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Limescale deposits reveal how ancient Arles adapted its aqueducts

Researchers from Johannes Gutenberg University Mainz (JGU), the University of Oxford, and the University of Innsbruck have deciphered the complex history of the ancient aqueduct system of Arles in Provence. This was made possible by aqueduct carbonates—limescale—that had deposited in the aqueducts, basins, and lead pipes, as well as lumps of aqueduct carbonate that had been used as building aggregate in the roof of the Baths of Constantine. The researchers published their in Geoarchaeology on June 28, 2025.

"This study clearly shows how a Roman aqueduct worked over several centuries and was transformed at different times by the Romans, efficiently maintained and modified, and hence this is one of the clearest examples of a sustainable water management system from antiquity," said Dr. Gül Sürmelihindi from the Institute of Geosciences at JGU who conducted the study.

"Unlike previous studies, where we mostly dealt with a single aqueduct, here we investigated the complex water supply of ancient Arles, consisting of several aqueducts, a basin and connected water structures in the city," added Prof. Dr. Cees Passchier from the Institute of Geosciences at Mainz University, who participated in the study.

While some relationships between different elements of the city's water supply system had been suggested for some time, the researchers have now succeeded in confirming the picture with solid archives from the Arles water system and proving the long-life cycle of the Roman aqueduct of Arles.

Initially, from 3 BCE onwards, an aqueduct from the south flank of the Alpilles hills supplied the city of Arles with water. However, after almost a hundred years, an additional aqueduct was built from the northern side of the same hills, whose water joined that of its southern counterpart in an existing basin that was part of the original aqueduct.

With the opening of the northern aqueduct, the southern one took on a new purpose: It was now diverted to power a huge 16-wheeled complex of water mills at neighboring Barbegal, as the researchers confirmed in an earlier study—also through the analysis of the carbonates.

The researchers further confirmed that the basin originally functioned as a header basin upstream of an aqueduct arcade bridge. Such basins were inserted so that sand and other suspended matter could settle. The northern aqueduct was then added later in an improvised manner, which can be seen from the architectural remains of its higher-level entrance to the basin.

Another piece of the puzzle came from collapsed ceiling pieces of the Baths of Constantine in Arles, whose water supplier was so far unknown. "We found aqueduct carbonates in these collapsed ceiling fragments that belonged to the northern aqueduct. Evidently, when the baths were built in the early 4th century AD on the orders of Emperor Constantine, the aqueduct was apparently restored and the chipped-off carbonates were used here as building material in the roof of the bath," said Sürmelihindi.

In this way, the researchers were able to answer the previously unanswered question of how the bath was supplied with water and how long the Roman aqueduct was in use—at least until the time when the baths were built, since the carbonate cleaned out of the aqueduct was used in its construction, and most likely the aqueduct worked well into the fifth century AD until the arrival of invading Franks and Burgundians.

The role of large lead pipes from Roman times that ran across the bed of the Rhône River and were discovered in the 19th century was also debated for a long time. In which direction did these pipes transport water?

The researchers were also able to solve this with the help of carbonates: deposits with similar isotopic composition to those found in the aqueducts in the north and south branches were also found in the lead pipes, confirming that an inverted siphon from the Arles aqueduct was supplying the Trinquetaille quarter on the opposite side of the river.

"Without the aqueduct carbonate archives, it would be impossible to reconstruct these relationships" said Passchier.

"But because the deposits are heavily contaminated with clay, they cannot be dated using standard dating technologies. Instead, we analyzed stable oxygen and carbon isotopes from the carbonates and cross-correlated the isotopic profiles to see the times of their simultaneous deposition," added Sürmelihindi.

"This allowed us to identify the same annual layers in the carbonates and thus determine their relative depositional periods and thus the historical timing of modifications and changes made to the Arles water supply system."