Groundwater ‘Ages’ Hide a Factor of 60 — And That Error Could Finally Reveal How Water Really Moves Underground

Beneath the surface of nearly every hydrogeology textbook sits an assumption so widely repeated that most researchers no longer question it: that the age of a groundwater sample, calculated from an environmental tracer such as radiocarbon or tritium, tells us how fast the water has travelled through the aquifer. A new theoretical study published in Hydrogeology Journal by Axel Suckow of CSIRO Environment in Australia dismantles that assumption with striking precision. The work shows that for realistic amounts of underground mixing, the difference between the age a tracer reports and the true advective travel time of the water can reach a factor of sixty. Far from being an inconvenient error, however, that discrepancy turns out to be a gift: it offers, for the first time, a practical way to measure one of the most elusive parameters in all of subsurface science — longitudinal dispersion — at scales of centuries to hundreds of millennia.

The confusion stems from the fact that ‘groundwater age’ is not one quantity but three. The first is the Peak Age, sometimes called the piston-flow or advective age, which describes how long it takes the centre of a concentration front — the peak of a tracer pulse — to travel a given distance. The second is the Mean Residence Time, the average age of all the water molecules in a sample, equivalent to the centre of mass of the age distribution. The third is the Tracer Age, the number actually calculated from a measured tracer concentration using standard formulas such as the conventional radiocarbon age or the tritium–helium age. In a perfectly ordered, dispersion-free aquifer these three values coincide. In the real world, where groundwater always undergoes at least some dispersive mixing, they diverge — sometimes dramatically.

Suckow’s analysis rests on the one-dimensional advection–dispersion equation, the workhorse model of solute transport, in which the balance between advective flow and dispersive spreading is captured by a single dimensionless quantity: the Peclet number. A high Peclet number means advection dominates and the water moves like a piston; a low Peclet number means dispersion smears everything extensively. Drawing on the classic field-data review by Gelhar and colleagues, the study notes that Peclet numbers observed in aquifers span an enormous range, from about 0.1 to 1000, with a best estimate near 10. When Suckow computed tracer output concentrations across that range, the consequences were sobering. For a Peclet number of 0.1, the Mean Residence Time exceeds the Peak Age by a factor of sixty; even at a Peclet number of 1, the gap is still a factor of six.

The behaviour of individual tracers adds further layers of complexity. For radioactive tracers with a constant atmospheric input — a reasonable approximation for argon-39, carbon-14 and krypton-81 — the calculated Tracer Age starts out equal to the Mean Residence Time and only gradually approaches the Peak Age as the water ages toward the tracer’s detection limit. For helium-4, a stable isotope produced underground at a roughly constant rate by the decay of uranium and thorium, the situation is different again: because helium mixes linearly rather than exponentially, its Tracer Age always reflects the Mean Residence Time and never the advective velocity. The practical result is that a single water sample, measured with a suite of modern tracers, will yield systematically different ages that increase in the order of tracer half-life: tritium, then argon-39, then carbon-14, then krypton-81, then helium-4.

Some of these discrepancies are large enough to rewrite interpretations of published datasets. The study highlights that reported contradictions between carbon-14 ages and helium-4 ages in major aquifers — differences previously explained by invoking a mysterious ‘crustal flux’ of helium rising from deep strata — may simply reflect low Peclet numbers. In the Great Artesian Basin of Australia, Torgersen and Clarke reported in 1985 that helium-based ages exceeded hydrologic age estimates by a factor of seventy-four, remarkably close to the factor of sixty that pure dispersion produces at a Peclet number of 0.1. Suckow is careful not to dismiss crustal degassing as a real phenomenon, but points out that such an external flux could only be quantified once the Peclet number is known — and under low-Peclet conditions the inferred flux would be substantially smaller than previously claimed.

The timing of this theoretical advance is no accident. For decades, the tracers needed to test these ideas simultaneously simply could not be measured on the same sample. That changed with the advent of Atom Trap Trace Analysis, or ATTA, a laser-based technique developed for krypton-81 and argon-39 that has turned isotopes once considered unmeasurable into routine tools. Before ATTA, only around 300 argon-39 groundwater measurements existed worldwide; today the two ATTA laboratories, in Hefei, China and Heidelberg, Germany, each process on the order of a hundred samples per year. Combined with the established workhorses tritium–helium, radiocarbon and chlorine-36, and supplemented by new stable noble-gas methods, researchers can now assemble continuous groundwater dating records spanning from a few decades to more than a million years — precisely the range needed to exploit Suckow’s framework.

The practical payoff is a new method for determining Peclet numbers on time scales of centuries to many millennia, something field tracer tests and contaminant plume studies — typically confined to transport distances of tens to hundreds of metres — have never been able to deliver. The logic is elegant: because each tracer’s age responds differently to dispersion depending on its half-life, the spread of Tracer Ages measured along a flow path encodes the Peclet number itself. For Peclet numbers below about 2, the differences between tracers become large and systematic; above 2, all tracers converge on the same age. By fitting the advection–dispersion equation directly to multi-tracer data from a series of wells, both the advective velocity and the Peclet number can be recovered simultaneously. Suckow demonstrates the approach on two published datasets — the Guarani aquifer in South America and the western Great Artesian Basin in Australia — deriving Peclet numbers of roughly 0.2 and 0.1, and correspondingly higher flow velocities than the original studies had estimated, a result with direct consequences for groundwater resource management in arid regions.

The findings also cast new light on palaeoclimate records preserved in groundwater. Noble-gas palaeotemperatures dissolved in recharging water have long been used to reconstruct past climates, and multiple studies report that land temperatures during the Last Glacial Maximum were about six degrees Celsius cooler than today. But dispersion acts as a low-pass filter on any signal carried by groundwater. Suckow’s calculations show that at Peclet numbers of 10 or less, temporal variations in the palaeotemperature record are progressively ironed out, and the reconstructed glacial cooling becomes biased warm — by nearly a degree at a Peclet number of 10, and potentially by several degrees at lower values. The widespread six-degree figure, in other words, may partly be an artefact of dispersive smoothing, and the true Last Glacial Maximum cooling on land could have been colder than commonly stated. The study also suggests that groundwater realistically preserves climate signals only up to about 100,000 years, even under favourable transport conditions.

For practitioners, the message is a call to methodological reform. Tracer Ages, the study concludes, should only be interpreted as advective or piston-flow ages when the Peclet number is known to exceed 2 — a condition rarely demonstrated in historical studies. Tritium–helium ages deserve particular caution: for all realistic dispersive conditions with Peclet numbers below 10, they do not represent the movement of a concentration front, as generally assumed, but instead follow or exceed the Mean Residence Time before levelling off with distance. The strongest recommendation is to measure as many tracers with different time scales as possible on every sample along a flow path, since no single tracer can disentangle velocity from dispersion. In an era when aquifers supply drinking water to billions and host critical infrastructure from nuclear waste repositories to carbon storage sites, knowing how water — and whatever it carries — actually moves underground has never mattered more. What was once dismissed as pedantic quibbling over definitions now looks like the key to one of hydrogeology’s oldest unsolved problems.

Subject of Research: The effect of dispersion on tracer-derived groundwater ages and its use for deriving Peclet numbers in large-scale groundwater transport

Article Title: The effect of dispersion on the “age” of groundwater calculated from environmental tracers allows a novel way to derive Peclet numbers for large-scale groundwater transport

Article References: Suckow, A. (2026). The effect of dispersion on the “age” of groundwater calculated from environmental tracers allows a novel way to derive Peclet numbers for large-scale groundwater transport. Hydrogeology Journal. https://doi.org/10.1007/s10040-026-03156-6

Image Credits: AI Generated

DOI: 10.1007/s10040-026-03156-6

Keywords: groundwater, environmental tracers, dispersion, Peclet number, hydrogeology, radiocarbon dating, krypton-81, argon-39, helium-4, advection, Mean Residence Time, paleoclimate