60 springs, 2 boreholes: where did Ax's hot water go?

The first difficulty is simply counting them. The town hall speaks of around sixty springs, spread across three groups. It also mentions more than 70 springs at the end of the 18th century, and more than fifty emergences in the old town centre alone. A local guide lists 63. A report presented to the Académie nationale de médecine counts 58 intakes, of which only 18 are authorised by ministerial order. The BRGM, the public geology agency, refers to more than sixty water sources identified and tapped at various periods, and notes that sixteen springs fed the baths until 1988.

These figures don’t necessarily contradict one another: they aren’t counting the same thing. An emergence is a spot where water naturally rises. An intake is a structure that captures it. An authorised intake is a structure recognised by the administration. A spring feeding the baths is yet another category. Temperatures likewise vary depending on the source cited, from 45 to 75°C for the BRGM and from 43 to 78°C for the town hall, which notes that these are the hottest waters in the Pyrenees.

Ax’s water is rainwater. It falls on the granite massif that overlooks the town, seeps into the rock, and descends slowly to a depth of roughly five kilometres. There, it reaches 120 to 130°C and becomes mineralised: it turns into a sulphurous, lightly mineralised water. The journey takes several hundred, even several thousand years, according to the BRGM.

It then rises back up through fractured zones to the lowest point of the massif, which happens to be Ax. To the north, an impermeable barrier of albitic schists forms the roof of the reservoir and blocks the water. It is at the contact between the granite and these schists that the emergences cluster, along the fracture lines. This partly explains why such a small area holds so many springs.

The final stretch of the journey is the trickiest. At the confluence of the Ariège, the Oriège and the Lauze, glaciofluvial deposits 15 to 20 metres thick shelter a body of cold groundwater, which the thermal water crosses just before emerging. This contact with a cold, shallow environment weakens the thermal fluid, the BRGM notes. This point matters for what follows: deep, very hot, very ancient water, emerging in the heart of a town, is exposed to everything the town does to it.

By the end of the 18th century, with more than seventy springs, thermalism had become Ax’s main economic activity, according to the town hall. Hot water was everywhere. Later, public fountains of sulphurous water let residents draw on it constantly for hygiene, housework and cooking, continuing the free, everyday use of water already established at the Bassin des Ladres.

Establishments multiplied around the three zones of emergence, as close as possible to the old town still enclosed within its ramparts, according to a study devoted to Ax’s thermal establishments. The town hall dates the founding of the Couloubret and the Teich to 1780 and 1787. From 1800 or 1801, depending on the source, surgeon Boulié opened an establishment on the Teich spring, where wooden shacks initially offered bathers only precarious shelter: 24 shacks and 51 bathtubs are counted there in 1821. The Breilh was built around 1815 by Jean Simon Sicre, at the back of his hotel’s courtyard, because the Teich and the Couloubret were no longer enough. The town hall states that today’s Teich was only inaugurated in 1893, while another source dates a more ambitious establishment to 1834.

The springs have names: Viguerie, Pyramide, Astrié Chaude, Grotte, Eau Bleue, Saint-Roch. The Teich group alone counts 14 springs, with temperatures ranging from 30 to 73.8°C and flow rates from 504 to 151,200 litres per 24 hours — a ratio of one to three hundred. The Breilh group, some thirty springs, is the most abundant, with the hottest and most sulphurous waters. In the 19th century, the mining service estimated the total natural flow at 60 m³ per hour.

The 20th century changed everything. According to the town hall, in this urban setting, the points of emergence and intake became subject to recurring pollution. The town’s expansion even led to the suspension of water-use authorisations from 1955 to 1964.

That’s when deep-water tapping began: in 1963 with the Diamant 1 borehole, 26 metres deep, then in 1984 with three boreholes, Bains Fort du Couloubret (64 m), Bain Fort du Modèle (30 m) and Grosse Sulfureuse. The solution of borehole extraction gradually established itself, the town hall notes.

In the 1990s, the shift became general: deep boreholes replaced the tapping of springs, ensuring better protection of the resource, the BRGM notes. The reasoning is easy to follow. A spring is a point where deep water meets the town. A borehole reaches the same water lower down, before it crosses the cold aquifer and the urban soil, then channels it through closed pipes.

Once mixed and transported, this water even carries a name: an authorisation request examined by the Académie nationale de médecine designates it as a natural mineral water under the name "Axéenne."

Since 2010, according to the BRGM, two structures supply the thermal water. FP1, at the Jardins du Coustou, is 167.5 metres deep and can deliver up to 60 m³ per hour at 76°C. F7bis, at Orlu, is 80.5 metres deep and authorised for 35 m³ per hour, at 68°C.

FP1’s water is stored in buffer basins — 220 m³ in the Teich park and 2 x 70 m³ beneath the Grand Tétras residence — then passes through heat exchangers that warm buildings before supplying the Teich and the Modèle. The rest is blended with F7bis water, and this water supplies the Couloubret thermoludic centre, the Saint-Louis hospital, the Château de Villemur and the casino, the Saint-Jérôme chapel, as well as the town’s basins and fountains. An older report, examined by the Académie de médecine, notes that this water also supplies the Bassin des Ladres.

The pumped flow averages 72.6 m³ per hour. The two thermal establishments receive around 6,360 patients a year, for cures of up to 18 days, and the Couloubret welcomes around 170,000 visitors a year, in a commune of around 1,200 residents. The springs belong to SEMTTAX, the mixed-economy thermal and tourism company of Ax, and the Eurothermes group operates the establishments. The flow, level, temperature and conductivity of both boreholes are recorded continuously.

The old intakes have not all disappeared, but their role has changed. According to the town hall, the old boreholes, emergences and intakes now serve to monitor the thermal aquifer, or are being phased out.

Three old operating boreholes, Bain-Fort, Boulié and Longchamp, have become piezometers — that is, measurement wells whose water level has been continuously monitored since 1998 for the last two and 2006 for the first. Two springs, Viguerie and the Puits d’Orlu, play a more distinctive role. Under the authorisation order of 3 May 2012, their natural flow is monitored, and they are required to keep flowing. They demonstrate that the deposit remains under pressure and allow a check that it is not being over-exploited. Two old springs thus serve as living indicators of the resource’s health.

One spring, the Grosse Sulfureuse, still flows beneath the Grand Tétras building. It is not tapped, its flow rate is unknown but probably below 7 m³ per hour, and its temperature is around 75°C. The Château de Villemur, meanwhile, has lost its springs: the 5 m³ per hour guaranteed to it under an old contract now comes from the boreholes, via a heat exchanger, whereas this water once rose near the château itself.

What was gained is clear. Water protected from urban pollution, continuously measured, distributed through a closed network. Water whose heat also serves to warm buildings and, on certain streets, to melt snow, according to the BRGM.

What was lost is less visible. The old spring groups each had their own character: the Breilh offered the hottest, most sulphurous waters, the Teich springs ranging from 30 to 74°C. Today, part of the distributed water is a blend of two boreholes. [TO VERIFY: comparative analyses before and after, to determine what this blending erased or did not.] The town has also become dependent on two production points. In 2021, one of the two reached its minimum level, and its flow had to be reduced for a time, with the deeper borehole compensating. The pumped flow already approaches the authorised maximum, the BRGM notes.

One open question remains: is a fountain fed by a borehole still a spring? For the visitor dipping their feet on Place du Breilh, the answer hardly matters. For the historian, it matters a great deal, since it determines what we call Ax’s thermal heritage.

What remains uncertain. The number of springs depends on what is counted. The dates of the first establishments vary between sources. The commissioning dates of the current boreholes remain to be pinned down. The town hall also gives two different dates for the founding of the hospital and the Bassin des Ladres, 1250 on one page and 1260 on another.

Sources consulted