the soil pollution levels of Cd, Pb, and As were classified as extremely contaminated. The statistical analysis indicated substantial anthropogenic contributions to groundwater contamination concentrated in the industrial area. The groundwater exhibits a range of δ18O and δ2H values, varying from 2.45 to -5.33‰ and 19.82 to -1.67‰ vs. the V-SMOW (Vienna-Standard Mean Ocean Water) standard, respectively. The mean isotopic composition is -2.50‰ for δ18O, -19.32‰ for δ2H, with a mean deuterium excess of 6.45. These values align with the Meteoric Water Line observed in the western Mediterranean region. It indicates a hydrological connection between recharge and groundwater which suggests a significant interaction between groundwater and surface water. Due to their proximity to the river and the shallow aquifer, the downstream area is more vulnerable to surface pollution. Furthermore, the young groundwater in the downstream can be attributed to the rapid recharge and replenishment of the aquifer caused by rainfall and quick infiltration and sustained by the shallow aquifer depth and sandy soil. Consequently, the rainwater and industrial wastes lead to faster pollutant infiltration. Anthropogenic sources, including industrial effluents from smelting, refining, manufacturing, steel and textile industries, electroplating, nickel-cadmium battery production, welding, PVC product and paint pigment manufacturing, were identified as major contributors to heavy metal pollution.

Investigating the enigma of an irregular groundwater age pattern in a confined, presumed “fossil” complex aquifer through mixing cell flow modeling

Significant fluctuations in the groundwater (GW) age along the eastern flow path of the Nubian Sandstone Aquifer's (NSA), as derived from Krypton-81 groundwater dating, have suggested that this aquifer, located in Israel's Negev Desert and previously presumed to be a fossil, is not entirely isolated but mixes with younger and even recent water. The intermittent rejuvenation and drastic increases in the GW age across short distances most likely imply hydraulic connectivity with the surrounding aquifers, which contribute both younger and more ancient water to the NSA. The current study aims at modeling the GW flow system to locate and quantify its water sources despite the aquifer's hydrogeological complexity and the scarcity of hydrological data. We implemented the Mixing Cell Modeling (MCM) approach, understanding that the alternating rejuvenations and increases in the GW age downstream of the NSA's eastern flow trajectory reflect the mixing of the NSA's groundwater with young and old GW bodies, respectively. Thus, prompted by the 81Kr water age distribution, yet independent of the Kr radioisotope data, a multi-tracer mixing cell flow model was adopted based on a set of balance equations of water, dissolved minerals, and stable environmental isotopes. The findings indicate that (1) there is a small, yet substantial, intrusion of old brackish GW from a deep-seated, highly pressurized aquifer into the NSA in the northeastern Negev; (2) the rejuvenation of GW in the NSA is due to significant mixing with water from nearby overlying carbonate and chert aquifers, and (3) the NSA is substantially replenished through the Nubian Sandstone (NS) outcrops along the Negev Desert anticlines. Most GW intrusions into the NSA occur near the intersections of the eastern flow path with some of the Negev's major faults and synclines, such as the Paran and Ramon Fault zones and the Zin Syncline. In light of the relatively young GW age at the end of the NSA's western flow path in the northern Negev, and based on the similarities in the hydrogeological structures in the Negev and northern Sinai Deserts, we propose that similar mixing processes with GW from the overlying carbonate aquifers and direct GW recharge through the NS outcrops also occur in the northern Sinai Peninsula. The approach presented in this study might apply to examining recharge processes and hydraulic connectivity in other aquifers that were formerly classified as “fossil,” such as the immense NSA found in the Arabian (Jordan & Saudi Arabia) and the Western (Egypt) Deserts.

Groundwater Age and Origin and Its Relation with Anthropogenic and Climatic Factors

Groundwater plays a major role in addressing the worldwide problem of water scarcity and food security. With a growing population and increasing urbanization, there is a rising demand for groundwater to meet agricultural and domestic water needs. A variety of advanced approaches are necessary to sustain groundwater management. This study investigated the age and origin of groundwater, as well as its relationship with anthropogenic and climatic factors. Stable isotopes were used, namely oxygen-18 (18O) and deuterium (2H) for the estimation of groundwater origin and radioactive isotopes of Tritium (3H) for the estimation of its age. The investigation of stable isotopes revealed that the aquifer is predominantly influenced by river water, with a minor contribution from rainwater. Furthermore, the analysis of radioactive isotopes revealed that the groundwater age ranges from 5 to 50 years old in most areas. Older groundwater is predominantly found in urban areas, while younger groundwater is present in agricultural and woodland regions. However, the presence of “old” water in the upper groundwater layers in urban areas is attributed to over-abstraction and limited natural recharge. The primary climatic factor that governs the age and origin of groundwater is rainfall upstream of the study area, which directly contributes to the river flows. The rainfall is high in the east but, due to urbanization, recharge is decreased. Consequently, old and river recharge groundwater is found in this area. These observations underscore the unsustainable and alarming use of groundwater in urban areas.

Environmental Tracers Tritium 3H and SF6 Used to Improve Knowledge of Groundwater Sustainability of a Crystalline Rock Island Aquifer of Tobago, West Indies

Understanding the residence time, flow velocities, and storage volumes in fractured rock aquifers is essential for assessing the sustainable use of groundwater in island aquifer systems. Environmental tracers such as tritium (3H) and sulfur hexafluoride (SF6) that enter the aquifer systems through natural hydrological processes are effectively used to quantify the “apparent age” of young groundwater (<60 years) to calculate these aquifer characteristics. The island of Tobago is predominantly a fractured rock aquifer system that contains groundwater sub-basins that produce 100–1000% more than their calculated recharge. In this study, we analyzed ten samples from groundwater wells throughout the island for environmental tracers to establish the “apparent age” of the groundwater. We then estimated the active flux and storage volume of the aquifer. The 3H results of ten wells throughout the island range between 0.02 and 0.60 TU. The groundwater samples to the south of the island possess more pre-modern groundwater ages (older ages) than samples in the north of the island. These tritium values reveal age ranges between 18 and 60–plus years. This finding suggests that fractures create flow paths oriented towards the southern parts of the basin that enable the transport of groundwater across significant topographic boundaries and watershed divides. Additionally, the data may support that southern wells are connected to isolated old sources of groundwater. SF6 values range between 0.91 and 7.97 (part per trillion volume) with an interpreted age range between 18 and 39 years. Both tracers support the original hypothesis that longer residence time waters are located south of the island except for three SF6 samples which are believed to be affected by “excess air”.

Using Tritium Isotope in Groundwater to Estimate the Reservoir Capacity at Erbil, Northern Iraq

Tritium is a radioactive isotope of hydrogen in water molecule can be applied in tracer studies to estimate water residence time. The tritium values of the groundwater in Erbil and the reservoir's capacity were the main subjects of this investigation. Three sub-basins, including, Kapran (720 km2), Central (1315 km2), and Bashtapa (625 km2) constructs Erbil City's groundwater basin. 27 groundwater samples from three sub-basins were taken for oxygen-18, tritium analysis and measurement of electrical conductivity. Tritium values divided the groundwater reservoirs into three possible groups: pre-bomb groundwaters that has ages older than 70 years due to lower tritium values, contaminated groundwater and mixture between young and old groundwaters. It is adequate to suppose that there were several substantial subsurface retention reservoirs in Dashty-Hawler basin. For instance, if two wells from the Kapan sub-basin were used as a point of reference, well No. 1 in Kalak region would have a tritium value of 1.7 ± 0.72 TU having groundwater that was approximately 16.2 years old. In contrast, the well No. 6 at Grd-Jutyar has a lower tritium value equal to 0.72 ± 0.68 TU indicating a greater residence time of 31.6 years. These two wells have differing reservoir sizes, and well No. 6 has a longer retention basin to store groundwater than well No. 1 According to these findings, younger groundwater and smaller retention basins were indicated by greater tritium levels, whereas older groundwater and larger retention basins were noted by lower tritium levels.

Hydrogen and Dark Oxygen drive Microbial Productivity in diverse Groundwater Ecosystems

Around 50% of humankind relies on groundwater as a source of drinking water. We investigated the age, geochemistry, and microbiology of 138 groundwater samples from 87 monitoring wells (<250 m depth) located in 14 aquifers in Canada (Fig. 1). Geochemistry and microbiology showed consistent trends suggesting large-scale aerobic and anaerobic hydrogen, methane, nitrogen, and sulfur cycling carried out by diverse microbial communities. Older groundwaters, especially in aquifers with organic carbon-rich strata, contained on average more cells than younger groundwaters, challenging current estimates of subsurface cell abundances. We observed substantial concentrations of dissolved oxygen in older groundwaters that could support aerobic lifestyles in subsurface ecosystems at an unprecedented scale. Metagenomics, oxygen isotope analyses and mixing models indicated that “dark oxygen” was produced in situ via microbial dismutation. We show that ancient groundwaters sustain productive communities and highlight an overlooked oxygen source in present and past subsurface ecosystems of Earth Ruff et al. 2023). Title, Abstract, and Figure 1 are reproduced from (Ruff et al. 2023) without adaptations, according to the terms of http://creativecommons.org/licenses/by/4.0/.

Hydrogen and dark oxygen drive microbial productivity in diverse groundwater ecosystems

Around 50% of humankind relies on groundwater as a source of drinking water. Here we investigate the age, geochemistry, and microbiology of 138 groundwater samples from 95 monitoring wells (<250 m depth) located in 14 aquifers in Canada. The geochemistry and microbiology show consistent trends suggesting large-scale aerobic and anaerobic hydrogen, methane, nitrogen, and sulfur cycling carried out by diverse microbial communities. Older groundwaters, especially in aquifers with organic carbon-rich strata, contain on average more cells (up to 1.4 × 107 mL−1) than younger groundwaters, challenging current estimates of subsurface cell abundances. We observe substantial concentrations of dissolved oxygen (0.52 ± 0.12 mg L−1 [mean ± SE]; n = 57) in older groundwaters that seem to support aerobic metabolisms in subsurface ecosystems at an unprecedented scale. Metagenomics, oxygen isotope analyses and mixing models indicate that dark oxygen is produced in situ via microbial dismutation. We show that ancient groundwaters sustain productive communities and highlight an overlooked oxygen source in present and past subsurface ecosystems of Earth.

Nitrate sources, timing, and pathways of a permeable volcanic aquifer system with mixed land use in Jeju Island, South Korea

The occurrence of various N-related human activities increases the difficulty in distinguishing the major sources of NO3− contamination in groundwater, especially in areas with mixed land uses. In addition, the estimation of the timing and pathways of NO3− is necessary to better understand the processes of NO3− contamination in the subsurface aquifer system. This study applied environmental tracers, such as stable isotopes and age tracers (δ15N and δ18O of NO3−, δ11B, chlorofluorocarbons, and 3H), to elucidate the sources, timing, and pathways of NO3− contamination in the groundwaters of the Hanrim area, which has suffered from illegal disposal of livestock wastes since the 1980s, and also characterizes them based on mixed N-contaminant sources such as chemical fertilizers and sewage. The combined use of δ15N and δ11B overcame the limitation of using only NO3− isotopes for the identification of overlapping sources of N and successfully identified the major source of N as livestock wastes. The lumped parameter model (LPM) estimated the binary mixing of the young (age: 23–40 years, NO3-N: 2.55–15.10 mg/L) and old (age: >60 years, NO3-N: <3 mg/L) groundwaters, and explained their age mixing behaviors. The young groundwater was highly affected by livestock-derived N loading during 1987–1998, which coincides with the period of improper dumping of livestock wastes. Furthermore, the young groundwater with elevated NO3-N followed the historical NO3-N curves with younger ages (6 and 16 years) than those derived from the LPM, suggesting the possibility of faster inflows of livestock wastes through the permeable volcanic structures. This study demonstrated that a comprehensive understanding of NO3− contamination processes can be achieved using environmental tracer methods, which enables the efficient management of groundwater resources in areas with multiple N sources.

Modeling groundwater transit time distributions and means across a Nebraska watershed: Effects of heterogeneity in the aquifer, riverbed, and recharge parameters

Groundwater transit time distributions (TTDs), the travel times through the aquifer from recharge at the water table to discharge at the surface water body, provide critical information on the timescales of hydrologic response of subsurface flow systems. We investigated the effects of spatial patterns of recharge, aquifer heterogeneity, and systematic variation in riverbed hydraulic conductivity on the mean transit times (MTTs) and TTDs of groundwater discharge to the Upper Middle Loup River (UMLR) and headwaters using a 3D-steady-state MODFLOW USG-MODPATHD3DU model. This 5436 km2 watershed overlies the High Plains Aquifer in the Sand Hills of Nebraska, USA. Modeled MTTs differed by up to three orders of magnitude from upstream to downstream in a 158 km section of the UMLR under varying recharge, aquifer, and riverbed heterogeneity scenarios. The simulated MTTs ranged from 1 to 397 years for upstream sites and 820 to 7968 years for the downstream sites. The TTDs at upstream sites were dominated by young groundwater from shallow flow paths and were sensitive to changes in riverbed hydraulic conductivity. Recharge parameterization had greater influence on the shape of the TTDs and magnitude of MTTs at the downstream sites, where much older groundwater discharged to the UMLR. Overall, spatial trends in transit times under varying model scenarios provided important information for refined conceptualization and calibration of future numerical models.

Residence times of groundwater along a flow path in the Great Artesian Basin determined by 81Kr, 36Cl and 4He: Implications for palaeo hydrogeology

Understanding the age distribution of groundwater can provide information on both the recharge history as well as the geochemical evolution of groundwater flow systems. Of the few candidates available that can be used to date old groundwater, 81Kr shows the most promise because its input function is constant through time and there are less sources and sinks to complicate the dating procedure in comparison to traditional tracers such as 36Cl and 4He. In this paper we use 81Kr in a large groundwater basin to obtain a better understanding of the residence time distribution of an unconfined-confined aquifer system. A suite of environmental tracers along a groundwater flow path in the south–west Great Artesian Basin of Australia have been sampled. All age tracers (85Kr, 39Ar 14C, 81Kr, 36Cl and 4He) display a consistent increase in groundwater age with distance from the recharge area indicating the presence of a connected flow path. Assuming that 81Kr is the most accurate dating technique the 36Cl/Cl systematics was unravelled to reveal information on recharge mechanism and chloride concentration at the time of recharge. Current-day recharge occurs via ephemeral river recharge beneath the Finke River, while diffuse recharge is minor in the young groundwaters. Towards the end of the transect the influence of ephemeral recharge is less while diffuse recharge and the initial chloride concentration at recharge were higher.

Characterization of groundwater types and residence times in the Verlorenvlei catchment, South Africa to constrain recharge dynamics and hydrological resilience

• The Verlorenvlei catchment is dominated by four groundwater types that belong to three aquifer systems. • 14 C and 3 H/ 3 He residence time constraints indicate distinctly different age spectrums in the three aquifer systems. • Discrepancies between the 3 H/ 3 He and 14 C results suggests mixing along the flow paths with repeated modern recharge packages entering shallower groundwater systems. • Groundwater resilience to climate change is dependent on understanding these mixing relationships. The Verlorenvlei catchment on the west coast of South Africa is a semi−arid region that is growing progressively more reliant on groundwater due to increased variability in precipitation and increasing agricultural productivity. The groundwater systems have been put under additional stress given the recent 2015–2017 El Ninõ system that led to drought conditions along the west coast of South Africa. This increased reliance puts the natural environment and human dependence on groundwater into direct conflict. Groundwater was sampled for δ 18 O, δ 2 H, δ 13 C, 14 C, 3 H and 3 H/ 3 He ratios in addition to cation and anion concentrations from seventeen production boreholes and two springs in the catchment. Residence times were estimated using tritium and radiocarbon, and in the case of the latter, the Pearson Model has been used to correct for possible carbonate dissolution. Hydrochemistry and residence times of groundwater in the Verlorenvlei catchment have successfully been used to distinguish between the primary−porosity alluvial aquifer, the secondary−porosity Malmesbury shale aquifer (MG) and the fractured rock aquifers associated with the Table Mountain Group (TMG). Groundwater mixing has been identified within the catchment and this plays an important role in the variation in groundwater chemistry and residence time between the aquifer systems. The discrepancy between the calculated radiocarbon and 3 H/ 3 He ages in the TMG and alluvial aquifers has been evaluated using a lumped parameter model that confirms that the TMG is strongly dominated by young groundwaters. Young 3 H/ 3 He residence times calculated for groundwater in the TMG (34–57 years) and alluvial aquifers (34–47 years) implies that these systems are most susceptible to reduced groundwater recharge. Given the interconnected nature of the aquifer systems, reduced recharge rates into the TMG aquifer will decrease groundwater flow to both the alluvial and MG aquifer and this will impact the long−term sustainability of the RAMSAR listed Verlorenvlei estuarine lake and wetlands at the catchment outlet. Future residence time studies should consider using a lumped parameter model (LPM) to calculate the age distribution of groundwater in these aquifers as this would provide additional insight into the sustainability of the groundwater systems.

Combining 14CDOC and 81Kr with hydrochemical data to identify recharge processes in the South German Molasse Basin

• Combination of 14 C DOC and 81 Kr provides new insight of recharge dynamics in the UJA. • Groundwater dating with 14 C DOC and 81 Kr proves to be useful for deep aquifers. • Groundwater dating revealed high spatial heterogeneity in groundwater ages. • The main recharge area of the UJA may be located at the southern border of the SGMB. • Glaciations may be the driving force for groundwater recharge. To face the challenges of ongoing climate change, renewable energy sources are needed to reduce CO 2 emissions to the atmosphere. The South German Molasse Basin (SGMB) contains an important geothermal aquifer in Germany; however, an enhanced understanding of the flow regime at the basin scale remains a fundamental limiting factor in assessing a long-term and sustainable strategy for using this green energy source. Here we report 14 C DOC and 81 Kr groundwater dating results together with hydrochemical data, stable water isotopes, noble gases and out-diffusion experiments that indicate that the thermal groundwater consists of at least two groundwater sources: a formerly unknown young groundwater component with apparent water ages between 9,800 and 18,700 a and an old groundwater component, which shows an apparent water age of up to approximately 300 ka. The results of the out-diffusion experiments with core material from the Upper Jurassic aquifer (UJA) suggest that diffusion processes between rock-matrix water and more mobile fracture water represent a controlling mechanism for groundwater flow and the observed extended apparent groundwater ages within the UJA. The observed elevated 14 C DOC activities and the high spatial heterogeneity in groundwater ages close to the southern margin of the SGMB were interpreted as a recharge zone of the system. The combination of the 14 C DOC and 81 Kr methods shows the potential as a powerful groundwater dating tool for deep aquifers.

Using helium-4, tritium, carbon-14 and other hydrogeochemical evidence to evaluate the groundwater age distribution: The case of the Neogene aquifer, Belgium

Apparent groundwater age dating has been proven useful and robust in understanding water origin and mixing processes, particularly when multiple tracers are considered. However, even though now extensively used, the age tracers have not been widely applied in the general practice of flow and transport model calibration. A multi tracer-study was carried out in the Neogene aquifer in Flanders to quantify the apparent age and construct a joint interpretation for the delineation of different groundwater flow systems. This understanding is critical as part of the safety and feasibility studies for the underlying Boom Clay Formation that has been considered as a potential host rock for the geological disposal of radioactive waste. In this study, we combine evidence from tritium/helium-3 (3H/3He), helium-4 (4He) and radiocarbon (14C) dating as well as stable isotopic (δ18O, δ2H) and hydrochemical signatures in combination with particle tracking-based age distributions from the 3D groundwater flow model. The results of the study indicate that mixing of groundwater with young and old fractions occurs predominantly in the central part of the aquifer which is made evident by the coexistence of 3H (pre and post-bomb pulse Era), 14C and 4He in several groundwater samples. The mixing between water of different origin is also supported by the sampled stable isotopic and hydrochemical composition of groundwater. Particle tracking residence time results show an acceptable agreement with apparent ages derived from age tracers for young (≤100 years) and old (>1000 years) groundwater. Groundwater with ages between 100 and 1000 years is likely a mixture of water with young/old fractions and shows the strongest discrepancies between advective model ages and age tracer based apparent ages. On the basis of our findings, we distinguish between three groundwater flow systems in the Neogene aquifer: i) a shallow/local flow system, with groundwater originating from modern meteoric water; ii) a deep/semi-regional flow system, characterized by old groundwater where the presence of 4Herad is significant; iii) a mixed zone of groundwater flow where the recently infiltrated meteoric water mixes with discharging old groundwater. These results have helped us to refine previously proposed conceptual models for the study area and will in the end reduce uncertainties relevant to the potential future geological disposal of radioactive waste.

Noble gases in shallow aquifers preserve signatures of boiling events beneath Weishan volcano of Wudalianchi volcanic field, northeast China

The study on whether there is a magma chamber under Wudalianchi is a hot topic recently, as it holds great significances for the theory of plate tectonics, early warning of volcano eruption and geothermal exploration. There is no near surface geothermal abnormality reported in Weishan, the widely used and well-studied tool, geochemistry of geothermal fluids and geothermal gases, cannot be applied to this area. To reveal the thermal conditions under Weishan, we conducted a study about the abundances and isotope ratios of dissolved noble gases in shallow groundwater and springs of Weishan. The concentrations of noble gases display obvious Ne excesses and mass-dependent differentiation, the isotopic ratios indicate that Ne, Ar, Kr, and Xe are atmosphere-derived noble gases (ANGs). We apply the closed-system equilibrium (CE) model to process the data of noble gases, and indicate that the excess ANGs are not caused by the increased hydrostatic pressure or diffusive influx. Neither meltwater mixing nor oxygen consumption alone could generate sufficient noble gas excesses in our samples. Based on the reconstructed abundances of noble gases in vapor phases produced at different boiling temperatures, which are calculated according to the boiling model established on Henry's law, we conclude that the abundances of noble gases in these samples were influenced by vapor–liquid partition caused by underground boiling. The diagrams of 84Kr/36Ar, 20Ne/36Ar and 130Xe/36Ar confine the temperature range where the boiling events occur in ∼100 °C–300 °C. We argued that the excess ANGs in shallow groundwater are geochemical evidence of boiling events, which seems to be continuously happening during the time range confined by the young groundwater in basalt aquifer and old groundwater in sandstone aquifer. Our findings also corroborate the recent results of geophysical survey works that a potential magma chamber exists under Weishan area. We suggest that noble gases dissolved in shallow aquifers represent a reliable geochemical tool to unveil subsurface geothermal state, where no near-surface thermal anomaly occurs.

Groundwater age dating using multi-environmental tracers (SF6, CFC-11, CFC-12, δ18O, and δD) to investigate groundwater residence times and recharge processes in Northeastern Ghana

The granitic aquifer of the area under investigation serves as an important source of water supply for the inhabitants. However, information about groundwater renewability and recharge process has not been defined. Multi-tracers were then applied to determine the residence times and recharge patterns of aquifer systems. In this study, sulphur hexafluoride (SF6) and chlorofluorocarbons (CFCs) were used to date shallow groundwater in Ghana for the first time. The tracer concentrations were measured in 38 groundwater wells from 38 sampling sites in Northeastern Ghana. A lump parameter model (LPM) describing the different age distributions reveals that the groundwater residence times are less than 50 years. The SF6 ages obtained were much younger than CFCs, an indication of terrigenous source enrichment. The young groundwater ages inferred from the dating process indicate modern recharge and, this collaborates with the results of the stable isotopes, in which there is a direct link between the isotopic composition of the rainfall and the groundwater. The results also indicate that the weathered and fractured granitic aquifer in the area enhances rapid and diffuse recharge after precipitation. In addition, a 3D numerical groundwater model has been developed to simulate groundwater flow under steady-state and obtain information on the key aquifer properties such as hydraulic conductivity and recharge rates. A comparison of the simulated and observed heads suggests a good match, and the developed model can satisfactorily describe groundwater flow in the area. A groundwater flow system has been delineated based on the multi-tracers (SF6, CFC-11, CFC-12, δ18O) and the numerical groundwater model. The findings of this study will be valuable for water managers and stakeholders to obtain relevant hydrogeological information for improved aquifer resource planning.

Behaviors of lithium and its isotopes in groundwater with different concentrations of dissolved CO2

Lithium (Li) isotopes have shown large potential in tracing weathering in various water bodies, but there is limited study on Li isotopes in subsurface conditions where CO2 has been largely consumed. In this study, we use a thick sandstone aquifer in the Ordos Basin, NW China, as a natural setting to investigate the behaviors of Li isotopes in hydrogeochemical conditions with different concentrations of dissolved CO2. For young groundwater in the recharge area (group R) where CO2 is abundant (mean PCO2 = 10-2.5 atm), clay formation accompanying with weathering leads to the enrichment of 7Li in groundwater. The four deep samples in the recharge area have uniform Li/Na ratios (with a mean of 2.52 μmol/mmol) and δ7Li (with a mean of 25.0‰), corresponding to a mean Li removal rate of 81.2% compared with the sandstone leachate. For groundwater in the shallow part of the discharge area (group D1), Li was firstly removed by clay formation during weathering in the recharge area and was later removed by physisorption when CO2 becomes much lower (mean PCO2 = 10-3.1 atm). Different degrees of weathering lead to a wide range of δ7Li varying from 19.7‰ in the deepest well to 33.0‰ in the shallowest well. The proportion of Li removal caused by physisorption is found to increase with groundwater age. After the stage of Li removal by adsorption, Li was released in the deeper part of the discharge area (group D2), and the positive correlation of δ7Li versus Li/Na is explained by a ternary mixing model. The endmember of water brought by cation exchange is inferred to have a heavier δ7Li than sandstone leachate, demonstrating that cation exchange could cause an enrichment of 7Li in water. This study enhances our understanding of the controlling factors of Li isotopes in deep groundwater with low dissolved CO2, which have implications for the application of Li isotopes in subsurface water.

Groundwater arsenic content in quaternary aquifers of the Red River delta, Vietnam, controlled by the hydrogeological processes

The relation between arsenic groundwater concentrations and hydrogeological processes was investigated in the proximal part of the Red River delta, Vietnam, west of the depression cone formed by the exploitation of groundwater in Hanoi. Flow paths in the Quaternary aquifers were modeled based on previously interpreted geological structure and hydrogeological data gathered during field work in 2014–2017. Sedimentary structures and simulated flow patterns were compared with the spatial distribution of the groundwater arsenic concentration.The regression of the sea in the area started 4 ka BP in the Holocene. The low tectonic subsidence rate of the Red River delta led to intensive erosion and replacement of fine grained sediments of the sea level high stand by sandy channel belts, resulting in hydraulic connections between the Pleistocene and Holocene aquifers. The Pleistocene aquifer is recharged by both regional flow paths and naturally occurring vertical recharge through Holocene sand and clay layers. Young groundwater (<40 a) in the shallow Holocene aquifer generally discharges to surface water bodies. The shallow flow system is also seasonally recharged with surface water, as indicated by δ18O enrichment of groundwater and oscillating groundwater ages in wells in the vicinity of water channels. The deeper flow system discharges into the Red River and Day River or flows parallel to the rivers, toward the sea.The overall pattern of arsenic groundwater concentrations (decreasing with increasing sediment age) is modified by groundwater flow. The arsenic contamination of the Pleistocene aquifer of the Red River delta is not only caused by the intensive groundwater abstraction in Hanoi, as reported previously, but also by the natural flow of high arsenic groundwater from Holocene to Pleistocene aquifers in areas located outside of the depression cone. Groundwater with < 50 µg L-1 arsenic is found in the Pleistocene aquifer close to the recharge zone in the mountains bordering the Red River delta and in the Holocene and Pleistocene aquifers where clay deposits were eroded. Close to the recent Red River channel, recharge of older Holocene and Pleistocene sediments occurs partially by arsenic-contaminated groundwater from the youngest Holocene aquifers, and here arsenic concentrations exceed 50 µg L-1. A high arsenic concentration is also present in the early Holocene-Pleistocene aquifer, beneath thick clay layers, indicating a limited extent of flushing and the inflow of fresh organic matter.