The number
Guillermina Bongiovanni, of CONICET and the Universidad Nacional del Comahue, sampled the waters of northwest Neuquén over six years. 350 samples, collected 2012 to 2018, published in Science of The Total Environment in 2020.
In Domuyo's thermal waters the study measured arsenic at up to 900 µg/L. At the confluence of the Estero Covunco and the Río Varvarco: 450 µg/L. Downstream in the Río Neuquén the concentration decays to about 100 µg/L. The finding was reported in Río Negro, which also carries the regulatory figures.
| Water | Arsenic | Against the WHO guideline |
|---|---|---|
| WHO drinking-water guideline | 10 µg/L | — |
| Argentine limit | 50 µg/L | 5× |
| Río Neuquén, downstream | ~100 µg/L | 10× |
| Covunco–Varvarco confluence | 450 µg/L | 45× |
| Domuyo thermal waters | up to 900 µg/L | 90× |
Read the bottom row again. That is ninety times the WHO guideline and eighteen times the Argentine national limit, in the water running down the valley system you walk into.
We could not find this mentioned in a single trekking source, in any language. Not Andeshandbook, not Wikiexplora, not the operator pages, not the English-language guides. The province promotes the springs. The science on the water was published in a first-tier journal six years ago. The two facts have never met on a page written for climbers.
Why a filter does nothing
This is the part that matters most, because the habit every trekker brings to a mountain is the wrong one here.
Arsenic in geothermal water is dissolved inorganic arsenic — arsenite, As(III), and arsenate, As(V). Not a particle. Not a microbe. Not a cyst. It is an ion in solution, chemically part of the water, at the same scale as the salt in it.
That single fact defeats every piece of field treatment in normal use:
| Method | Removes | Arsenic? |
|---|---|---|
| Hollow-fibre or ceramic filter (0.1–0.2 µm) | particles, bacteria, protozoa | No — dissolved ions pass straight through |
| Activated carbon | taste, organics, some chemicals | No — negligible affinity for arsenite |
| Boiling | pathogens | Worse — arsenic is non-volatile, the water leaves as steam and the arsenic stays behind, more concentrated than when you started |
| Chlorine dioxide, iodine, UV pen | pathogens | No — they attack organisms, not a dissolved metalloid |
| Reverse osmosis | dissolved ions | Yes, though As(III) is uncharged at these pH values and is removed far less well than As(V) unless oxidised first |
| Distillation | dissolved non-volatiles | Yes — for the same reason boiling fails: the arsenic cannot follow the steam |
| Specific adsorbent media (iron oxide/hydroxide, activated alumina, arsenic-selective ion exchange) | arsenic | Yes, with the same As(III) caveat |
Nothing in the right-hand column of that table fits in a pack in a form that works. There is no field arsenic filter you can hang from a tree. The three methods that do work are a household RO unit, a still, or a treatment cartridge sized for a village supply.
So the rule on Domuyo is not "treat the water". It is know which water, and carry the rest.
Continue reading Termas de Domuyo — the destination the summit is attached to →
Why the arsenic is there at all
It is not pollution. Nothing spilled it. The arsenic is leached from rock by a hydrothermal system that is one of the most powerful measured anywhere on Earth, and it has been doing this for a very long time.
Domuyo is not a stratovolcano. It is a Middle Pleistocene silicic dome complex sitting on a basement anticline that reactivated faulting pushed upward — Galetto et al. (2021) describe the core as a porphyritic granitic–dioritic stock, the top of a magma chamber. That shallow body is the heat source. The old basement faults are the plumbing. Water works down through fractured rock, sits at depth against hot rock, strips soluble elements out of it, and comes back up.
Chiodini et al. (2014), in the Journal of Volcanology and Geothermal Research, measured the advective thermal energy release of the system at 1.1 ± 0.2 GW, with equilibrium fluid temperatures of 190–230 °C. The provincial geothermal project working the same field quotes a geothermometric 220 °C at a reservoir 800–1,000 m down.
Be precise about what the 1.1 GW figure ranks. It is the second-highest measured advective heat output of any hydrothermal system on Earth, after Yellowstone — among systems that have actually been measured. It does not rank area. It does not rank volume. Domuyo is not "the second-largest hydrothermal system in the world", and the guides that say so have compressed a heat-flux measurement into a size claim it never made.
The surface expression is concentrated on the western slope: Los Tachos, El Humazo, Rincón de las Papas, Las Olletas, La Bramadora, Aguas Calientes. The protected-area record counts 18 hot springs and 2 streams above 40 °C, with published temperatures across the field running from 40 °C up to around 90 °C — many of them at or near boiling.
Los Tachos is four small geysers on the edge of the Covunco stream, throwing water and vapour over a metre, at 85–90 °C. There is a five-metre boardwalk and an 898 m approach path, which is exactly the infrastructure the temperature demands. That is boiling ground, not warm ground.
El Humazo carries a blunt note on its OpenStreetMap node, in Spanish: "Zona riesgosa. No se recomienda acceder." Risky area. Access not recommended. Somebody who knows the site wrote that.
The water map on the route
The normal route starts at El Playón, crosses the Estero Covunco on a footbridge, and follows the south bank up the valley. The Covunco drains the mountain's south side and runs down to the Río Varvarco, and so to the Río Neuquén. That is the drainage the 450 µg/L confluence sample sits at the bottom of.
Note the geometry before drawing conclusions from it. The trailhead crossing is at about 2,461 m. The Los Tachos geyser field sits on the Covunco's edge at 2,043 m — downstream of the crossing, not above it. The 450 µg/L sample is downstream of both. No published measurement exists for the Covunco at the footbridge itself.
| Point | Elevation | Water | Status |
|---|---|---|---|
| El Playón trailhead | 2,461 m | Estero Covunco at the footbridge | Unmeasured. Geothermal input to the Covunco is below this point, not above it |
| Campamento Base | 3,064 m | Spring 200–300 m northwest of camp | The one to use. Not visible from the campsite — you have to walk out and look for it |
| Campamento Base | 3,064 m | The lagoon beside camp | Brackish. Not drinkable |
| Campamento Base | 3,064 m | The spring feeding the lagoon | Not drinkable. Andeshandbook is explicit about this. It is a different spring from the one above |
| Campamento Los Pozos | 3,448 m | — | No source documents water here |
| Campamento 3.800 | 3,804 m | None | Exposed pircas, no water at all |
| Above 3,800 m | — | Snowmelt, or what you carried | Nothing running |
Two springs at base camp. One you drink, one you do not. They are not adjacent and the good one is not in sight of the tents. This is the single most useful sentence in the article for anyone actually walking up: the drinkable spring is 200 to 300 metres northwest of the campsite, and you will not see it from the campsite.
The commercial dome refuge at base camp confirms the arrangement from the other side. Guests fetch their own water from a nearby spring and are told to bring a container. There is no tap, no tank, and no treatment.
Above base camp, plan on carrying. The 3,800 m camp is dry. The summit day runs from there to about 4,690 m and back with nothing to draw from except snow, and snow cover varies sharply between seasons — one party reports "poca nieve a partir de los 4.000 msnm", another reports heavy penitentes in the same band. Melting snow costs fuel and time you may not have on a day with a hard turnaround.
Bathing is a different question from drinking
This needs saying carefully, because it would be easy to read the arsenic figure and conclude the springs are dangerous. No source we found says that, and we are not going to imply it.
The 10 µg/L WHO figure is a drinking-water guideline. It is a limit for water that is swallowed, applied over a lifetime of daily consumption. It is not a bathing standard, and it was never written as one. No source at any tier assesses the bathing risk at Aguas Calientes in either direction.
What the record does say is that bathing access at Villa Aguas Calientes is free — "acceso libre y gratuito" — that the ISSN runs five cabins and a camping area there, that the complex received a satellite antenna and new lodging in 2025, and that the season closes on 31 March. It is a legitimate destination, promoted by the province, and it is what most people who come to this valley actually come for. The 940 climbers the Army registered across the 2021–22 season are a rounding error attached to a hot-springs destination.
The practical line is narrow and easy to hold:
- Bathe where the province says you can bathe, at the temperatures the boardwalks and signage allow.
- Do not drink thermal water, do not fill bottles from it, do not cook with it, and do not brew tea with it.
- Do not go near Los Tachos or El Humazo off the marked path. At 85–90 °C the burn hazard arrives long before any chemistry does, and El Humazo's own map note tells you not to approach.
What is not known
Three gaps, stated plainly rather than papered over.
The base-camp spring has not been published as tested. Bongiovanni's sampling covered thermal waters and the river system. We found no arsenic measurement for the drinkable spring 200–300 m northwest of Campamento Base. Its status rests on Andeshandbook's field guidance, which is a good T2 source and is not a laboratory result. Treat it as the best available answer, not a certified one.
Snowmelt on the upper mountain is also unmeasured. It falls from the atmosphere onto a hydrothermally altered mountain and runs over sulphur-bearing, acid-altered ground. Nobody has published what that does to it.
The management plan is being rewritten right now. A working session in Varvarco brought the provincial environment ministry, the ranger service, Recursos Hídricos, the thermal-baths agency E.Pro.Te.N. and the local comisiones de fomento to the same table, with stated aims including the protection of "vertientes, ambientes termales" — springs and thermal environments. Rules about water access could change between seasons without a national gazette notice.
The working rule
Carry water out of Chos Malal or Varvarco for the drive. At El Playón, fill from what the Army post and your guide use. At base camp, walk northwest and find the spring — not the lagoon, and not the spring that feeds it. From base camp upward, carry everything, because the 3,800 m camp has none and the summit day has none.
And drop the reflex. On most mountains, water risk is biological and a filter answers it. Here the risk is a dissolved element that filters cannot see, boiling concentrates, and chemical tablets ignore entirely. The mountain that makes the springs worth travelling to is the same mountain that puts 900 µg/L of arsenic in them.
10 Sources
- Bongiovanni et al., arsenic in northwest Neuquén waters — Science of The Total Environment (2020); 350 samples 2012–2018. Reported in Río Negro — https://www.rionegro.com.ar/sociedad/alerta-por-arsenico-en-el-agua-al-noroeste-de-neuquen-se-debe-a-los-60-volcanes-activos/
- Andeshandbook, Domuyo Ruta Normal o Sur (base-camp springs, brackish lagoon, dry 3,800 m camp) — https://www.andeshandbook.org/montanismo/ruta/179/Normal_o_Sur
- Galetto et al. (2021), CONICET — Domuyo as a granitic–dioritic stock on a reactivated basement anticline — https://ri.conicet.gov.ar/handle/11336/178097
- Chiodini et al. (2014), Journal of Volcanology and Geothermal Research 274:71–77 — 1.1 ± 0.2 GW advective heat flux, equilibrium fluid temperatures 190–230 °C. Read via abstract and secondary summaries; ScienceDirect returned 403 to us.
- ISSN, end of season at the Domuyo and Copahue thermal complexes — https://www.issn.gov.ar/finaliza-la-temporada-de-los-complejos-turisticos-ubicados-en-copahue-y-en-domuyo/
- Río Varvarco, drainage of the Estero Covunco — https://es.wikipedia.org/wiki/R%C3%ADo_Varvarco
- Ministerio de Ambiente de Neuquén, update of the Domuyo protected-area management plan — https://ambiente.neuquen.gov.ar/avanza-la-actualizacion-del-plan-de-manejo-del-area-natural-protegida-domuyo-con-participacion-local/
- Centro Cultural Argentino de Montaña, ficha Volcán Domuyo — https://revistadigital.culturademontania.org.ar/articulo/646cc94cfc0d3efac6f6e74b
- Lundgren et al. (2020), Scientific Reports — deformation of the Domuyo system — https://pmc.ncbi.nlm.nih.gov/articles/PMC7363862
- OpenStreetMap: El Humazo node/7196698562 (note "Zona riesgosa. No se recomienda acceder."), Los Tachos node/2602383785, trailhead node/7196308386, camp nodes 12070593282 / 12070590451 / 12070588182