# The Hidden Hydrogeosphere: The Contribution of Deep Groundwater to the Planetary Water Cycle

Barbara Sherwood Lollar, Oliver Warr, Peter M. Higgins

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Barbara Sherwood Lollar, Oliver Warr, Peter M. Higgins. The Hidden Hydrogeosphere: The Contribution of Deep Groundwater to the Planetary Water Cycle. Annual Review of Earth and Planetary Sciences, 2024, 52, pp.443-466. 10.1146/annurev-earth-040722-102252. insu-05081570

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*Annual Review of Earth and Planetary Sciences*

# The Hidden Hydrogeosphere:
The Contribution of Deep
Groundwater to the Planetary
Water Cycle

Barbara Sherwood Lollar,<fnref n="1" />,<fnref n="2" /> Oliver Warr,<fnref n="3" /> and Peter M. Higgins<fnref n="1" />

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Annu. Rev. Earth Planet. Sci. 2024. 52:443–66

First published as a Review in Advance on
January 25, 2024

The *Annual Review of Earth and Planetary Sciences* is
online at earth.annualreviews.org

https://doi.org/10.1146/annurev-earth-040722-102252

Copyright © 2024 by the author(s). This work is
licensed under a Creative Commons Attribution 4.0
International License, which permits unrestricted
use, distribution, and reproduction in any medium,
provided the original author and source are credited.
See credit lines of images or other third-party
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**Keywords**

deep groundwater, subsurface biosphere, water resources, hydrogen, helium, residence times

**Abstract**

The canonical water cycle assumes that all water entering the subsurface to form groundwater eventually reenters the surface water cycle by discharge to lakes, streams, and oceans. Recent discoveries in groundwater dating have challenged that understanding. Here we introduce a new conceptual framework that includes the large volume of water that is estimated to account for 30–46% of the planet's groundwater but that is not yet incorporated in the traditional water cycle. This immense hidden hydrogeosphere has been overlooked to date largely because it is stored deeper in the crust, on long timescales ranging from tens of thousands to more than one billion years. Here we demonstrate why understanding of this deep, old groundwater is critical to society's energy, resource, and climate challenges as the deep hydrogeosphere is an important target for exploration for new resources of helium, hydrogen, and other elements critical to the green energy transition; is under investigation for geologic repositories for nuclear waste and for carbon sequestration; and is the biome for a deep subsurface biosphere estimated to account for a significant proportion of Earth's biomass.

<footnote n="1">¹Department of Earth Sciences, University of Toronto, Toronto, Ontario, Canada; email: barbara.sherwoodlollar@utoronto.ca</footnote>

<footnote n="2">²Institut de Physique du Globe de Paris, Université Paris Cité, Paris, France</footnote>

<footnote n="3">³Department of Earth and Environmental Sciences, University of Ottawa, Ottawa, Ontario, Canada</footnote>
* We provide a new conceptual framework for the hidden hydrogeosphere, the 30–46% of groundwater previously unrecognized in canonical water cycles.

* Geochemico-statistical modeling groundwater age distributions allows deconvolution of timing, rates, and magnitudes of key crustal processes.

* Understanding and modeling this deep, old groundwater are critical to addressing society's energy, resource, and climate challenges.

# 1. INTRODUCTION

## 1.1. The Importance of Understanding Groundwater Age

What do we mean when we report a groundwater age? When told the aquifer supplying their community's drinking water had been radiocarbon-dated at 10,000 years before present, any reasonably astute member of the public might ask, "How can a substance containing just two hydrogens and one oxygen be dated using radiocarbon?" While ¹H is intrinsic to the water mass itself, the majority of groundwater age tracers are geochemical tracers of the water mass, either reflecting atmospheric components dissolved in water and entrained from the surface hydrologic cycle (e.g., ¹⁴C) or added to the groundwater by subsurface water-rock interactions (WRIs) during the time since recharge (e.g., ⁴He, ⁴⁰Ar). The public cannot assume that scientists even mean the same thing when they use the word water. While hydrogeologists mean H₂O (and those of a more geochemical inclination include solutes dissolved within that aqueous fluid), petrologists use the word water to include H bound to and within the rock minerals and matrix. The latter practice lends to a common source of confusion when discoveries of water on Earth's Moon (He et al. 2023) or even the deep Earth (Schmandt et al. 2014) are reported, as, in many cases, the water referred to is H bound to minerals [e.g., ringwoodite (Schmandt et al. 2014)], not H₂O in liquid form. Hence, even within the scientific research community, and between different scientific disciplines in particular, long-standing paradigms and assumptions—and, at times, imprecise vocabulary—all contribute to significant challenges in our understanding of the water cycle on a planetary scale. Further, we argue here that the most pressing societal challenges related to the availability and quality of water necessitate a reexamination of the traditional conceptual model of the water cycle.

This new review builds on the previous Bethke & Johnson (2008), Gleeson et al. (2020), and Templeton & Caro (2023) *Annual Review of Earth and Planetary Sciences* reviews to respond to the increasing number of pressing scientific and societal challenges and opportunities in the study of ancient groundwater. We introduce a new conceptual framework for the water cycle that includes the hidden hydrogeosphere (Sherwood Lollar 2016, Warr et al. 2018, Lollar et al. 2019, Phillips et al. 2022) and demonstrate why this not only is critical for understanding fundamental issues of groundwater resource volumes, water quality, and subsurface habitability but also has essential implications for society's critical challenges arising from the energy transition and global change. We provide new perspectives on thinking about groundwater age, moving beyond the focus on mean ages to consider the constituent components that collectively contribute to groundwater age distributions. In this context we consider the degree of connection between the conventional water cycle and the hidden hydrogeosphere, in light of newly available information on the expanded spectrum of timescales for preservation and sequestration of groundwater in the crust. We highlight emerging trends in groundwater dating that are particularly relevant to old groundwaters. Finally, we emphasize the pressing need to better understand the chronological implications the hidden hydrogeosphere presents, as we pivot toward developing more
robust estimates of the rates of subsurface processes. These process- and rate-based understand-
ings are essential to resolving both the fundamental and applied challenges facing society and its
relationship to and understanding of the planetary water cycle.

**1.2. Challenges of the Canonical Water Cycle**

The very definition of groundwater age, or more accurately residence time, is still evolving. In canonical models, groundwater residence time is often defined as the amount of time since water entered the groundwater system (Sprenger et al. 2019, Ferguson et al. 2020) via precipitation, runoff, infiltration, and recharge. All these inputs, due to their contact with the atmosphere, are referred to as meteoric water. The canonical model further incorporates that assumption that groundwater ultimately all returns to the surface (Figure 1). The majority of the canonical images of the water cycle imbed this concept, shown here in Figure 1, with even so-called old groundwater simply depicted as following longer, deeper flow paths, above some conceptually impermeable boundary at depth, before reentering the surface hydrologic cycle (United Nations 2022). Treating the surface-groundwater cycle as a single interconnected system like this is entirely appropriate and serves as a powerful model for understanding spatiotemporal variability in near-surface groundwater resources, particularly in the case of dynamic climate-driven changes in rainfall and recharge, and the impacts of groundwater extraction and contamination from anthropogenic activities (Sprenger et al. 2019, Gleeson et al. 2020, McIntosh & Ferguson 2021, Scanlon et al. 2023). However, this review focuses on recent discoveries that demonstrate that the traditional water cycle conceptual model neglects a substantial and critical component of groundwater, estimated to account for 30–46% of the planet’s subsurface water (Sherwood Lollar 2016, Warr et al. 2018, Ferguson et al. 2021).

In a previous review on this topic Bethke & Johnson (2008) provided important insights that challenged long-standing oversimplifications in groundwater and age-dating, specifically high-lighting that “A groundwater sample is composed not of water that recharged at a point in the past, but a mixture of waters that have resided in the subsurface for varying lengths of time” (p. 148) and “Conventional age-dating techniques applied to mixed waters predict sample ages that in most cases are strongly biased toward youth; the bias can be extreme” (p. 148). In the 2008 review and additional works (Goode 1996; Bethke & Johnson 2002a,b) a sophisticated set of concepts were articulated that even today require reinforcement and more rigorous incorpora-tion into common practice, including that (a) the traditional definition of groundwater residence time based on piston-flow models (defined in **Supplemental Figure 1a**) is overly simplistic and groundwater age is better understood as a distribution of water ages (**Supplemental Figure 1b**); (b) different geochemical solutes or tracers provide additional insights into different components of the groundwater system; and (c) models of age mass to address 3D advective, diffusive, and dis-persive transport, and mixing, need to replace piston-flow concepts that embed the oversimplified assumption of 1D advective flow of closed packets of water along streamtubes (**Supplemental Figure 1a,b**), which implicitly assume diffusion, dispersion, and cross-formational flow are neg-ligible. Piston-flow models particularly break down when applied to fractured rock settings, the geologic terrain in which the vast majority of so-called ancient groundwaters that are the focus of this review are found.

## 2. EVOLVING CONCEPTS OF THE NATURE OF GROUNDWATER AND AGE-DATING

**2.1. Beyond Young and Old**

For the purposes of this review, we use the Gleeson et al. (2016) definition of young ground-
water as less than 50–100 years old (based on the presence or absence of ³H), and hence old

Supplemental Material >
Canonical water cycle

<figure><img src="image_4.png" /></figure>

(Caption appears on following page)
**Figure 1** (*Figure appears on preceding page*)

An expanded conceptual framework for the water cycle, moving beyond the canonical model where all groundwater circulates above an impermeable boundary and returns to the surface. Blue arrows indicate movement of flow in the canonical water cycle, where fluid transport between recharge and discharge is often approximated using a piston model (Supplemental Figure 1a). This new schematic extends Earth's hydrogeosphere to include the vast quantities of hidden groundwater that circulates only on much longer geologic timescales and in some cases is stored in the crust for hundreds of millions to billions of years. Circles illustrate additional components of such groundwaters rarely considered to date including contributions from water-rock reactions such as (a) radiolysis and (b) hydration/dehydration reactions, as well as (c) diagenesis/metamorphic processes, (d) fluid inclusions, and (e) water present at the time of rock/sediment formation. Opening and closing of fracture systems, tectonic processes, erosion, and impact events are some of the processes that enable mixing and transport within the hidden hydrogeosphere and control the degree of mixing between the meteoric, paleometeoric, and nonmeteoric waters of the crust.

groundwater generally defines all else. Because estimates demonstrate such old groundwater represents 30–46% of Earth’s entire groundwater inventory between 2 and 10 km depth (Sherwood Lollar 2016, Warr et al. 2018, Ferguson et al. 2021), young versus old is clearly a necessary but insufficient differentiation for a detailed understanding of the planet’s subsurface water. Further, such relative definitions are prone to misunderstandings given that the use of the word young versus old varies depending on the scale and scope of the study in question. Groundwaters of several hundreds to thousands of years old may be referred to as old in the context of understanding the subsurface biome of microbial communities (Ruff et al. 2023) or multicellular life such as nematodes (Borgonie et al. 2019). In other settings, such groundwaters are considered young, compared to groundwaters whose noble gas concentrations provide multiple lines of evidence for components stored over tens of millions of years (Lippmann et al. 2003, Lin et al. 2006, Kietäväinen et al. 2014) or, at the extreme, billions of years (Holland et al. 2013; Warr et al. 2018, 2022).

A more powerful and informative set of definitions for contextually constraining the subsurface water cycle differentiates instead between meteoric water, paleometeoric water, and nonmeteoric water (see definitions in **Figure 2**). Even within old groundwaters where short-lived atmospheric tracers such as ³⁷Ar, ⁸⁵Kr, and ³H from surface recharge have decayed below detection limits, multiple tracers can still be applied to determine the meteoric and paleometeoric characteristics of the groundwater. Isotopes such as ¹⁴C, ³⁶Cl, ³⁹Ar, and ⁸¹,⁸⁵Kr are present as trace components in the atmosphere either due to natural cosmic radiation or due to anthropogenic activities (e.g., weapons testing). Once introduced to the subsurface through recharge, they persist until decaying below detection depending on their respective half-lives (discussed in more detail in Section 3.1) (Lu et al. 2014), thereby providing constraints on the time since recharge (Warr et al. 2023a). Where timescales of groundwater age exceed these half-lives, the δ¹⁸O and δ²H values of the water molecules themselves can provide critical evidence of the relationship to the surface meteoric water cycle when their correlation still falls along the Global Meteoric Water Line (GMWL) (**Figure 2**). Not all paleometeoric waters fall on the GMWL, as evaporation, geothermal activity, and both low- and high-temperature exchange processes can cause the δ¹⁸O and δ²H signatures to evolve away from the characteristic meteoric correlation line. However, in such cases, the ultimate meteoric origin can typically still be inferred by context or evidence from isotopic evolution, geochemical trends, and mixing relationships (Clark & Fritz 1997, Ward et al. 2004, Kietäväinen et al. 2013, Warr et al. 2021). Further, atmospheric-derived noble gases provide a precise determination of the residual contribution of atmospheric components to such meteoric and paleometeoric fluids (Ballentine et al. 2002, Warr et al. 2022).

## 2.2. The Hidden Hydrogeosphere

Much less attention to date has been focused on nonmeteoric water components of so-called old or ancient groundwaters, although the characteristic pattern that many of these fall above the

**Supplemental Material** >
<figure><img src="image_5.png" /><figcaption>Figure 2</figcaption></figure>

Water isotope plot indicating the relationship between δ¹⁸O and δ²H of meteoric and paleometeoric waters (which typically fall along the GMWL), and nonmeteoric waters, whose isotopic ratios may be shifted by several processes in the crust. Meteoric waters are components of the hydrologic cycle (rivers, lakes, snow, ice, oceans, and meteoric groundwaters) whose δ¹⁸O and δ²H isotopic composition is controlled by liquid-vapor equilibrium in the atmosphere with a linear trend (Craig 1961) resulting from the dual controls of the temperature effect (due to the temperature-dependent isotopic fractionation between water in the gas and liquid phase) and degree of rainout (or condensation, e.g., the amount effect). Recently recharged groundwaters retain this atmospheric (meteoric) signature unless significant evaporation, high- or low-temperature exchange with surrounding rocks, or mixing with other fluids occurs. Paleometeoric waters are meteoric groundwaters that recharged under past climatic conditions. As recharge is largely an isotopically conservative process, groundwaters that have recharged in the recent geologic past continue to reflect their meteoric origin via signatures that still fall along the GMWL (Clark & Fritz 1997, and references therein). Nonmeteoric waters are typically found in the more than 70% of Earth's lithosphere that is Precambrian in age (Goodwin 1996) and are saline groundwaters and brines that lie above the GMWL due to the effects of a variety of low-temperature WRIs (Frape et al. 1984, Onstott et al. 2006, Warr et al. 2021). While δ¹⁸O and δ²H signatures are not a dating tool, for these nonmeteoric groundwaters, the oldest groundwaters typically fall farthest above the line as the effects of WRIs increase with residence time. Hence, this displacement above the GMWL can be used as a qualitative proxy for extended residence times (Ward et al. 2004, Lippmann-Pipke et al. 2011, Heard et al. 2018, Warr et al. 2021). In all cases, mixing between fluids of different origin (meteoric and nonmeteoric) can produce mixing and evolution trends (dashed lines) that reflect the relative contribution of each end member to the groundwater system under investigation. Abbreviations: GMWL, Global Meteoric Water Line; VSMOW, Vienna Standard Mean Ocean Water; WRI, water-rock interaction. Figure adapted from Warr et al. (2021).

GMWL was first identified decades ago (Fritz & Frape 1982; Frape et al. 1984, 2014) (Figure 2). The hidden hydrogeosphere contains a variety of contributing components that have hitherto rarely been considered due to the traditional focus on a canonical water cycle. These not only include paleometeoric waters but also include nonmeteoric water components, including juvenile or formation waters trapped at the time of the rock formation or compaction (e.g., Warr et al. 2021, and references therein); fluids introduced during diagenesis/metamorphism (Bottomley & Clark 2004, Onstott et al. 2006, Lippmann-Pipke et al. 2011, Ingebritsen & Gleeson 2017); groundwaters whose H₂O molecules have participated in water-rock reactions that cycle elemental O and H between the rock and water over long time periods (Warr et al. 2021); and
even those groundwaters for which fluid inclusions account for a non-negligible contribution to the groundwaters and their dissolved solutes (Guha & Kanwar 1987, Nordstrom et al. 1989, Lippmann-Pipke et al. 2011, Piribauer et al. 2011, Holland et al. 2013, Klein et al. 2019) (Figure 1). All of these nonmeteoric water components (defined in Figure 1) may mix with groundwaters originally derived from the surface hydrologic cycle, but the complexity of components and the geochemical and isotopic alteration over long time geologic periods, typically in settings with high rock/water ratios that maximize the impact of water-rock alteration reactions and cycles of hydration/dehydration, together represent groundwater systems for which there is no longer evidence of modern atmospheric components and that no longer recirculate back into the canonical surface water cycle on geologically short timescales. Figure 1 presents an expanded framework for Earth's water cycle that moves beyond the canonical concept of all water reentering the surface water cycle via discharge and for the first time incorporates the vast terrain and multiple components of Earth's hidden hydrogeosphere.

**2.3. Beyond the Impermeable Boundary**

To fully develop an understanding of the hidden hydrogeosphere, it is helpful to consider the geologic environments most relevant to the groundwater characteristics described above. This includes settings where low porosity and permeability and low net hydraulic conductivity restrict the penetration of more modern fluids and constrict mixing, even between various compartments of the groundwater (e.g., different fracture networks, or vugs). Intuitively the hidden hydrogeosphere is preferentially located in systems of large hydrogeologic discontinuity where the typical assumptions of a continuous porous medium (either homogeneous or even heterogeneous) and flow path continuity are insufficient to describe the hydrogeology because materials are characterized by low porosities and variable permeabilities that change the flow of water and restrict mixing. The hidden hydrogeosphere lies in fractured rock in particular, the canonical impermeable basement occupying the bottom zone in the traditional water cycle schematic—"beyond which there be dragons" (Figure 1). However, important settings also include deep sedimentary basement rocks, in addition to metamorphic and igneous basement rocks. While it is also possible to imagine an extremely low hydraulic conductivity system such as a clay that meets the definition criteria, intuitively the major terrains of the hidden hydrogeosphere are fractured basement rock—particularly at depths where lithospheric pressure, long-term precipitation of permeability-clogging fracture minerals, and other processes of compaction and consolidation mean that fracture networks are increasingly discontinuous, patchy, and hydrogeologically disconnected not only from the surface but also from each other on long spatiotemporal scales (Sherwood Lollar et al. 2006; Sleep & Zoback 2007; Holland et al. 2013; Kietäväinen et al. 2013, 2014; Warr et al. 2018, 2022; Lollar et al. 2019).

Previous work has discussed the difficulties of extending conventional hydrogeologic mapping and dating to Earth's deep subsurface groundwaters. Bethke & Johnson (2002b, 2008) specifically highlighted fractured rock as a system within which the challenges they identified in their review defy simple solutions, due, in particular, to 3D subsurface heterogeneity. Gleeson & Ingebritsen (2017) provided a detailed collection of essays examining what has come to be referred to as the fourth dimension (i.e., time) in their examination of the profound effects of dynamic permeability, as changes in time are layered onto already complex 3D subsurface heterogeneity. Ingebritsen & Gleeson (2017) focused on the crust below the brittle-ductile transition (BDT) zone (typically at 10–15 km below surface) where they note "hydrogeologists rarely venture" (p. 2221). Presciently they noted the absence of an apparent discontinuity in permeability in the crust at the BDT and focused on the potential for internally derived fluids produced by metamorphism or (de)volatilization reactions to transit from below to the upper crust and hence enter
the more conventional water cycle (Ingebritsen & Gleeson 2017). While they did not explicitly address the contribution of waters entering this zone from above via surface recharge, they note “the depth range between the BDT and roughly 2-km depth is still largely terra incognito from a hydrogeologic perspective” (p. 2222).

This review continues the exploration of that terra incognito by focusing on groundwaters no longer dominated by meteoric waters from surface recharge but wherein evidence of modern surface recharge has been overprinted by geochemical and isotopic alteration, and the significant contributions, or even dominance, of nonmeteoric-derived waters due to processes as depicted in **Figure 1**. A goal is developing an understanding of the hydrogeochemical and geochronologic characteristics of fluids that may occur anywhere between surface and the BDT, but for which their defining characteristic is that the groundwater's complex geochemical and isotopic character is derived from multiple contributing components over long timescales such that evidence of any modern meteoric component is no longer readily discernable. The dominant story these waters tell is no longer that of the surface hydrologic cycle but that of the deep hidden hydrogeological processes, of geochemical alteration, of long-term storage, and of transport in the crust. The hidden hydrogeosphere is hence a conceptual framework that significantly evolves our understanding of Earth's water cycle, and we feel it provides a more powerful interpretational context for understanding the full spectrum of the planet's old groundwater than either a spatial definition (based on depth) or a temporal definition (based on age or residence time) alone.

## 2.4. Reexamining Concepts of Representative Elemental Volume

The hydrogeological principle of representative elemental volume (REV) defines the appropriate or meaningful scale of measurement as the scale at which the same value will be calculated regardless of position in the 3D subsurface space (Figure 3). This approach to scaling typically increases the scale of measurement to determine the scale at which the variability in smaller individual properties no longer creates variability in the calculated output for the whole system (e.g., Freeze & Cherry 1979; Warr et al. 2023b, and references therein). This foundational tenet of hydrogeology is focused on determining the mean value that is most representative of the whole system and is applied for physical properties (porosity, permeability, hydraulic conductivity, etc.), for modeling transport and flow, and even to interpret variation in isotopic signatures in contaminant remediation studies (Gilevska et al. 2019). But by definition, the approach sacrifices detail.

What if it is the details that matter most? Contaminant hydrogeology perhaps embraces this insight most consistently with travel time distributions (e.g., Sprenger et al. 2019), which define when the average groundwater front reaches an environmental receptor, examine the breakthrough curves (e.g., the distribution in time of arrival at a receptor), and even, in some cases, calculate individual breakthrough curves for different contaminants carried in the groundwater. The differences in first arrival time, mean arrival time, and last arrival time are critical for assessing environmental impact and designing protective strategies for contaminated sites.

To date, such an approach has rarely been applied to other properties such as groundwater age. This issue is particularly important to consider for the hidden hydrogeosphere. First and foremost this is because, as in contaminant hydrogeology, the urgent questions under investigation may mean it is more important to know the oldest contributing fluid, or youngest contributing fluid, in addition to the mean age of the groundwater. Further, determining appropriate scale and REV for the hidden hydrogeosphere is particularly difficult as few empirical measurements are available. Detailed mapping of fractures, porosity, permeability, and hydraulic conductivity is few and far between in the deeper crust, with the exception of underground research laboratories and other sites under investigation for future deep geologic repositories (Murdoch et al. 2012, Kuva et al. 2015, Aaltonen et al. 2016, Snowdon et al. 2021). In the absence of detailed site-specific data,
<figure><img src="image_6.png" /><figcaption>Figure 3</figcaption></figure>

Example of the concept of REV, here using porosity of a given lithology (based on the concepts outlined by Freeze & Cherry 1979). The gray line represents porosity measurements (y axis) made at variable spatial locations in a given setting at steadily increasing scales (x axis). Gray shading represents the schematic population of values that would result from measurements on a given volume selected for REV. At the extreme, an inappropriately small scale (e.g., *a*, *i*; *a*, *ii*) results in a bimodal distribution of measurements—all either 100% porosity (within a pore space) or zero porosity (within a grain). Subpanels *b*, *i* and *b*, *ii* illustrate a scale still too inappropriately low to yield measurements representative of the entire groundwater system (e.g., the same value regardless of point of measurement). Only the large-scale panel *b* (circle shown in brown) would ensure that regardless of the spatial location in the groundwater formation, the same representative measure of porosity would be made. At the other extreme, particularly in heterogeneous systems with different lithologies, the scale can again become nonrepresentative (e.g., *c*, *i*; *c*, *ii*; *c*, *iii*) if the scales capture only part of the overall heterogeneity. Abbreviation: REV, representative elemental volume.

to date in the literature, large assumptions are typically made based on either assuming physical
parameters for key rock types or applying generic models of, for instance, porosity and perme-
ability functions with depth (Bethke 1985, Ingebritsen & Gleeson 2017). Such approaches may
provide a useful estimate of macroscopic features of the hidden hydrogeosphere that can facilitate,
for instance, first approximations of the volume of deep groundwater (Warr et al. 2018, Ferguson
et al. 2021) or initial global estimates of the deep subsurface biomass (Magnabosco et al. 2018).
However, such global or regional averaging simultaneously obscures the localized details that may
be critical to addressing some of the most pressing questions facing society as it struggles to un-
derstand subsurface processes, resources, and biomass as we transition to a new global climate
and energy future. As discussed with examples provided below, these questions critically require
moving beyond mean or global averages.

**2.5. Moving Beyond the Mean**

**2.5.1. Insight from other disciplines: population demographics.** By and large, groundwater dating literature focuses on reporting mean ages, but the same is not true of all disciplines. Perhaps more than any other discipline, the subtleties of age are most explicitly addressed, and most intuitive to consider, in population demographics. Certainly, the fact that populations range from newborns to centenarians reinforces the simple concept that a mean age for a population may obscure much more than it informs. Interpretation of populations explicitly incorporates age structure including distributions around a mean; constraints on the left-hand tail or right-hand tail (i.e., youngest and oldest segments of a population, respectively); consideration of the degree to which a population is open versus closed (immigration and emigration rates); and, in particular, the changes in the relative rates of population growth, decline, and redistribution about the mean over time. Such a nuanced approach is also required in the context of groundwater dating.

**2.5.2. The role of multiple tracers.** Closer to the discipline of groundwater, oceanographers also have a well-developed paradigm for addressing the age of the oceans, largely drawing on the concept of residence time (amount of water in the oceans divided by the estimated input from rivers and other sources, versus the rate of loss due to evaporation and other processes). Most importantly oceanography distinguishes between the residence time of water itself (defined as above) versus the residence time of the different dissolved components in water. Residence times range from hundreds of years (iron, aluminum) to 1 million years (calcium) and 100 million years (chlorine) based on the average concentration of the particular ion in seawater divided by the input rate (Chester & Jickells 2012). The input rate itself is defined as the average concentration of the ion in river water multiplied by the estimated river discharge to the oceans. Means and averages are still deeply embedded in this approach, but the concept of sets of residence times based on different components in the oceans (reactive ions, conservative ions, and H₂O itself) is well established. Bethke & Johnson's (2008) review advocated a similar approach for groundwater dating—recognizing that ages calculated from physical flow [typically 1D estimates of time from recharge to a given subsurface point (Supplemental Figure 1a)] are inherently different than ages derived from chemical tracers (based on dissolved constituents within the water flow). Their review advocated that integration of both physical and chemical approaches, and sets of age markers, is essential for a comprehensive understanding of the system. This current review takes this a step further, advocating that for many of the current scientific and societal challenges of hydrogeology and planetary sciences, not only should multiple tracers be used but also they should be strategically combined to explicitly target and define both the oldest (right-hand tail) and youngest (left-hand tail) components as well as the mean properties of the system (Figure 4).

There remain compelling reasons in hydrogeology to focus on the age of residence time that best reflects the mean. For groundwater resource assessment in particular, this approach is entirely appropriate as it best allows assessments of groundwater flow, storage, or yield that transcend smaller-scale heterogeneity and provide an aquifer-scale or regional estimate of groundwater resources. For example, Gleeson et al.'s (2011) goal of mapping groundwater to 1 km depth at a

Supplemental Material >
<figure><img src="image_7.png" /><figcaption>Figure 4</figcaption></figure>

Conceptual schematic of residence time distribution of age mass approximated as a Gaussian bell curve (though in reality likely to be non-Gaussian). Most age-dating approaches return a mean value for a given groundwater, and most interpretations focus on that information. Here we emphasize the importance of more detailed consideration of the age distribution in any given groundwater system. The LHT represents the youngest drop, or younger contributions to the overall system—critical, for instance, when determining the most recent possible introduction of microbes. The RHT meanwhile represents the oldest drop—critical, for instance, for determining the longest timescales on which fluids may be preserved in the subsurface of Earth and other planets. Abbreviations: LHT, left-hand tail; RHT, right-hand tail.

regional to global scale used a spatial scale on the order of 5 km specifically to ensure they were above the scale at which heterogeneities such as discrete fractures and lithologies were major controlling features of groundwater flow. Similarly, Scanlon et al. (2023) apply an even larger scale of hundreds of cubic kilometers to take an integrated look at surface water-groundwater resources using water storage data from the Gravity Recovery and Climate Experiment (GRACE) satellite to constrain spatiotemporal variability in light of global water stress and climate change. While arguably these approaches work better for sedimentary sequences closer to the canonical homogeneous porous media so beloved in hydrogeology, and are less appropriate for fractured basement rock, they nonetheless provide excellent examples of the appropriateness of focus on a mean or REV when estimation of groundwater resources is the main objective.

Where other scientific questions are being asked, however, the mean residence time, like a population age mean, may obscure more than it enlightens, and different approaches are required. A few illustrative examples are provided here from environmental groundwater resource science, from deep geologic repository and waste disposal investigations, from the rising interest in exploration for nonconventional and novel sources of helium and hydrogen, and finally from exploration of the deep biosphere and the implications for astrobiology and the search for life in the universe.

### 2.5.3. The youngest drop.
For applications to groundwater contamination, the most important information might not be mean transport time (where the bulk of the fluid reaches a critical point such as an off-site receptor) but instead the first arrival time—the earliest point at which environmentally sensitive receptors might be impacted by contaminants transported in groundwater. Where protection of aquifers is concerned, this issue comes very much to the fore as illustrated by the study by Jasechko et al. (2017) pointing out that even “fossil groundwater aquifers” (i.e., those
whose mean age or residence time is estimated to be greater than 12,000 years old, in this case) cannot be assumed to be protected from surface contamination. In such cases, sets of groundwater dating tools can reveal the youngest recharge contribution to these otherwise paleometeoric waters. This further emphasizes the importance of multiple tracers as, while ¹⁴C dating might provide information on the bulk average age of the aquifer, ³H may provide constraints on the youngest contributions from surface recharge and hence best estimate of resource vulnerability. As societies continue to focus on deep geologic repositories for high- and low-level nuclear waste (Tsang et al. 2015) and increasingly investigate subsurface storage sites for carbon sequestration (Leung et al. 2014) or for novel sources of helium or hydrogen (Danabalan et al. 2022, Warr et al. 2022, Cheng et al. 2023), here too the most relevant question is not the mean age of isolation of the geologic system selected for storage or exploration but the youngest contribution and potential for fluid transport/loss from the system. Prior to emplacement of any subsurface storage system, understanding the youngest components is critical to assessing the potential routes and rates for leakage of sequestered materials and evaluating any adverse impacts from their mobilization. Similarly, understanding the potential for an economic exploitation of resources of helium or hydrogen requires understanding trapping effectiveness and the timescale of any transport and loss due to advection or diffusion (Cheng et al. 2021, 2023; Warr et al. 2022).

**2.5.4. The oldest drop.** In other cases, the most pertinent question is not what is the mean age of a groundwater system, nor the youngest fluid contribution, but what is the oldest drop? The saline fracture waters at 2.4 km below surface in the Kidd Creek Observatory of the Canadian Shield were the first groundwaters to show mean residence times of more than one billion years in age (1.1–1.7 Ga) based on ⁴He, ²¹Ne, ⁴⁰Ar, and ¹³⁶Xe radiogenic noble gases (Holland et al. 2013). Because microbial communities in groundwater could be as young as the most recent groundwater that penetrated a rock or fracture system, determining the youngest possible groundwater component in such systems is a critical question for understanding the timing of life's colonization of the subsurface (Lollar et al. 2019), but an equally intriguing question arises from constraining the oldest fluid contribution. At Kidd Creek, the conservative, inert noble gases demonstrated both the presence of a characteristic neon isotopic signal first identified in more than 2 Ga old regional metamorphic fluids in the Kaapvaal Craton, South Africa (Lippmann-Pipke et al. 2011), and the presence of an ¹²⁹Xe excess indicative of a preserved component within the Kidd Creek system from approximately 2.64 Gyr (Holland et al. 2013, Warr et al. 2018). While fluid inclusions have long been known to preserve components of fluids and gases of such advanced age, these studies were the first to demonstrate the presence of such components in macroscopic-scale groundwater systems within these deep subsurface fractures. The significance of these systems is still being explored as more sites with billion-year-old fluids are identified (Warr et al. 2022), but they provide both an opportunity for a novel set of samples to investigate the ancient fluid history for the planet, as well as indications of the geologic timescale on which fluids can be preserved in the deep fractured rocks of the terrestrial crust. Such ancient groundwaters are providing new insights not only into Earth's history and coevolution of the planet with life but also into the possible timescale of preservation of fluids in the deep subsurface of astrobiology targets such as Mars (NASEM 2019, 2022).

# 3. EVOLVING CONCEPTS IN APPLICATIONS OF GROUNDWATER DATING

## 3.1. Beyond the Million-Year-Old Challenge

**Figure 5** shows the major isotopic geochemical tracers used to infer groundwater age. These tracers can be broadly divided into two categories: (a) degradative tracers originating from the
<figure><img src="image_8.png" /><figcaption>Figure 5</figcaption></figure>

The major isotopic geochemical tracers most commonly used to infer groundwater age and their respective applicable age ranges (Phillips & Castro 2003, Schilling et al. 2017, Warr et al. 2023a). While $^{21}$Ne can also be used alongside the other cumulative noble gases, the production of this isotope is dependent on the oxygen and fluorine content of the host rock in the vicinity of U-bearing minerals, which can result in considerably different production rates of this tracer and therefore introduce uncertainty in its application as a hydrochronologic tracer (Ballentine & Burnard 2002; Lippmann-Pipke et al. 2011; Warr et al. 2018, 2023a). Likewise, while $^{86}$Kr is produced in the crust through U decay, the production rates are so low that this tracer typically cannot be measured above baseline concentrations in most geologic settings (Ballentine & Burnard 2002, Warr et al. 2022).

atmosphere that enter the subsurface via groundwater recharge and decrease in concentration over time due to natural radioactive decay and (b) cumulative tracers that are produced over time due to subsurface production and accumulate in groundwater via a range of WRIs. Input functions are based on atmospheric concentrations that are entrained in groundwater during recharge. Within the first category, the most established tracers include tritium ($^{3}$H), radiocarbon ($^{14}$C), and $^{36}$Cl, as well as $^{37}$Ar, $^{39}$Ar, $^{81}$Kr, and $^{85}$Kr. Subsurface (bio)geochemical processes for the first category can introduce additional complexity, most notably addressed in the case of $^{14}$C, where processes such as organic carbon oxidation, carbonate dissolution, and matrix exchange can affect the calculated age (e.g., Wigley et al. 1978, Fontes & Garnier 1979, Sudicky & Frind 1981, Andrews et al. 1989, Maloszewski & Zuber 1991, Wassenaar et al. 1991, Clark & Fritz 1997, Phillips & Castro 2003, Han & Plummer 2013). Importantly, the spectrum of radioactive (degradative) tracers can address a range of groundwater ages, from decades ($^{3}$H) through to the Ma timescale ($^{36}$Cl, $^{81}$Kr). After ~1 million years, however, this suite of degradative tracers can no longer be applied due to natural decay reducing concentrations to below modern detection limits, or, in the case of $^{36}$Cl, subsurface processes overprinting any initial signal.
Hence, while multiple tracers are available to date groundwater up to an age of about 1 million years, a long-standing gap has been dating of groundwaters of greater antiquity. This presents a particular challenge for investigation of the hidden hydrogeosphere and is likely one of the reasons so many aspects of this deep component of Earth's water cycle have been underappreciated for so long. In these situations, the second category, the cumulate tracers, becomes increasingly important. Cumulative tracers most notably include components of the noble gases (e.g., <sup>4</sup>He, <sup>40</sup>Ar, <sup>136</sup>Xe) produced via the decay of naturally occurring radioelements in the crust (U, Th, K) that progressively concentrate in the water over time (Ballentine & Burnard 2002). In recent years these have successfully extended the ability to date groundwaters from Ma to Ga timescales in deep crustal systems (e.g., Bottomley et al. 2005; Greene et al. 2008; Lippmann-Pipke et al. 2011; Holland et al. 2013; Kietäväinen et al. 2014; Heard et al. 2018; Warr et al. 2018, 2022). This expansion of these tools to investigate old groundwater, including the right-hand tail (Figure 4), has catalyzed a range of investigations, including the contribution of ancient atmospheric and metamorphic fluids to deep groundwaters (Kendrick et al. 2011, Lippmann-Pipke et al. 2011, Avice & Marty 2020) and investigation of the in situ processes producing radiogenic noble gases via water-rock reactions, of which a few examples are highlighted below.

## 3.2. Cycling Elements Beyond Hydrogen and Oxygen in Groundwater

The canonical water cycle considers groundwater to be only a transient visitor to the subsurface. In this approach, interaction between groundwater and host rock is considered principally from a mechanical perspective (in terms of fluid flow), and with notable exceptions such as the case of <sup>14</sup>C (see Section 3.1), is generally appropriate for settings where groundwaters cycle on short to moderate timescales. Where groundwaters have sufficiently high residence times, however, water-rock-driven processes can modify the properties of groundwater to a remarkable extent, increasing salinity; trace metal content; and dissolved, reduced, and other gases (H<sub>2</sub>, CH<sub>4</sub>, and higher hydrocarbons, as well as He, N<sub>2</sub>, and Ar), and driving extensive elemental cycling of hydrogen, carbon, oxygen, nitrogen, sulfur, and other elements, including driving isotopic signals off the GMWL as discussed previously (e.g., Frape & Fritz 1982; Frape et al. 1984, 2003; Sherwood Lollar et al. 1993, 2002, 2021; Kloppmann et al. 2002; Ward et al. 2004; Onstott et al. 2006; Holland et al. 2013; Kietäväinen et al. 2013; Li et al. 2016; Bomberg et al. 2021; Warr et al. 2021). Such subsurface water-rock reactions have been quite extensively studied in the context of generating electron acceptors and donors such as H<sub>2</sub> and sulfate for the subsurface biosphere (e.g., Lefticariu et al. 2010, Sherwood Lollar et al. 2014, Li et al. 2016, Sauvage et al. 2021) and exploring habitability models on Earth and beyond (Onstott et al. 2019; Tarnas et al. 2018, 2021; Templeton & Caro 2023), or the role of radiolysis in generating and accumulating essential societal resources such as He and H<sub>2</sub> in the crust (Sherwood Lollar et al. 2014, Warr et al. 2019, Danabalan et al. 2022, Cheng et al. 2023). To date, however, the implications of these water-rock reactions for the underpinning assumptions of the isotopic tools to investigate groundwater ages have received much less attention.

Warr et al. (2023a) recently demonstrated there are underappreciated complexities regarding the basic assumptions concerning <sup>3</sup>H, <sup>14</sup>C, <sup>36</sup>Cl, <sup>39</sup>Ar, and <sup>81</sup>Kr dating (Figure 5). Specifically, in situ production reactions may affect the concentrations and subsurface baselines of these degrada-tive short-lived tracers. This challenges the prevailing assumption in traditional groundwater dating methods, wherein the starting baseline is defined by atmospheric concentrations of these tracers, and after recharge their concentrations are assumed to be controlled simply by their ra-dioactive half-life decay times. With the notable exception of <sup>14</sup>C (see Section 3.1), the influence of the host rocks and WRIs have typically been assumed to be negligible in most settings (e.g., Zito et al. 1980; Andrews et al. 1989, 1991; Lehmann et al. 1993; Lehmann & Purtschert 1997; Bethke & Johnson 2008). Warr et al. (2023a) reexamined this issue and documented the role of
neutron capture reactions controlling subsurface production of tracers such as ³H, ¹⁴C, ³⁶Cl, ³⁹Ar, and ⁸¹Kr, with production occurring both in the host matrix and in associated fluids. Crucially, the study found that while applications of ³H and ⁸¹Kr are largely unaffected, in situ production of ³⁶Cl, ³⁹Ar, and ¹⁴C can produce an elevated subsurface production baseline that, if unrecognized, leads to false young ages for groundwaters that, in reality, may be more than ~1 million years in age. While invoked in some previous studies (Fontes et al. 1984, Lehmann et al. 1993, Purtschert et al. 2021), and most thoroughly examined in Andrews et al. (1989) for a case study of the Stripa nuclear waste disposal lab in Sweden, Warr et al. (2023a) provides a systematic demonstration of the impact for the degradative tracers being applied to groundwater dating and demonstrates the quantitative steps required to identify and address any discrepancies in groundwater ages. The high rock/water ratios, fractured crystalline rock settings, and likelihood of ancient groundwaters make these issues particularly critical for the hidden hydrogeosphere.

### 3.3. Integrating Statistical and Combinatorial Approaches for Probabilistic Modeling and Uncertainty Evaluation

The applications of cumulative tracers such as the radiogenic noble gases for groundwater dating (Figure 5) are also impacted by the effects of host rock lithology, water-rock reactions, and subsurface production processes. This is particularly important in the context of error analysis. Bethke & Johnson (2002a, 2008) flagged the importance of addressing not only measurement (analytical) error in developing uncertainty analysis but also the still-today underdeveloped concept of conceptual error. The radiogenic noble gas tools provide an important example of this, where data are still too often reported as mean values only (see Section 2.5), and with total uncertainty expressed as only the analytical uncertainty. In actual practice, groundwater ages are calculated by integrating noble gas data with variables including host rock mineralogy and fluid geochemistry, of which radioelement concentrations, density, and porosity introduce the largest uncertainty (Warr et al. 2023b). We echo Bethke & Johnson's (2002a,b) recommendations and suggest consistent integration of these additional levels of uncertainty into the error bars used to express groundwater ages is required in the future (e.g., Holland et al. 2013; Heard et al. 2018; Warr et al. 2018, 2022). In the next section, we also raise recommendations concerning best practice in groundwater dating using radiogenic noble gases. These are issues that are increasingly significant for the many exciting research questions that are being pursued in the hidden hydrogeosphere but that are illustrated here, for brevity, in the context of radiogenic production of H₂ and He—elements not only relevant to the deep biosphere and habitability models on Earth and off but also of potential use for society's pressing energy transition.

Given the typical paucity of site-specific information, even for basic parameters such as porosity, permeability, or U, Th, and K concentrations, as discussed in Section 2.4, current practice in the substantial literature investigating these issues (including groundwater dating studies and also studies estimating subsurface production of H₂, He, and SO₄, abiotic formation of CH₄, etc.) is to apply global or regional average values for the important input parameters. Examples include, for instance, using average porosity values published for certain lithologies, or global porosity and permeability functions with depth (e.g., Ingebritsen & Manning 1999, Ehrenberg & Nadeau 2005, Sherwood Lollar et al. 2014, Achtziger-Zupančič et al. 2017), or average radioelement concentrations based on specific lithologies (e.g., Lin et al. 2005a,b, 2006; Onstott et al. 2006; Dzaugis et al. 2016; Warr et al. 2019; Sauvage et al. 2021). However, taking this approach results in only an average for any modeled process. This harks back to the previous discussion of the significance of scaling in terms of REV and how to appropriately incorporate scaling, as such approaches can lose their relevance to the important local site information. This can be particularly critical if the issue includes, for instance, a question such as is the He or H₂ found in a geologic trap sufficient
for economic exploitation? Relying, as current approaches do, on global or regional averages as input parameters introduces significant risk in such estimations.

We highlight a recent emerging trend in Earth and planetary sciences to employ a spectrum of statistical approaches (e.g., Monte Carlo, Bayesian models, machine learning) to incorporate both uncertainty and variability, thereby to provide a more quantifiable and probabilistic approach when considering crustal processes related to both groundwaters and Earth and planetary sciences broadly (e.g., Sambridge & Mosegaard 2002; Gallagher et al. 2009; Warr et al. 2015, 2023b; Liu & Liang 2017; Sahoo et al. 2017; Sakan et al. 2019; Wang et al. 2020; Zounemat-Kermani et al. 2021; DeWitt et al. 2022; Karolytė et al. 2022; Warren-Rhodes et al. 2023). Statistical approaches can be used to define not only the upper- and lowermost estimates of groundwater ages or production rates but also the distribution within that range and its most probable value. They also allow evaluation of the combinatorial effect of multiple variables and quantification of the net effect on the model outputs. Further, these approaches can be used to develop more comprehensive estimates of total uncertainty that embrace the conceptual error recommendation championed by Bethke & Johnson (2002a,b). Ultimately these more statistical and quantitative approaches will not only improve delineation of time and age distributions of groundwaters but also critically enable the translation of process-level understanding of Earth's subsurface into quantitative rates of elemental production and transport, as discussed below.

### 3.4. Building on Groundwater Age Estimates to Extrapolate to Rates

Over the past 40 years the discovery of the role of water-rock reactions producing electron donors and acceptors that support a deep hot biosphere at hydrothermal vents and ocean floor spreading centers, as well as a deep cool biosphere to kilometer depths in the continental lithosphere, has revolutionized our understanding of habitability and the range of metabolic strategies for life (Templeton & Caro 2023). The marine and continental subsurface biospheres have been estimated to account for some 10–20% of Earth’s total biomass (Whitman et al. 1998, Kallmeyer et al. 2012, McMahon & Parnell 2014, Parkes et al. 2014, Bar-On et al. 2018, Magnabosco et al. 2018), but a wealth of questions that require constraints on time and rates remain unresolved:

* What are the production rates of critical electron donors and acceptors required to sustain deep subsurface communities, and on what timescale are such communities viable?

* How long can these communities survive in the subsurface? This question is relevant not only to understanding life on Earth but also to the search for life on other planets and moons.

* What do production rates of electron donors and acceptors from radiolysis and serpentinization tell us about the potential habitability of other planets and moons such as Enceladus or Europa?

The conceptual and methodological advances in the understanding and dating of groundwater laid out in this review are essential components required to extend existing habitability models. These will catalyze the necessary progression from initial identification and characterization of these subsurface communities and the processes on which their metabolisms are dependent to the vital next step of quantification of production rates of essential chemical species, of temporally evolving factors that affect habitability and metabolic rates, and of other fluxes relevant to transport of fluids and the evolution of the deep biosphere.

Further, recent studies are coupling an increasing wealth of genomic data to new hypotheses concerning the adaptive evolution of subsurface microbial life, the timing of ancient metabolisms and ancestral pathways, and the rate of differential evolution of biomes isolated from the surface or from each other (e.g., Spang et al. 2015, Boyd et al. 2020, Becraft et al. 2021, Templeton & Caro 2023). But a critical gap remains in terms of integrating quantitative time constraints on these
evolutionary and metabolic hypotheses. Studies must rigorously evaluate the assumption that mi-
crobial life is synchronous with its host rocks/sediments, as simply relying on sediment/rock age
or deposition rate as a proxy may result in a significant overestimation of ecosystem residence
time if penetration of these systems by potentially younger groundwaters and pore fluids is not
considered (Bengtson et al. 2014, Jaakkola et al. 2016, Bradley et al. 2020, Dai et al. 2021). Increas-
ingly, studies have taken the next step—recognizing that establishing the temporal constraints on
subsurface life required to develop evolutionary interpretations requires understanding not only
the relationship between microbiology and host rock/sediments but also the fluids that may have
much more recently inoculated the environment and/or enabled mixing between subsurface sys-
tems. Several recent studies have addressed this through sedimentological arguments (Morono
et al. 2020) or dating of fracture veins or minerals (Heim et al. 2012; Tillberg et al. 2019; Drake
et al. 2021, 2023). Other studies applied stable or radioactive tracers such as ³H, ¹⁴C, or ⁸¹Kr
(Purtschert et al. 2021, Trembath-Reichert et al. 2021), but the half-lives for these tracers are too
short to be applicable to deep fracture waters in the range of millions to billions of years explored
by the emerging data from many of the continental subsurface systems, for instance (Lin et al.
2006, Magnabosco et al. 2018).

Even where fluid residence times are incorporated (through either direct sampling of associ-
ated groundwaters or the record of fluid penetration of the system preserved in fracture minerals
and/or fluid inclusions), a sensitivity test incorporating both the youngest and oldest residence
time estimates is often lacking. Becraft et al. (2021) provide an example of this approach in their
study that challenged fundamental evolutionary paradigms by suggesting some deep subsurface
lineages may exhibit evolutionary stasis over 55–165 Ma. This was based on the closely related
genomes of a chemolithotrophic sulfate-reducing bacterium, *Candidatus* Desulforudis audaxviator,
identified in a set of subsurface locations dispersed thousands of kilometers apart on three
continents physically separated since the breakup of the Pangean supercontinent. Becraft et al.
(2021) insightfully note the risk associated with assuming breakup of the rocks at 55–165 Ma was
the only possible time constraint and also evaluate the possibility that subsequent groundwater
transport could control the colonization and penetration of the rocks in these three locations.
They note that based on the published age of one of the groundwaters [20 Ma Mponeng (Lin
et al. 2006)], the time of divergence (and hence evolutionary timescale) for *Candidatus* Desul-
forudis audaxviator might be shorter than their preferred timescale of 55–165 Ma. We note that
even this 20 Ma residence time may be an overestimation, however, given that the other two sites
from the Witwatersrand Basin included in the study have been shown to contain mixing with
much younger fluids and may have a left-hand tail an order of magnitude lower, on the order of
only thousands of years (Simkus et al. 2016, Heard et al. 2018). While the study raises important
questions about the explicit and implicit assumptions about microbial evolution based on findings
in the deep subsurface biosphere, a robust resolution of the timescales controlling the systems
under investigation is essential to resolving them. This is a classic example of the need not only
to investigate the age constraints from the rocks as well as the mobile fluid (or gas) phases but
also to integrate contextual understanding beyond the mean residence time that also includes the
right-hand tail and left-hand tail (see Section 2.5).

# 4. CONCLUSIONS

The urgent pressures society faces from rapid climate change and global warming and the pressing
need for a green transition require us to understand and predict the impact of global groundwater
resource distribution and changing recharge patterns (Taylor et al. 2022) for society's drinking
water and agricultural and industrial needs. Even if not a source of drinking or agricultural water
due to its deep and often saline nature, the hidden hydrogeosphere remains at the front lines of
society's challenges with respect to the climate and energy transition. Helium and hydrogen are both produced in the lithosphere and, with rapidly depleting documented global sources of helium vital to medical, nuclear, and cryogenic applications, and the rising interest in finding hydrogen sources that are carbon neutral (i.e., not linked to conventional hydrocarbon gas exploitation), there is substantial interest in geologic settings where the two gases are found together (Cheng et al. 2023). Saline ancient groundwaters have hence become important targets for coexploration for helium and hydrogen (and potentially additional dissolved components of economic interest such as lithium). This coproduction is linked to decay of radioactive elements in the crust directly producing helium, as well as hydrogen indirectly via radiolytic decomposition of water. Depending on the mineralogy, hydration of ultramafic and mafic rock may also serve as an additional hydrogen production route (Sherwood Lollar et al. 2014). Evaluating the volumes and potential economic exploitation of such He- and H₂-rich systems requires constraining the rates of production, accumulation, and transport from these systems—all of which fundamentally hinge on the understanding of residence times of the fracture fluids (Warr et al. 2022). Investigations of the hidden hydrogeosphere have allowed science to discover the characteristics of systems where radiogenic chemistry has had a previously unrecognized impact, resulting in production of electron donors and acceptors capable of sustaining life and a basis for subsurface habitability previously overlooked, where chemoautotrophic communities may dominate, or where novel organisms adapted to extremes of temperature, pressure, or survival time may yet be discovered. Further, as society wrestles with the urgent challenges of energy transition, geologic repositories are needed for energy storage (e.g., H₂) or waste sequestration (e.g., carbon capture and storage or radioactive waste). This requires robust understanding of the age and associated migration rates of the groundwater in order to determine whether systems are hydrogeologically tight enough such that surrounding subsurface environments and groundwater resources are sustainably protected. The competing demands modern and future society will put on the hidden hydrogeosphere require rapid expansion of our understanding of these least explored but vast compartments of Earth's hydrogeologic cycle.

## DISCLOSURE STATEMENT

The authors are not aware of any affiliations, memberships, funding, or financial holdings that might be perceived as affecting the objectivity of this review.

## ACKNOWLEDGMENTS

This review was financially supported by Natural Sciences and Engineering Research Council of Canada Discovery and New Frontiers in Research Fund grants to B.S.L., and a Natural Sciences and Engineering Research Council of Canada Discovery Grant to O.W. B.S.L. is a Director of the Canadian Institute for Advanced Research Earth 4D Subsurface Science and Exploration program. Ideas articulated in this review were originally developed for the Royal Society UK meeting on the origin, history, and role of water in the evolution of the inner Solar System in February 2016, and thanks are acknowledged for discussions with colleagues at that meeting and subsequent discussions with the Earth 4D group.

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