Tritium and helium isotopes as hydrologic tracers in a shallow unconfined aquifer

doi:10.1016/0022-1694(88)90002-9

Journal of Hydrology

Volume 103, Issues 1–2, 15 November 1988, Pages 1-9

https://doi.org/10.1016/0022-1694(88)90002-9 Get rights and content

Abstract

Measurements of tritium and ³He in groundwater can be used to calculate the age of tritium in groundwater. Such combined measurements on water samples thus provide a powerful tool towards understanding fluid flow in the near-surface saturated zone. We report tritium and helium isotope measurements on waters from an unconfined aquifer associated with a low-level nuclear waste storage area. These values are used to calculate ages of tritium in groundwater from 0.1 to 6 years. From the groundwater ages and a three-dimensional flow net hydraulic conductivities of 3.3 × 10⁻⁶ to 5.5 × 10⁻⁶ ms⁻¹ for five wells, and extreme values from a single well of 18 × 10⁻⁶ and 97 × 10⁻⁶ms⁻¹, are calculated. These values agree with hydraulic conductivities obtained from conventional aquifer tests. The high conductivity value may indicate an area of preferred fluid flow.

References (11)

T.H.E. Heaton et al.
Excess air in groundwater
J. Hydrol.
(1981)
W. Broecker et al.
Tracers in the Sea
T.E. Cerling et al.
Use of tritium and helium isotopes in the study of a shallow unconfined aquifer
EOS
(1987)
H. Craig et al.
Helium-3 and mantle volatiles in the ocean and the oceanic crust
W. Rison et al.
Helium isotopes and mantle volatiles in Loihi Seamount and Hawaiian Island basalts and xenoliths
Earth Planet. Sci. Lett.
(1984)

There are more references available in the full text version of this article.

Cited by (86)

Short-lived natural radionuclides as tracers in hydrogeological studies – A review
2024, Science of the Total Environment
Fundamental approaches to the study of groundwater rely on investigating the spatial and temporal distribution of stable and radioactive isotopes and other anthropogenic compounds in natural waterbodies. The most often used tracers for estimating groundwater flow paths and residence times, groundwater/surface water interaction as well as tracing chemical (contamination) sources include stable isotopes of water (δ ¹⁸O and δ ²H), radiocarbon (¹⁴C; t_1/2 = 5730 a), tritium (³H; t_1/2 = 12.43 a) as well as unreactive fluorine-containing gases (e.g., chlorofluorocarbons CCl₃F or CFC-11; CCl₂F₃ or CFC-12; C₂Cl₃F₃ or CFC-113; and SF₆). While gas tracers are usually referred to as transient tracers and are appropriate for investigating modern flow systems, the isotopic tracers are often used to investigated paleo or regional flow systems. Stable isotopes of water can also be used to investigate groundwater/surface water interactions. Another, thus far been less frequently used group of groundwater tracers, are cosmo- and geo- genic short-lived radioisotopes. These isotopes are uniquely suited for studying a wide range of groundwater problems that have short time scales including high aquifer vulnerability to quantitative and qualitative impacts and groundwater discharge to surface waters. Here, we discuss and compare the applications of radio‑sulphur (³⁵S; half-life t_1/2 = 87 d), radio‑beryllium (⁷Be; t_1/2 = 53 d), radio‑phosphorus (^32/33P; combined t_1/2 = 33 d), natural tritium (³H; t_1/2 = 12.43 a), radon (²²²Rn; t_1/2 = 3.8 d) and short-lived radium (^224/223Ra; combined t_1/2 = 5.2 d). The paper discusses the principles of the individual tracer methods, focusing on the isotopes' input functions or values, on sampling techniques, and on methods of analyses. Case studies that applied a combined use of the tracers are referred to for readers who wish to learn more about the application of the so far underused cosmo- and geo- genic radioisotopes as aquatic tracers.
Groundwater age distribution in a highly dynamic coastal aquifer
2021, Advances in Water Resources
Citation Excerpt :
Due to its relatively short half-life of 12.32 years (Lucas and Unterweger, 2000), the ³H concentration has been continuously decreasing since reaching a concentration peak (so-called tritium bomb-peak) with concentrations of about 10,000 TU in precipitation in 1962-1963 (Poreda et al., 1988; Schlosser et al., 1988). Since 3H is incorporated in the water molecule, it is washed out from the atmosphere by precipitation and deposited in oceans, lakes, rivers, and groundwater (Poreda et al., 1988; Sültenfuß and Massmann, 2004). Dissolved 3Hetrit originating from 3H decay in the unsaturated zone can be released back to the atmosphere, while it accumulates in the groundwater within the saturated zone where gas exchange with the atmosphere is inhibited (Solomon et al., 1993; Sültenfuß et al., 2010).
Besides other environmental tracers, the use of the ³H-³He method enables the estimation of modern groundwater ages. Widely used in fresh groundwater studies, the combination of tritium (³H) and its stable daughter product tritiogenic helium-3 (³He_trit), however, has so far only been applied in a few studies to date groundwater in coastal aquifers. The coastal aquifer presented in this study is located at the very young eastern part of the North Sea barrier island Spiekeroog, the so-called ‘Ostplate’, that is characterized by a currently developing freshwater lens. Previously field and numerical modeling studies showed that the groundwater salinity distribution within this coastal aquifer is largely determined by seawater infiltration during storm tides. The main aim of this study was to characterize the groundwater age distribution in a highly dynamic coastal aquifer using the ³H-³He age dating method. In addition, a transient 2-D density-dependent flow and transport model that was calibrated to groundwater heads and salinity was utilized to assess whether measured ³H-³He ages can be reliably simulated despite the mixing of distinctly different and time-variable ³H endmembers, i.e., freshwater and seawater. The numerical calculation of the ³H-³He age distribution, thereby, was based on the simulation of the fictive tracer ‘stable tritium’ (sum of ³H and ³He_trit) and ³H. Even though the freshwater lens is very young, both field observations and simulation results indicate notably lower ages within the freshwater lens as compared to the age of the lens itself. Simulation results show that frequent seawater infiltration, most importantly during winter storm tides, and discharging water from the freshwater lens lead to a disturbance of the vertical age stratification below the beach. A comparison with direct ages shows that the chosen 2-tracer approach improves system understanding, especially with regard to dispersion effects.
Complexity of groundwater age mixing near a seawater intrusion zone based on multiple tracers and Bayesian inference
2021, Science of the Total Environment
Citation Excerpt :
Atmospheric tracers such as the chlorofluorocarbons (CFCs) and sulfur hexafluoride (SF6) have been commonly used to study groundwater with modern recharge dates (< 60 y) (Ayraud et al., 2008; Böhlke and Denver, 1995; Cook et al., 1995; Kaown et al., 2009, 2018; Koh et al., 2006). The 3H/3He ratio has been used successfully in dating groundwater less than 50 y old (Plummer et al., 1998; Poreda et al., 1988; Schlosser et al., 1988; Szabo et al., 1996), while 4He has been used for quantifying relatively old groundwaters from 103 to 108 y (Ballentine et al., 1991; Mazor and Bosch, 1987; Torgersen, 1980) and even for young groundwaters with age ranges of 10 to 103 y in systems with high production rates (Solomon et al., 1996; Green et al., 2016). Each tracer characterizes a different range of residence times, which has led to the use of multiple age tracers for complementary age-dating (Ekwurzel et al., 1994; Waugh et al., 2003).
Aquifer flow systems near seawater interfaces can be complicated by density-driven flows and the formation of stagnation zones, which inevitably introduces uncertainty into groundwater age-dating. While age-dating has proved effective to understand the seawater intrusion and aquifer salinization process in coastal aquifers, further efforts are needed to propagate model and data uncertainty to the uncertainty associated with the inferred age distributions. This study was performed in a coastal aquifer located close to the Yellow Sea, South Korea, where there is a decreasing trend of groundwater levels due to recent heavy exploitation, raising a warning of induced seawater intrusion. We inferred the groundwater age distributions in wells around the intrusion zone and estimated the uncertainty associated with the inference based on multiple age tracers including ³H, tritiogenic ³He, radiogenic ⁴He, CFC-11, CFC-12 and CFC-113 using Bayesian inference. We examined various models representing the age distributions including traditional parametric Lumped Parameter Models (LPMs) and two non-parametric “shape-free” models. The results showed that the mean ages at the study site ranged from 10.9 to 522.5 y. Complex, multimodal distributions of ages occurred near a seawater intrusion area and upland recharge zones, implying converging paths of a wide range of different ages in those regions. In particular, the age distributions estimated near the seawater intrusion interface were characterized by heavy-tailed mixing structures with elevated concentrations of ⁴He. This likely indicates density-driven upward flow at the seawater intrusion interface, forcing old groundwater rich in ⁴He into the shallow aquifer. The Bayesian inference estimated large uncertainties particularly for the old age distributions, which was attributed partly to the gradual accumulation of ⁴He in groundwater. The Bayesian inference improved understanding of flow dynamics at a complex seawater interface and identified opportunities to further reduce uncertainty of old water age estimates that characterize upwelling groundwater near the interface.
Noble gas composition and <sup>3</sup> H/ <sup>3</sup> He groundwater ages in the Gardermoen Aquifer, Norway: Improved understanding of flow dynamics as a tool for water management
2019, Science of the Total Environment
This study presents a novel data set of noble gas compositions and ³H/³He measurements in groundwater samples from the Gardermoen Aquifer in Norway. The motivation was to test the applicability of noble gases as tracers in constraining the conceptual model and improve the understanding of deeper parts of the aquifer. Flow models have been used as tools for water resource management at the Oslo international airport, which is located within the aquifer recharge area, and bordering towards protected nature reserves. Current models are well calibrated for upper parts of the aquifer, and in line with new noble gas data. However, in some areas large inverse pressure gradients are observed. Interpretation of noble gas data proved useful in understanding these pressure deviations as a result of water retention in low permeability layers of clay and silt. Noble gas composition, isotopic ratios and tritium water ages define different water “types”. Mixing along flow path or as a consequence of pumping were found to be indicators of geological heterogeneity, and were used to evaluate degree of flow separation as well as vertical versus horizontal flux. ³H/³He water ages varied between 0 and 50 years, increasing with depth below the groundwater table and along flow paths. These data indicate lower vertical permeability and longer residence times in deeper parts of the aquifer than suggested by previous models. This study is of high relevance in the continued water resource management scheme at Gardermoen, for improving existing models and in sound, long-term monitoring of groundwater, ensuring sustained influx towards sensitive biotopes.
Using environmental tracers to determine the relative importance of travel times in the unsaturated and saturated zones for the delay of nitrate reduction measures
2018, Journal of Hydrology
Citation Excerpt :
Multiple pumping wells and piezometers in the Dünnerngäu, Solothurn, Switzerland, were sampled to determine how quickly already implemented nitrate reduction measures will result in a trend reversal of nitrate concentrations. Multiple tracers were used to characterize the groundwater age distribution on a time scale of a few years to decades: 3H, tritiogenic 3He (Poreda et al., 1988; Schlosser et al., 1988), radiogenic 4He (e.g. Torgersen and Stute, 2013), 85Kr (Althaus et al., 2009; Loosli et al., 2000; Smethie et al., 1992) and 39Ar (Loosli, 1983; Loosli et al., 2000). The Bayesian statistical framework of Alikhani et al. (2016) allows us to estimate confidence intervals for the age distributions and predicted nitrate concentrations.
Groundwater quality in many regions with intense agriculture has deteriorated due to the leaching of nitrate and other agricultural pollutants. Modified agricultural practices can reduce the input of nitrate to groundwater bodies, but it is crucial to determine the time span over which these measures become effective at reducing nitrate levels in pumping wells. Such estimates can be obtained from hydrogeological modeling or lumped-parameter models (LPM) in combination with environmental tracer data. Two challenges in such tracer-based estimates are (i) accounting for the different modes of transport in the unsaturated zone (USZ), and (ii) assessing uncertainties. Here we extend a recently published Bayesian inference scheme for simple LPMs to include an explicit USZ model and apply it to the Dünnerngäu aquifer, Switzerland. Compared to a previous estimate of travel times in the aquifer based on a 2D hydrogeological model, our approach provides a more accurate assessment of the dynamics of nitrate concentrations in the aquifer. We find that including tracer measurements (³H/³He, ⁸⁵Kr, ³⁹Ar, ⁴He) reduces uncertainty in nitrate predictions if nitrate time series at wells are not available or short, but does not necessarily lead to better predictions if long nitrate time series are available. Additionally, the combination of tracer data with nitrate time series allows for a separation of the travel times in the unsaturated and saturated zone.
The study of the interactions between groundwater and Sava River water in the Ljubljansko polje aquifer system (Slovenia)
2018, Journal of Hydrology
River basin aquifers are common sites for drinking water wells as bank filtration can be a cost effective pretreatment technology. A groundwater vulnerability to pollution depends on a groundwater mean residence time and on a relative contribution of river water versus local precipitation to groundwater. Environmental isotopes of oxygen and hydrogen (δ¹⁸O and δ²H), tritium (³H) and concentrations of nitrate (NO₃⁻) were used to investigate hydrological pathways, mean residence time and interactions between surface water and groundwater in the Ljubljansko polje aquifer system in Slovenia. δ¹⁸O and δ²H values indicate a spatial variability of the influence of individual groundwater sources inside the aquifer – local precipitation and the Sava River water. Fractions of river water in groundwater depend on the depth of perforated screens in the pumping wells and their distance from the Sava River. It was estimated that groundwater at wells Kleče 11, Hrastje 3, and Hrastje 8 is mostly composed of recently infiltrated local precipitation, while the Sava River is the dominant source of groundwater at the well Jarški prod 1. Groundwater at wells Kleče 8, Kleče 12, and Jarški prod 3 contains on average between 41% and 48% of the Sava River water. The ³H and ³H/³He methods indicate short mean residence time of groundwater present at Jarški prod (2–7 years) and Hrastje (7–8 years). A small fraction (<10%) of old groundwater is present at Kleče. Furthermore, infiltration of local precipitation influenced the levels of NO₃⁻ at Hrastje. These data extend our understanding of groundwater flow in the Ljubljansko polje aquifer system, interactions between the Sava River water/local precipitation and groundwater, and the utility of isotope tracers in evaluating the spatial distribution of groundwater vulnerability to pollution.

View all citing articles on Scopus

^∗: Present address: Department of Geological Sciences, University of Rochester, Rochester, NY 14627 (U.S.A.)

View full text

Research paperTritium and helium isotopes as hydrologic tracers in a shallow unconfined aquifer

Abstract

J. Hydrol.

Tracers in the Sea

Use of tritium and helium isotopes in the study of a shallow unconfined aquifer

EOS

Helium-3 and mantle volatiles in the ocean and the oceanic crust

Helium isotopes and mantle volatiles in Loihi Seamount and Hawaiian Island basalts and xenoliths

Earth Planet. Sci. Lett.

Research paper
Tritium and helium isotopes as hydrologic tracers in a shallow unconfined aquifer