The isotopic signature of El Chaltén's water: how the nivoglacial system works and why it needs continuous monitoring

Every day, the De Las Vueltas River receives water from glaciers, streams, and lagoons throughout the northern basin. But where does every drop that forms it actually come from? Within that current, waters of different origins coexist: snow accumulated in the highlands, glacial ice formed over centuries, rain from last week, and water that spent time in a lagoon under the Patagonian sun. To the naked eye, it is impossible to tell them apart. However, water carries a mark of its origin—and for the first time, a scientific study has read it in the Northern Zone of Los Glaciares National Park.

The study, coordinated among the Institute of Marine and Coastal Research (IIMyC, CONICET-UNMdP), the "Dr. Enrique Jorge Schnack" Institute of Coastal and Quaternary Geology (IGCyC, UNMdP-CICPBA), the National Parks Administration, and the BOANA Association, surveyed 81 sampling sites across approximately 850 km²—ranging from surface snow and glacial ice to streams, rivers, lagoons, and Lake Viedma—covering an altitude gradient between 252 and 1,510 meters. This is the first comprehensive isotopic characterization of this system, one of the most important freshwater reserves in southern Patagonia.

The fieldwork was conducted between 2021 and 2022, when the 81 sampling points were surveyed. Processing the samples, laboratory analysis, and data interpretation required careful and sustained work. It is only now, in 2026, that the full results are available, which will be presented at the XIII Argentine Congress of Hydrogeology and the IX Spanish-Latin American Seminar on Current Topics in Groundwater Hydrology. This journey—from the field campaign for sample collection, through laboratory analysis, to the interpretation of results—illustrates why scientific research takes time: each stage is necessary for the conclusions to be robust, reproducible, and reliable.

What follows is what this snapshot reveals about how El Chaltén's water functions, and why a single snapshot—no matter how complete—is not enough.

What is the water's "signature"?

Although all water molecules have the same chemical composition (H₂O), they do not all possess exactly the same isotopic composition. This is because oxygen (O) and hydrogen (H) atoms can exist in different variants called stable isotopes, which share the same chemical behavior but have a slightly different mass.

Consequently, there are also water molecules that are slightly heavier or lighter, depending on the isotopes they contain. In nature, lighter molecules widely predominate, while heavier ones are much less abundant. However, the ratio between them changes in predictable ways depending on the processes the water undergoes, such as evaporation, condensation, or snow formation.

The logic is simple: water that falls as snow in high mountains becomes relatively "depleted" of heavy isotopes because heavier water molecules tend to condense and precipitate first. Conversely, when water evaporates from a lagoon, lighter molecules escape into the atmosphere more easily. As a result, the water remaining in the lagoon becomes relatively "enriched" with heavy isotopes. Thus, each type of water leaves a characteristic "isotopic signature," measurable in the laboratory and expressed through δ¹⁸O and δ²H values.

It is worth clarifying that, in this context, the terms "depleted" and "enriched" do not refer to water quality or purity, but rather to the proportion of heavy isotopes relative to light ones. Depleted water contains a lower proportion of heavy isotopes and presents more negative δ¹⁸O and δ²H values; conversely, enriched water contains a higher proportion of those heavy isotopes and presents less negative δ¹⁸O and δ²H values. It is simply a way to describe the water's isotopic composition, which functions as a fingerprint of origin: high-altitude snow, with δ¹⁸O values near -22‰, is the most depleted in the system, while lagoons, with δ¹⁸O values up to -5.6‰, are the most enriched.

This signature functions like an identity document for water: it allows scientists to reconstruct where it comes from and what processes it went through, without needing to have physically followed its path.

A gradient from snow to rivers and the lake

The first finding of the study is that the system displays a clear isotopic gradient. Surface snow shows the most depleted signal, with an average δ¹⁸O value of -22‰. Glacial ice appears one step above (δ¹⁸O = -15.5‰), followed by meltwater streams (δ¹⁸O = -13.1‰). Rivers sit at -14.3‰, and lagoons reach the most enriched values in the entire system, with an average of -11.7‰ and extreme cases of up to -5.6‰ of δ¹⁸O.

This journey tells a story: the isotopic composition of water changes as it flows down the mountain and moves through the system. And behind this entire gradient, there is a dominant factor. Monthly samples of local precipitation, collected over a full hydrological year, showed a wide range (-19‰ to -6.5‰) that encompasses almost all the variability of the other environments. In other words: the system's isotopic signal originates from precipitation. Glaciers store and regulate it, but this atmospheric input is the primary source feeding snow, ice, rivers, and lagoons.

The twist: still water changes the most

Here is the most counterintuitive result. One would expect a calm, crystal-clear lagoon to be the most stable water body in the system, and a churning, constantly moving river to be the most variable. The data says exactly the opposite.

Rivers are the most homogeneous environments in the entire system: just ±0.3‰ of variation across 24 different sites distributed throughout the basin. Lagoons, on the other hand, are the most variable, with a dispersion ten times greater (±3.6‰).

The explanation lies in the processes acting on each environment. The river presents a more stable isotopic signal: its composition changes very little between different sampling sites because it integrates multiple contributions at the basin scale, thereby averaging out the differences between them. The lagoon, by contrast, remains more exposed to local processes like evaporation. The Patagonian sun and wind favor the evaporation of lighter molecules and relatively concentrate the heavier ones.

But there is an important nuance to this finding: size matters. Lake Viedma, despite being a lentic (still) body of water, displays a stable isotopic signal (±0.7‰). Its large volume and depth reduce the relative impact of evaporation that heavily affects shallow, exposed lagoons. It is not "still water versus moving water": it is the combination of exposed surface area, volume, and environmental conditions that defines how much the water's isotopic signal can transform.

Don't judge water by a single measurement

The study did not stop at stable isotopes of the water molecule: it also integrated physicochemical and microbiological variables. A key lesson for water management emerges from this combination.

Some high-mountain streams presented electrical conductivity values higher than the rest of the aquatic ecosystems—above 350 µS/cm—a measurement related to the amount of dissolved minerals in the water (salinity), yet they recorded no significant presence of Escherichia coli (a bacterium indicating fecal contamination). These values do not necessarily indicate pollution: in this case, they can be explained by water coming into contact with naturally mineralized rock formations that release salts. If water quality were evaluated solely based on conductivity, these environments could be mistakenly interpreted as problematic.

The reverse case is also possible. Water can present physicochemical parameters within expected ranges and still be affected by contamination. This is why it is necessary to combine different types of information: the isotopic signature reveals the origin and path of the water, conductivity provides data on its mineralization, and microbiological analysis detects biological contamination indicators. No single variable tells the whole story on its own.

From a snapshot to a movie: what is a baseline?

So far, we have a snapshot of the hydrological system. But the true value of this work lies not just in what it reveals today, but in what it allows us to build for the future.

What the study established is known as a baseline: the first systematic characterization of a system, a reference point from which to compare changes that occur over time. In water resource management, having a baseline allows researchers to identify trends, evaluate alterations, and recognize potential threats before their effects become irreversible.

The idea is intuitive: a single photo cannot tell you if something is changing. Multiple measurements over time are needed to detect trends. Without a baseline, any change in the system is difficult to interpret because there is no point of comparison. With it, every future measurement adds a new piece to reconstruct the story of the water.

Being a National Park protects the landscape, not necessarily the water

There is a widespread misconception: thinking that because an area is inside a National Park declared a World Heritage Site, the water is already protected and monitored. The reality of managing protected areas is more complex.

International research shows that the design and management of terrestrial protection are often inadequate for the specific processes of freshwater ecosystems. Furthermore, many aquatic ecosystems within protected areas are not sheltered from impacts arriving from upstream. Water crosses map boundaries—and so do impacts. Protecting the landscape does not equate to monitoring the water.

The study itself provides a proof of concept of why this matters. Downstream from El Chaltén's wastewater treatment plant, on the Fitz Roy River, the highest E. coli values in the entire system were recorded: 370 CFU/mL, along with increases in the lower basin of the De Las Vueltas River. A single campaign already detected a signal of negative impact near the urban area, in a zone of growing tourism pressure. If a single snapshot revealed this, one can imagine what could be anticipated with continuous, long-term monitoring.

Why isotopes, in particular, need time

There is a reason why environmental isotopes are an especially valuable parameter for long-term monitoring. Temperature, pH, or conductivity describe the state of the water at the exact moment of measurement. Isotopes provide different information: they preserve a signal related to the origin of the water and the hydrological processes it underwent. In that sense, they are a parameter with a "memory of origin."

And precisely because of this, their value unfolds through temporal comparison. If, in a few years, the rivers show an isotopic signature closer to that of recent precipitation and less like glacial ice, it could indicate a change in the relative contribution of different water sources. An isolated measurement cannot interpret this evolution. A time series can.

To be precise: isotopes alone do not define phenomena like peak water—the point at which the contribution of a retreating glacier begins to decline—which requires glacial mass balance models, flow series, and climate data. Isotopes are just one piece within a broader water monitoring system that combines natural tracers, ice measurements, and hydrological observations. But they are a piece that only truly makes sense when sustained over time.

Measure, compare, correct

This is how a changing system is cared for: through what environmental management calls adaptive management, an iterative process of monitoring and evaluation that allows decisions to be adjusted as data reveals where the system is heading. Nivoglacial systems change slowly and silently; often, the damage only becomes visible once it has already occurred—unless it is being measured beforehand.

The first snapshot of El Chaltén already exists. And it was built in an unusual way: the study was an unprecedented participatory experience in Argentina's protected areas management system, with neighbors, mountaineers, and park rangers collecting samples alongside scientific teams and facilitating access to remote sectors.

What follows is sustaining that gaze over time. Because the water of this territory—the water that sustains ecosystems, supplies the town, and forms part of the landscape that attracts people from all over the world—deserves a whole movie, not just a single snapshot. And that movie, just like the first image, is best built together.

Acknowledgments

We thank the Association of Friends of Los Glaciares National Park for purchasing the sampling materials, and the El Chaltén Health Post for lending the use of their laboratory. The author thanks Asunción Romanelli and Valeria Crosa for reviewing the text and providing feedback.

References

• Romanelli, A., Quiroz Londoño, O. M., Martínez, L., Crosa, V., Aniere, M., Martínez, D. & Esquius, K. S. Isótopos estables del agua en ambientes acuáticos de montaña y glaciares del Parque Nacional Los Glaciares (Patrimonio Mundial UNESCO). IGCyC (UNMdP-CICPBA) / IIMyC (CONICET-UNMdP) / Administración de Parques Nacionales (APN) / Asociación BOANA.

• On water management and monitoring in protected areas:

  • Durable Freshwater Protection: A Framework for Establishing and Maintaining Long-Term Protection for Freshwater Ecosystems and the Values They Sustain. Sustainability, 2021.

  • From assessment to action: informing water resource management in protected areas amid global change. Frontiers in Water, 2025.

  • IUCN World Commission on Protected Areas — guide on inland water protection.