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Understanding connectivity between the soil and deeper bedrock groundwater is needed to accurately predict a watershed's response to perturbation, such as drought. Yet, the bedrock groundwater dynamics in mountainous environments are typically under-constrained and excluded from watershed hydrologic models. Here, we investigate the role of groundwater characterized with decadal and longer water ages on the hydrologic and mass-transport processes within a steep snow-dominated mountain hillslope in the Central Rocky Mountains (USA). We quantify subsurface and surface water mass-balance, groundwater flowpaths, and age distributions using the ParFlow-CLM integrated hydrologic and EcoSLIM particle tracking models, which are compared to hydrometric and environmental tracer observations. An ensemble of models with varied soil and hydrogeologic parameters reproduces observed groundwater levels and century-scale mean ages inferred from environmental tracers. The numerical models suggest soil water near the toe of the hillslope contains considerable (>60 % of the mass-flux) contributions from bedrock flowpaths characterized with water ages >10 years. Flowpath connectivity between the deeper bedrock and soil systems is present throughout the year, highlighting the potentially critical role of groundwater with old ages on processes such as evapotranspiration and streamflow generation. The coupled numerical model and groundwater age observations show the bedrock groundwater system influences the hillslope hydrodynamics and should be considered in mountain watershed conceptual and numerical models.

Groundwater is a critical resource in the Upper Chao Phraya basin, providing consistent water supplies for agricultural, domestic, and industrial activities, especially during the dry season. This study utilized tritium and carbon-14 dating techniques to investigate groundwater age, analyzing 273 samples collected in 2021 from various wells and depths across the basin. Tritium and carbon-14 were measured using liquid scintillation counting (LSC). The results revealed a wide range of groundwater ages, including recently recharged water with tritium concentrations up to 2.4 TU, corresponding to groundwater ages ranging from a few months to 44.17 years BP (Before Present), with an average of 18.26 years BP. Older groundwater was identified with carbon-14 concentrations as low as 3.22 pMC, indicating ages of up to 22,899 years, with a mean age of 6687 years BP. Correlation analysis showed a positive relationship between tritium and carbon-14 concentrations (r = 0.52). Spatial distribution patterns indicated that tritium concentrations were higher in northern and mountainous areas, identifying these as critical recharge zones. In contrast, lower carbon-14 concentrations in the central and southern areas suggested the presence of older groundwater, emphasizing the need for careful management of these ancient water reserves. The spatial variation in tritium and carbon-14 concentrations highlights differences in groundwater circulation and recharge patterns, enabling the identification of key recharge zones in the northern and highland regions. This highlights the importance of conserving these areas from pollution and over-extraction. The presence of old groundwater in the central and southern areas further emphasizes the need for ongoing monitoring to sustainably manage these long-term water resources. This study enhances the understanding of groundwater dynamics in the Upper Chao Phraya basin and provides valuable insights for improving water resource management strategies. [ABSTRACT FROM AUTHOR]

The age of shallow groundwater in Shenyang-Fushun section of Hunhe River Basin is calculated with 3H isotopic method. Through the age zoning of groundwater and analysis of its circulation characteristics, the paper provides a scientific basis for the rational development and utilization of groundwater resources and study on carrying capacity of urban resources and environment. The results show that the groundwater in the Shenyang-Fushun section is mainly the modern water replenished in the past 60 years, and the age of groundwater flow increases gradually from the eastern mountainous area to the western plain. The distribution area of groundwater aged less than 30 a, 30-60 a and more than 60 a account for 34%, 31% and 35% of the total of the study area, respectively. The upper reaches of the basin are hilly and mountainous area, with single aquifer structure, coarse particles, groundwater age less than 30 a and strong groundwater circulation. The aquifer in the middle reaches thickens gradually, the alternation of groundwater circulation weakens gradually, and the groundwater age increases to 30-60 a. The aquifer in the lower reaches gradually changes from single layer to double layer and multilayer, with fine particles and slow water circulation, and the groundwater age increases to more than 60 a. [ABSTRACT FROM AUTHOR]

The volcanic aquifer of Bilate River Basin (BRB) situated on the southwestern side of the Main Ethiopian Rift (MER) serves as the sole reservoir of water supply for urban and rural people found in the basin. This study investigates residence time and groundwater recharge process in the Bilate River Basin. For the first time, this research work delivers information on the resident time and groundwater recharge on the basis of multi-tracer approach (CFCs, Cl, δ18O and δD). The δ18O and δD values of groundwater are drawn along or close to local Addis Ababa Meteoric Water Line (AAMWL), indicating that groundwater recharge is of meteoric origin. CFC data show that the oldest component of groundwater was recharged before 1950, while the younger component recharged in different time as of 1950. The assigned apparent age for groundwater range from 24 to more than 50 years. Binary mixing model is used to estimate the age and fraction of the younger component. Mixing model suggest that groundwater consists young groundwater that mainly recharged the aquifer between the mid-1980s and mid-1990s. The proportion of young water in the groundwater shows decreasing trend from highland towards the rift indicating the main recharge area is the highland part. The chloride mass balance employed to Bilate River Basin calculated yearly average groundwater recharge to be 217 mm/y which accounted for 20% of the mean annual areal rainfall. The estimated recharge decrease from highland to the rift floor highlighting the importance of preferential flow recharge mechanism. The analysis of apparent age, young proportion and recharge rate, can aid in understanding complex groundwater flow system and residence time of groundwater in the complex hydrogeological setting of rift volcanic aquifer. [ABSTRACT FROM AUTHOR]

Groundwater-resource quality is assumed to be less responsive to drought compared to that of surface water due to relatively long transit times of recharge to drinking-supply wells. Here, we evidence dynamic perturbations in aquifer pressure dynamics during drought and subsequent recovery periods cause dramatic shifts in groundwater quality on a basin scale. We used a novel application of time-series clustering on annual nitrate anomalies at >450 public-supply wells (PSWs) across California's San Joaquin Valley during 2000-22 to group subpopulations of wells with similar water-quality responses to drought. Additionally, we statistically evaluated the direction and magnitude of multi-constituent water-quality changes across the San Joaquin Valley using a broader dataset of >3000 PSWs with data during two select hydrologic stress periods representing an extreme drought (2012-16) and subsequent recovery (2016-19). Results of time-series clustering and stress-period change analyses corroborate a predominant regional response to pumping stress characterized by increased concentrations of anthropogenic constituents (nitrate, total dissolved solids) and decreased concentrations of geogenic constituents (arsenic, fluoride), which largely reversed during recovery. Cluster analysis also identified a secondary, less commonly occurring group of PSWs where nitrate decreased during drought, but explanatory factor analysis was not able to discern hydrogeologic drivers for these two divergent response patterns. Long-term tracer data support the hypothesis that the predominant regional signal of nitrate increase during drought is caused by enhanced capture of modern-aged groundwater by PSWs during periods of pumping stress, which can drive rapid changes in water quality on seasonal and multiannual timescales. Pumping-induced migration of modern, oxic groundwater to depth during drought may affect geochemical conditions in deeper portions of regional aquifers controlling the mobility of geogenic contaminants over the long term. [ABSTRACT FROM AUTHOR]

The western U.S. is experiencing shifts in recharge due to climate change, and it is currently unclear how hydrologic shifts will impact geochemical weathering and stream concentration–discharge (C–Q) patterns. Hydrologists often use C–Q analyses to assess feedbacks between stream discharge and geochemistry, given abundant stream discharge and chemistry data. Chemostasis is commonly observed, indicating that geochemical controls, rather than changes in discharge, are shaping stream C–Q patterns. However, few C–Q studies investigate how geochemical reactions evolve along groundwater flowpaths before groundwater contributes to streamflow, resulting in potential omission of important C–Q controls such as coupled mineral dissolution and clay precipitation and subsequent cation exchange. Here, we use field observations—including groundwater age, stream discharge, and stream and groundwater chemistry—to analyse C–Q relations in the Manitou Experimental Forest in the Colorado Front Range, USA, a site where chemostasis is observed. We combine field data with laboratory analyses of whole rock and clay x‐ray diffraction and soil cation‐extraction experiments to investigate the role that clays play in influencing stream chemistry. We use Geochemist's Workbench to identify geochemical reactions driving stream chemistry and subsequently suggest how climate change will impact stream C–Q trends. We show that as groundwater age increases, C–Q slope and stream solute response are not impacted. Instead, primary mineral dissolution and subsequent clay precipitation drive strong chemostasis for silica and aluminium and enable cation exchange that buffers calcium and magnesium concentrations, leading to weak chemostatic behaviour for divalent cations. The influence of clays on stream C–Q highlights the importance of delineating geochemical controls along flowpaths, as upgradient mineral dissolution and clay precipitation enable downgradient cation exchange. Our results suggest that geochemical reactions will not be impacted by future decreasing flows, and thus where chemostasis currently exists, it will continue to persist despite changes in recharge. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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Study region: A 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 focus: Hydrogeochemical 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 region: Stable 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.

Groundwater is a critical resource in the Upper Chao Phraya basin, providing consistent water supplies for agricultural, domestic, and industrial activities, especially during the dry season. This study utilized tritium and carbon-14 dating techniques to investigate groundwater age, analyzing 273 samples collected in 2021 from various wells and depths across the basin. Tritium and carbon-14 were measured using liquid scintillation counting (LSC). The results revealed a wide range of groundwater ages, including recently recharged water with tritium concentrations up to 2.4 TU, corresponding to groundwater ages ranging from a few months to 44.17 years BP (Before Present), with an average of 18.26 years BP. Older groundwater was identified with carbon-14 concentrations as low as 3.22 pMC, indicating ages of up to 22,899 years, with a mean age of 6687 years BP. Correlation analysis showed a positive relationship between tritium and carbon-14 concentrations (r = 0.52). Spatial distribution patterns indicated that tritium concentrations were higher in northern and mountainous areas, identifying these as critical recharge zones. In contrast, lower carbon-14 concentrations in the central and southern areas suggested the presence of older groundwater, emphasizing the need for careful management of these ancient water reserves. The spatial variation in tritium and carbon-14 concentrations highlights differences in groundwater circulation and recharge patterns, enabling the identification of key recharge zones in the northern and highland regions. This highlights the importance of conserving these areas from pollution and over-extraction. The presence of old groundwater in the central and southern areas further emphasizes the need for ongoing monitoring to sustainably manage these long-term water resources. This study enhances the understanding of groundwater dynamics in the Upper Chao Phraya basin and provides valuable insights for improving water resource management strategies.

The depth of active groundwater circulation is a fundamental control on stream flows and chemistry in mountain watersheds, yet it remains challenging to characterize and is rarely well constrained. We collected hydraulic conductivity, hydraulic head, temperature, chemical, noble gas, and 3H/3He groundwater age data from discrete levels in two boreholes 46 and 81 m deep in an alpine watershed, in combination with chemical and age data from shallow groundwater discharge, to discern groundwater flow rates at different depths and directly observe active and inactive groundwater. Vertical head gradients are steep (average of 0.4) and thermal profiles are consistent with typical linear conductive continental geotherms. Groundwater deeper than ∼20 m is distinct from shallow groundwater and creek water in its chemistry, noble gas signature, and age (dominantly >65 years compared to

Krypton-81 was applied to investigate the age of groundwater in the aquifer system in the Bangkok metropolitan and vicinity areas. Stable (2H, 18O and 13C) and radioactive (3H, 85Kr and 14C) isotopes and noble gases were applied in parallel. Low levels of 14C and significant radiogenic 4He confirm that groundwater in the deep aquifers is older than 30 ka. 81Kr analysis identified groundwater with ages ranging from 17 to 300 ka. At some sites, large age discrepancies between 81Kr and 14C indicated that inter-aquifer mixing is likely occurring. The interpretation of the noble gases suggests that groundwaters in the deeper aquifers, with apparent ages of 300 to 10 ka, have recharged in slightly colder and wetter climates than those found in the upper aquifers with apparent ages < 10 ka. Degradation of water quality from seawater intrusion was identified in the upper four aquifers. This was also evidenced by higher δ18O and δ2H values, typical of seawater. The four deeper aquifers contain high quality water characterised by less enriched 18O and 2H. This work presents new findings of very old groundwater in the Bangkok aquifer system. [ABSTRACT FROM AUTHOR]

The complex Krupaja karst system, located in Serbia, which consists of a strong karst spring, a thermal karst spring, and a borehole that captures sub-thermal waters, has been studied. A phenomenon which gives rise to the occurrence of karst waters of three different temperatures within a relatively short distance (about 200 m) led us to assume that these karst waters are formed within the same karst aquifer and circulate through different levels of karst channels that are zonally distributed within a vertical profile; this was confirmed by assessing the groundwater ages. Based on the development of karstification, as presented in this paper, as well as on previous investigations, explanations of four types of circulation within same karst system are provided. The results of measurements of stable isotopes (H, 18O, and 13C) and radioactive isotopes of tritium and T+He and 14C, as well as hydrochemical water analyses, speleogenetic spatial modeling, speleodiving research, etc., confirmed the assumption that these groundwaters belong to the same karst system. However, these karst waters have different residence times, i.e., from only a few days to over several decades and even up to approximately 4000 years, circulating through channels with various distributions along the depth of the karst aquifer. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Groundwater Hydrology is the property of Japanese Association of Groundwater Hydrology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Management of groundwater quality is assisted by an understanding of reference conditions, which describe the concentration ranges expected for key substances in the absence of human impact. This study evaluates reference conditions for NO3–N in New Zealand groundwater based on three complementary methods: hierarchical cluster analysis, relationships to groundwater age, and regression against a measure of land-use impact. The three methods result in very similar national-scale estimates of reference conditions for NO3–N concentration in oxic, minimally impacted groundwater, with the 80th, 90th and 95th percentiles found to be 1.65 ± 0.12, 1.97 ± 0.14 and 2.32 ± 0.14 mg/l, respectively (weighted average ± 95% confidence level), in good general agreement with previous studies from New Zealand and overseas. Anoxic groundwaters were treated separately for definition of reference conditions, with the 80th and 90th percentiles of NO3–N found to be 0.04 ± 0.01 and 0.16 ± 0.01, respectively (the 95th percentile could not be estimated reliably). For both oxic and anoxic groundwater, where a site-specific investigation has not been conducted to estimate reference conditions at a local scale, we suggest that the 80th percentile is an appropriate national-scale default threshold, to match the thresholds used for surface waters under the Australian and New Zealand Guidelines for Fresh and Marine Water Quality. [ABSTRACT FROM AUTHOR]

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As a part of Hexi Corridor in Gansu Province, the Zhangye Basin is located in the middle reaches of the Heihe River Basin, with high topography in the southeast and low topography in the northwest. The existing exploration data show that the Zhangye basin is rich in hydrothermal geothermal resources. Based on the study of geophysical exploration, geothermal drilling, geo-temperature measurement and hydrogeochemistry, this paper analyzes the occurrence characteristics of geothermal resources and discusses the genetic mode in Zhangye Basin. The geothermal field in the Zhangye basin belongs to the sedimentary basin type of low-medium temperature. The geothermal reservoir is composed of sandstone and glutenite of Neogene Baiyanghe Formation with layered distribution. Temperature of the geothermal reservoir ranges from 47 to 82 °C calculated by using the potassium magnesium geochemical temperature standard. The caprock consists of mudstone of Neogene Shulehe Formation and Quaternary unconsolidated sediments. The geothermal water type is mainly clastic pore water. The characteristics of hydrogen and oxygen isotope indicate that the main recharge source is atmospheric precipitation in the southern Qilian Mountains. The deep faults in the northern margin of Qilian Mountains and NNW-trending basement faults in the basin are good conduits for deep circulation of the geothermal fluids. After receiving recharge, groundwater migrates along the water-conducting fault zones or rock pore fractures. Heating by the deep heat conduction, it occurs in the pores of clastic rocks and forms geothermal resources in this area. The results of hydrochemical analyses show that the geothermal water in this area belongs to continental sedimentary water containing dissolved water in rock salt formation, and the hydrochemical type is of Cl·SO4—Na. The contents of fluorine, SiO2, total dissolved solids and total hardness increase with the increasing water temperature. The tritium value of hot water in the area is generally less than 2.0 TU, indicating that the formation age is relatively early. The results of carbon-14 analysis further confirm that the formation age of the regional geothermal water is more than 20,000 years, reflecting the characteristics of a long geothermal fluid supply path and slow runoff. The research results can provide important reference for exploration and utilization of geothermal resources in the Zhangye Basin.

The karst groundwater flow system in the ejective folds area in the eastern Sichuan Basin shows the characteristics of a hierarchically nested structure. However, previous studies were mainly focused on hydrogeological conditions analysis, lacking qualitative evidence directly from hydrochemistry. In this study, the authors collected and sorted the chemistry data of representative karst springs and boreholes in the study area, namely, 41 shallow karst waters and 19 deep ones. The Pipper chart shows that the shallow karst water is HCO3-Ca·Mg and low Mg2+ concentration, while the deep karst water is SO4-Ca·Mg and high Mg2+ concentration. For both shallow and deep karst groundwater, the analysis of the groundwater flow system of a typical profile shows that the Mg2+ concentration value can be used to evaluate the groundwater residence time. The shallow karst groundwater system is highly controlled by geomorphology, among which the shallow incised valley springs are the discharge points of the local groundwater flow system. However, springs in deep gullies correspond to intermediate or even regional flow systems. This pattern is verified by the groundwater residence time evaluation from the Mg2+ concentration, which also reflects the movement characteristics of deep-circulation groundwater flow revealed by hot springs and boreholes.The hydrochemical characteristics can effectively indicate the pattern of groundwater flow systems. This paper provides a reference for identifying the hierarchically nested structure of groundwater flow systems in similar karst areas.

Copyright of Hydrogeology Journal is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Hydrogeology Journal is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Hydrogeology Journal is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Groundwater dynamics near the saltwater–freshwater interface were investigated in an island of the Seto Inland Sea, using multiple tracers (δD, δ18O, Cl−, SF6, and 14C) at two coastal groundwater monitoring wells at depths of 10–40 m. The groundwater recharge area and age were also estimated using these tracers. Additionally, bedrock groundwater at a depth of 40 m at the 2.7 m altitude was brackish and considered to be near the saltwater–freshwater interface, and a mixture of seawater (2–3.5%) and fresh groundwater (97–98%) was estimated by the Cl− concentration. Based on the δ18O of fresh groundwater estimated from the seawater mixing ratio, the recharge area was estimated to range from near to above the summit; however, this region is unlikely to be the actual recharge area, as the groundwater may be old freshwater that was recharged during a previously colder period. Groundwater dating using SF6 and 14C suggests that the fresh groundwater originated during the last glacial period (assumed 20,000 years ago) and that the 40 m deep bedrock groundwater is a mixture of old water (0–28%), 30 m deep groundwater (76–100%), and stagnant seawater (1–3%). [ABSTRACT FROM AUTHOR]

Copyright of Hydrogeology Journal is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

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.

The crystalline basement aquifers host significant groundwater resources for the communities in Vea catchment for public and irrigational water supply. As part of measures to understand the hydrogeological processes of the area, multi-tracers (SF6, CFC-12, CFC-11, 3H, δ18O and δ2H) were employed to elucidate the groundwater residence time, recharge mechanism and mixing process of the shallow groundwater. Ambient concentrations of these tracers were taken from 10 sampling locations in the Vea catchment of Upper East Region of Ghana. A purge and trap method was used to take the samples and electron capture detector gas chromatography (GC-ECD) was used to analytically measure the SF6 and CFCs gases in the groundwater. The SF6, CFC-11 and CFC-12 dating complimented with 3H revealed that the groundwater age in the catchment was relatively young ranging from 5 to 42 years with a mean age of 23 years. This corroborates well with the stable isotopes data , which showed that the catchment receives modern recharge from meteoric origin. The results also indicated that the estimated SF6 ages were younger compared to the CFCs ages, suggesting additional inputs of SF6 from terrigenous sources. Similarly, a comparison of 3H and CFCs showed little variations in ages in some wells, a possible indication of CFCs degradation. The conceptual groundwater flow model developed showed three groundwater flow regimes along the topographical line. Despite some uncertainties in age interpretations, the multi-environmental tracers have been successfully used to date shallow groundwater in the Vea catchment. To reduce the uncertainties in the age determination, the use multi tracer approach is highly recommended.

• Nitrate concentrations in groundwater and surface water in Dorset (UK) were studied. • Both seasonal and event-based variations were found in groundwater and surface water. • Nitrate peaks in springs and streams lag groundwater nitrate peaks by days to weeks. • Lag times reflect celerity and velocity of preferential flow in the chalk aquifer. Short-term nitrate variations in streams are common, but the extent of groundwater contributions to such variability has been unclear. Analysis of well, spring and stream nitrate concentrations plus well water levels and stream flow for a chalk aquifer in Dorset (UK) shows that nitrate variation in streams lags discharge peaks by days to weeks. Spring monitoring also shows a short lag time, showing that high nitrate concentrations are rapidly transmitted from aquifers to streams. The lag time reflects the difference between the celerity (pressure wave speed) and the advective velocity of groundwater flow through the fracture network. Results give celerities of ∼500 m/d, advective velocities of 100–200 m/d, and kinematic porosities of ∼0.001. In addition, long-term nitrate data gives time lags of several decades, reflecting the matrix porosity of ∼0.3. Consequently, nitrate can be used an environmental tracer for both preferential flow through fracture networks and for retarded matrix flow and storage in this dual-porosity bedrock aquifer. Dual porosity is common in bedrock aquifers, so it is possible that this methodology may be widely applicable to understand the hydraulic characteristics of bedrock aquifers and to explain short-term nitrate variation in streams. [ABSTRACT FROM AUTHOR]

This paper examines the influence of simplified vertical discretization using 50- to four- layer models and ensemble size on history matching and predictions of groundwater age for a national scale model of New Zealand (approximately 265,000 km2). A reproducible workflow using a combination of opensource tools and custom python scripts is used to generate three models that use the same model domain and underlying data with only the vertical discretization changing between the models. The iterative ensemble smoother approach is used for history matching each model to the same synthetic dataset. The results show that: 1) the ensemble based mean objective function is not a good indicator of model predictive ability, 2) predictive failure from model structural errors in the simplified models are compounded by history matching, especially when small (

We combine physics‐based groundwater reactive transport modelling with machine‐learning techniques to quantify hydrogeological model and solute transport predictive uncertainties. We train an artificial neural network (ANN) on a dataset of groundwater hydraulic heads and 3H concentrations generated using a high‐fidelity groundwater reactive transport model. Using the trained ANN as a surrogate model to reproduce the input–output response of the high‐fidelity reactive transport model, we quantify the posterior distributions of hydrogeological parameters and hydraulic forcing conditions using Markov chain Monte Carlo calibration against field observations of groundwater hydraulic heads and 3H concentrations. We demonstrate the methodology with a model application that predicts Chlorofluorocarbon‐12 (CFC‐12) solute transport at a contaminated field site in Wyoming, United States. Our results show that including 3H observations in the calibration dataset reduced the uncertainty in the estimated permeability field and infiltration rates, compared to calibration against hydraulic heads alone. However, predictive uncertainty quantification shows that CFC‐12 transport predictions conditioned to the parameter posterior distributions cannot reproduce the field measurements. We found that calibrating the model to hydraulic head and 3H observations results in groundwater mean ages that are too large to explain the observed CFC‐12 concentrations. The coupling of the physics‐based reactive transport model with the machine‐learning surrogate model allows us to efficiently quantify model parameter and predictive uncertainties, which is typically computationally intractable using reactive transport models alone. [ABSTRACT FROM AUTHOR]

Delingha is located in the northeast margin of Qaidam Basin. Bayin River alluvial proluvial fan is the main aquifer of Delingha, in which groundwater generally flows from north to south. The hydrochemistry results showed that two different hydrochemical evolution paths formed along southeast and southwest directions, respectively. Cl-Na type groundwater was formed in front of Gahai Lake, and SO4·HCO3-Na·Ca type groundwater was formed in front of Keluke Lake. The results of deuterium (D) and 18O revealed that the groundwater mainly originated from the continuous accumulation of precipitation during geological history under cold and humid climate conditions. In addition, results of 14C indicated that the groundwater age was more than 1140 years, implying relatively poor renewal capability of regional groundwater. Moreover, our numerical modeling results showed that the regional groundwater level will continue to rise under the warm and humid climate conditions.© 2021 China Geology Editorial Office.

In order to study the groundwater flow system and effective groundwater management in Turpan Basin, a total of 57 samples including 8 spring samples and 8 river samples were collected for isotopic compositions analysis. Based on stable isotopes, the groundwater flow system in Turpan Basin can be divided into several secondary flow systems. Groundwater in the northern sub-basin flows from north to south and recharges southern sub-basin through the bedrock gap of the Flaming Mountain. In the southern sub-basin, groundwater flows from all sides to Ayding Lake which is the lowest point of the basin. Surface water has δ18O − 11.3 to − 8.7‰ and δ2H − 78 to − 56‰. The isotopic compositions of the groundwater samples collected vary from –11.5 to 6.9‰ for δ18O and from –85 to 7‰ for δ2H. The groundwater 14C value is relatively low (22.94–32.92) in the middle part of the southern sub-basin, which indicates groundwater is older; while the groundwater 14C value is relatively high (76.65–77.94) in the eastern part of the northern sub-basin, which indicates the groundwater is younger. As groundwater flows toward south, groundwater becomes older. The Turpan Basin can be divided into three main groundwater flow systems. One is located in the northern sub-basin, where groundwater is recharged from the northern mountainous areas, and flows toward south. The other two are located in the southern sub-basin. Under natural conditions, groundwater on the east and west sides of southern sub-basin drains to Aiding Lake which locates in the center of the basin. In Turpan city, where human activities are concentrated, a variety of water sources have been drawn from other areas for living and irrigation, including river water, groundwater and overflow spring water. These mixed water in Turpan city recharges groundwater by infiltration, which strengthens groundwater vertical movements. [ABSTRACT FROM AUTHOR]

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Hydrogeological barriers can significantly impact groundwater model predictions. They are, however, often excluded from, or misrepresented in, groundwater models if their presence is unknown or their properties are poorly constrained. Here we show that sharp barriers can be included in groundwater model inversion, even where their presence is uncertain. We describe an approach utilizing "phantom structures"—randomly located, linear groups of model cells assigned a unique hydraulic conductivity value—to improve identifiability of barriers. Algorithmic parameter estimation using model‐independent parameter estimation code (PEST) is implemented to determine model structures that best match the hydraulic head and groundwater age observation data from a hypothetical aquifer. Our results show that for a series of case studies, this method was successful in inferring the appropriate location and properties of hydrogeological barriers, when that barrier was not aligned with the dominant flow direction. We compare these results to model inversion using traditional pilot points. The phantom structures approach shows promise in identifying hydrogeological structures and in reproducing groundwater flow across a model domain. Our results demonstrate that the geometric properties of geological structures can remain flexible in a model inversion. This is a step toward reducing conceptual model uncertainty where the presence and properties of hydrogeologic barriers are undefined. Plain Language Summary: Understanding the geological characteristics of a region can be very challenging. This is particularly true if the region contains narrow, linear structures known as faults and dykes. However, these structures can be important when it comes to defining the flow of groundwater. Many faults and dykes obstruct groundwater flow, so may inhibit groundwater availability to rivers and ecosystems or to other groundwater users. In this paper, a methodology is introduced that uses commonly collected hydrogeological datasets (known as hydraulic head and groundwater age) to discover the location, shape and physical properties of certain geological structures in the subsurface. The method relies on "inverse approaches", where data observed in the field is compared to a modeled equivalent. This method of incorporating narrow, linear structures in groundwater models results in a better match between field and modeled data when compared to another approach that relies on representing faults and dykes as broader, flatter, smoother geological anomalies. Key Points: A method for including hydrogeological barriers in model inversion is developed using a regularization device we call phantom structuresNarrow, low‐permeability structures such as faults and dykes are included in model inversions with no prior knowledge of their geometriesInverting models with phantom structures provide improved results compared to using pilot points for a wide range of conditions [ABSTRACT FROM AUTHOR]

This study presents an almost entirely unpublished dataset of 121 samples of groundwater analyzed for helium concentration and its isotopic ratio (3He/4He) in two adjacent watersheds of the St. Lawrence Lowlands, in a region with intensive agricultural activities in the southern Québec Province, Eastern Canada. Most of the samples were collected in the regional bedrock fractured aquifer hosted in mid-Ordovician siliciclastic shales, on a total surface of 7500 km2. Even with this low-density sampling, and in a heterogeneous and fractured aquifer, the helium isotopes bring precious information on the recharge conditions and on chemical evolution of water. The helium spatial interpolation does not show a clear isotopic gradient through the basin. However, it shows progressive enrichment of radiogenic 4He in the confined part of the aquifer. The atmospheric and/or tritiogenic-rich helium occurs at the recharge in the Appalachians and in the middle of the plain, where impermeable cover is limited, and local infiltration of meteoric freshwater reaches the bedrock aquifer. The relation between the total dissolved solids (TDS) and 3He/4He ratios remains elusive. However, on discriminating the samples with the dominant chemistry of water, a clear trend is observed with 3He/4He ratio, suggesting that radiogenic 4He accumulates together with dissolved solids and with increasing time (indicated by progressively older 14C ages). Finally, the noble gas temperatures (NGTs) obtained from concentrations of the other noble gases (Ne, Ar, Kr and Xe) brings constraints on the earlier recharge conditions during the Holocene. Particularly, the NGTs showed that the studied aquifers were continuously replenished, even under ice-sheet cover in the last 10,000 years. [ABSTRACT FROM AUTHOR]

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 CO 2 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 CO 2. For young groundwater in the recharge area (group R) where CO 2 is abundant (mean P CO2 = 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 CO 2 becomes much lower (mean P CO2 = 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 CO 2 , which have implications for the application of Li isotopes in subsurface water. [ABSTRACT FROM AUTHOR]

• A new groundwater tritium and carbon isotope dataset is established for China. • The time series of the precipitation 3H and atmospheric 14C are reconstructed for China. • The depth profiles of modern groundwater and fossil groundwater in China are presented. • In China, fossil groundwater is dominated (>50 %) in most wells (>50 %) below 300 m. • In China, precipitation after 1953 can hardly recharge groundwater below 420 m. Older groundwater is generally more susceptible to overdraft and also more difficult to recover once contaminated. To understand the prevalence and vulnerability of fossil groundwater infiltrated before Holocene, a global assessment has been conducted recently, but the analysis results for China have been limited by the collection of data. In this study, we combined the groundwater isotope data from the literature databases of CNKI (China national knowledge infrastructure) and SCIE (Science citation expanded) and established the groundwater age dataset of China by virtue of the water samples from 1441 wells. By reconstructing the historical precipitation 3H records of China and using the late-Quaternary atmospheric 14C, the fractions of the modern groundwater that infiltrated after the year 1953 and the fossil groundwater that infiltrated before 12,000 years in the groundwater samples have been calculated based on the straight-forward mixing models. Then we evaluated the extent and depths of the modern and fossil groundwater in the major 16 groundwater-supply areas of China. The results show that the exploitation of deep groundwater mainly occurs in northern China, such as in the Ordos Basin, the North China Plain, and the Hexi corridor, etc. The average critical depth of the vertical transition zone between modern infiltration water and fossil groundwater in China is about 300 m, beneath which more than half of the water samples are dominated by fossil groundwater. Meanwhile, at depth greater than 420 m, the groundwater can rarely be recharged by the modern precipitation. Besides, in China, modern and fossil groundwater, respectively, share 15–45 % and 20–85 % of the total groundwater storage in the uppermost 1000 m of the crust. [ABSTRACT FROM AUTHOR]

Groundwater dynamics near the saltwater–freshwater interface were investigated in an island of the Seto Inland Sea, using multiple tracers (δD, δ18O, Cl−, SF6, and 14C) at two coastal groundwater monitoring wells at depths of 10–40 m. The groundwater recharge area and age were also estimated using these tracers. Additionally, bedrock groundwater at a depth of 40 m at the 2.7 m altitude was brackish and considered to be near the saltwater–freshwater interface, and a mixture of seawater (2–3.5%) and fresh groundwater (97–98%) was estimated by the Cl− concentration. Based on the δ18O of fresh groundwater estimated from the seawater mixing ratio, the recharge area was estimated to range from near to above the summit; however, this region is unlikely to be the actual recharge area, as the groundwater may be old freshwater that was recharged during a previously colder period. Groundwater dating using SF6 and 14C suggests that the fresh groundwater originated during the last glacial period (assumed 20,000 years ago) and that the 40 m deep bedrock groundwater is a mixture of old water (0–28%), 30 m deep groundwater (76–100%), and stagnant seawater (1–3%).

The Okaloosa darter (Etheostoma okaloosae) is a diminutive, perch-like, benthic fish that inhabits only six small, clear, and shallow creek systems that flow almost entirely within Eglin Air Force Base in the panhandle of northwest Florida. Listed as Endangered by the U.S. Fish and Wildlife Service (USFWS) in 1973, improvements in erosion control and habitat restoration led to the Okaloosa darter being downlisted from Endangered to Threatened in 2011. However, the long-term management of the species is hampered by the lack of knowledge of the spatial extent of the recharge areas that ultimately support creek flow through groundwater discharge. To address this lack of data, we collected groundwater samples from the sand and gravel aquifer beneath 11 headwater and 11 downgradient sites across six creek basins during February and December 2020. The groundwater samples were collected from 1 to 1.2 m beneath the creek bottom. Concentrations of sulfur hexafluoride (SF6) were analyzed and used to calculate groundwater age (residence time), and indicated that at the 11 headwater sites, recharge occurred between 11 and 28 years ago. Groundwater ages in downgradient parts of the same creeks indicated that recharge occurred between 5 and 25 years ago. When combined with representative values of hydraulic conductivity for the sand and gravel aquifer, the ages reveal that the extent of the maximum recharge distance from the sampling sites ranged from about 222 to 2011 m from the creeks. This new information can be used by natural resource managers as additional evidence to support the USFWS Recovery Plan and proposed delisting of the Okaloosa darter from the Endangered Species List. Moreover, these results may also be useful to fisheries biologists to incorporate groundwater inputs to facilitate fisheries management. [ABSTRACT FROM AUTHOR]

Sustainable management of groundwater relies on robust estimates of aquifer fluxes. As such, recharge can be estimated from groundwater age using analytical models assuming a homogeneous aquifer. We propose to assess how this assumption affects recharge estimates in fluvial deposits. First, we use CHILD, a landscape evolution model, to generate plausible deposits after tens of thousands of years of river evolution. Then, we use the fractional packing model to compute porosity and permeability of unconsolidated sediments assuming a mixture of a coarse and a fine fraction. Finally, we use PFLOTRAN to simulate groundwater flow and mean age over two thousand years of spatially uniform recharge. This reproduces the boundary conditions underlying Vogel's analytical model to estimate recharge rates. Our results are based on 20,000 realizations from varying inputs: grain sizes, aggradation rate of the river, porosity, recharge, etc. For most models, the mean estimate of the recharge rate remains below an absolute error of 25%. But fluvial heterogeneities can lead to local errors ranging from close to zero to several thousand percent, which means that estimates may vary drastically depending on sampling locations. A sensitivity analysis using the δ‐importance measure and CUSUNORO curves exposes how fluvial processes influence the range and distribution of the error. For instance, a high bank erodibility and small coarse grain size under low aggradation rates favor the reworking of deposits, which increases heterogeneity and leads to high errors. This work paves the way for a more reliable estimation of recharge by considering the sedimentary context. Plain Language Summary: Despite being hidden underground, groundwater is a major source of freshwater worldwide. But population growth and global warming put an ever‐increasing stress on this vital resource. Its sustainability depends on reliable estimates of the amount of water replenishing groundwater systems. One way to estimate this recharge uses groundwater age: the time since a parcel of water entered a system. It assumes a homogeneous system, so the rocks or sediments bearing the water must have the same properties everywhere. This assumption is unlikely to be met, so we created virtual aquifers to assess the impact of geological heterogeneities on recharge estimates. We started by simulating the evolution of meandering rivers and their deposits over tens of thousands of years using different parameters to obtain 20,000 3D models, each with different distribution and extent of heterogeneities. Then, we simulated groundwater flow and age and computed the error when estimating recharge. Local errors range from close to zero to several thousand percent, so sampling from a few locations ‐ a common setting in hydrogeological studies ‐ could lead to high discrepancies. Simulating so many models highlights how river evolution affects recharge estimates, pointing out the importance of considering the geological context. Key Points: Analytical models to estimate recharge rate from groundwater age neglect heterogeneitiesA landscape evolution model simulating fluvial deposits reveals variable but prevalent absolute errors up to thousands of percentA sensitivity analysis on 20,000 realizations exposes how fluvial processes influence errors through the heterogeneity of fluvial deposits [ABSTRACT FROM AUTHOR]

Streamwater transit time distributions display a variable proportion of old waters (≥1 year). We hypothesize that the corresponding long transit times result from groundwater contributions to the stream and that seasonal streamwater transit time variations result from (a) the variable contributions of different flowpaths (overland flow, seepage flow and baseflow) and (b) the stratification of groundwater residence times. We develop a parsimonious model to capture the groundwater contribution to the stream discharge and its effect on transient transit times. Infiltration is partitioned according to the aquifer saturation between Boussinesq groundwater flow and overland flow. Time‐variable transit time distributions are obtained with a new 2D particle tracking algorithm. Hydraulic conductivity, total and drainable porosities are calibrated by using discharge and CFC tracer data on a crystalline catchment located in Brittany (France). The calibrated models succeed in reproducing CFCs concentrations and discharge dynamics. The groundwater flow contribution to the stream is controlled by the aquifer hydraulic conductivity, while its age is controlled by the drainable and total porosities. Old groundwater (≥1 year) is the source for approximately 75% of the streamflow with strong seasonal variations (between 40% and 95%). Mean transit times are approximately 13 years, varying between 6 and 20 years, proportional to the groundwater contribution. These seasonal variations are driven by the groundwater versus overland flow partitioning. The stratification of groundwater residence times in the aquifer plays a minor role in the streamwater transit times but is key for the transit time dynamics of the groundwater contribution to the stream. Plain Language Summary: Water entering a catchment as precipitation can take multiple paths with different transit times to the stream. While a significant fraction of water has short transit times (i.e., is "young") when it reaches the stream, there is also an important contribution of "old" water with long residence times in the subsurface. The age distribution of this old component is important for understanding the resilience of watersheds to climate change, the behavior of persistent pollutants and chemical weathering processes. We developed a model, informed with discharge time series and atmospheric age tracer (CFCs), to constrain the age distribution in both groundwater and streamwater and how they vary seasonally. In our temperate test catchment with crystalline bedrock (Brittany, France), we find that the mean age of streamwater exiting the catchment is approximately 13 years, but varies seasonally from 6 to 20 years as the relative contributions of older groundwater and younger runoff change. Groundwater stratification further influences the mean age of the groundwater contribution arriving at the stream. Key Points: Hydraulic conductivity controls groundwater flow contributions to streams while porosity directly scales the streamwater mean transit timesSeasonal groundwater contributions to streams modulate the mean transit times dynamics from 6 years at high flows to 20 years at low flowsStratified groundwater ages lead to a significant fraction (>75%) of old water to streamflow (>1 year) [ABSTRACT FROM AUTHOR]

Synoptic sampling of three rivers for a suite of environmental tracers is shown to be an efficient way to gain an understanding of groundwater flow paths for a previously unstudied large area in Alberta, Canada. For regional‐scale characterization, classical hydrogeological techniques are limited by the location and number of groundwater wells. This study demonstrates that rivers can become an easily accessible location to sample the distribution of groundwater flow paths discharging to surface water. Modelling of groundwater discharge to the rivers and groundwater mean age helps generate knowledge of groundwater circulation for a large area, which is useful for conceptual model development and focusing future characterization efforts. Results indicate that the benchland areas in this region, with higher topographic relief, had hydrogeological conditions that favoured deeper groundwater circulation with a modelled mean age greater than 100 years from recharge to discharge. Lower relief areas, which coincide with a transition in bedrock formations in this region, appeared to have much shorter and shallower groundwater circulation. The approach required a field program completed in 5 days and financial budget approximately equivalent to drilling a single borehole and installing a monitoring well. It is concluded that under the right conditions, where few classical observation points exist and knowledge is limited, synoptic sampling of rivers can be used to develop scientifically defensible conceptual models at a comparable scale to regional planning and resource management. [ABSTRACT FROM AUTHOR]

A comparison of two methods of radiocarbon age determination of groundwater is presented. The simplest Pearson model and the “user-defined” option of the NETPATH program were considered. Both methods were used to determine the age of water from a PZ-2 piezometer that is situated in the foreground of chamber Z-32 in Wieliczka Salt Mine. Results of these calculations clearly demonstrate that 14C ages obtained by the Pearson model can be significantly overestimated in comparison with those determined by the NETPATH code. Without additional data, such as the stable isotope composition of the water, conclusions on the age of the groundwater based solely on the Pearson model may be highly inadequate.

Groundwater age-dating is an important tool for quantifying and managing water resources. Groundwater age is the elapsed time between recharge (at the land surface or water table) and the time when groundwater is sampled. If groundwater is sampled at the point of discharge from an aquifer, then the age represents the groundwater transit time. Groundwater that has recharged in recent decades is considered young groundwater. In many areas, the quality and quantity of young groundwater has been impacted by human activities and groundwater age-dating is useful for quantifying current and historical water and contaminant fluxes into and through aquifers. This review is focused on the tritium-helium (3H/3He) method, which is a robust and widely applied age-dating technique for young groundwater. We present the development of the 3H/3He method and practical considerations for sampling groundwater in shallow unconfined aquifers. Along the way, we highlight available resources: (1) educational software for building intuition around groundwater age-dating and selection of sampling sites and (2) software that can be used to calculate 3H/3He age from noble gas and 3H data. We also highlight strengths and potential uncertainties associated with the method. For example, while other age-dating techniques require a known historical record of tracer concentration in the atmosphere, the 3H/3He age-dating technique does not require such historical records. However, the 3H/3He method requires measurement of two tracers to produce a groundwater age estimate (“apparent age” or “tracer age”). Precise measurement of 3H and noble gases, plus careful analysis of noble gas data to calculate the tritiogenic 3He (i.e., the portion of 3He derived from decay of 3H in the aquifer) is required to calculate the groundwater apparent age. Sampling for noble gases is sometimes challenging and requires specialized sample containers and technique. We also introduce basic sampling methods in this review but highlight how practitioners should work closely with a noble gas laboratory to obtain the correct containers and assess field conditions and/or the overall feasibility of projects. Lastly, the review highlights recent applications of the 3H/3He method, including recharge rate estimation, characterization of contaminant input histories for aquifers, quantifying groundwater transit times by sampling at aquifer discharge points, and the use of isotope data to constrain and inform numerical and statistical models of groundwater and contaminant movement in the subsurface.

Abstract Hierarchically nested groundwater flow systems have been widely investigated by numerical modeling and laboratory experiments but seldom recognized from in situ observations and testing. Groundwater age and geochemical profiles were obtained by the authors using a packer system along two wellbores drilled in the Ordos Plateau, China. Groundwater age profiles were constructed from 3H, 85Kr, 14C, and 81Kr data, with the apparent age generally increasing with depth, while exhibiting several inflection points. These inflection points were compared with geochemical profiles and numerical modeling results to delineate groundwater flow systems. Inflection points with significant increases in the gradients of age and Cl− concentration versus depth may indicate interfaces among flow systems. A concurrent decrease‐increase turning of groundwater age and Cl− concentration is influenced by a stagnation point. The groundwater age profiles are quite different from previous studies, indicating that the method can greatly reduce the uncertainty in model construction.

Hierarchically nested groundwater flow systems have been widely investigated by numerical modeling and laboratory experiments but seldom recognized from in situ observations and testing. Groundwater age and geochemical profiles were obtained by the authors using a packer system along two wellbores drilled in the Ordos Plateau, China. Groundwater age profiles were constructed from 3H, 85Kr, 14C, and 81Kr data, with the apparent age generally increasing with depth, while exhibiting several inflection points. These inflection points were compared with geochemical profiles and numerical modeling results to delineate groundwater flow systems. Inflection points with significant increases in the gradients of age and Cl− concentration versus depth may indicate interfaces among flow systems. A concurrent decrease‐increase turning of groundwater age and Cl− concentration is influenced by a stagnation point. The groundwater age profiles are quite different from previous studies, indicating that the method can greatly reduce the uncertainty in model construction. Plain Language Summary: The provision of renewable groundwater, a crucial source of drinking water worldwide, relies on a circulation path from recharge to discharge zones with a subsurface flow in reservoirs (aquifers). Basin‐scale groundwater circulation is speculated to form hierarchically nested flow systems with differing capabilities for renewal. However, flow systems are invisible and difficult to detect through in situ observations on groundwater. In this study, nested groundwater flow systems were identified from groundwater age and geochemical profiles along wellbores. Inflection points on these profiles provide significant insights into the interfaces and stagnation points among nested groundwater flow systems. Uncertainties could be brought by the aquifer heterogeneity in the hydraulic conductivity and localized soluble salts. This adopted method is expected to be improved by incorporating geophysical survey and high‐resolution geochemical analyses to eliminate uncertainties in model construction. Key Points: Groundwater age and geochemical profiles were obtained to investigate groundwater flow systemsInflection points on profiles are used to infer interfaces or stagnation points among different groundwater flow systemsGroundwater flow models constrained by inflection points greatly reduce the uncertainty of flow system characterization [ABSTRACT FROM AUTHOR]