Water man­age­ment in ancient Arles

Calcium deposits that had settled in the aqueducts, basins, and lead pipes of the ancient aqueduct system in Arles, Provence, as well as lumps of aqueduct carbonate used as building material in the roof of the so-called Constantine Baths, made it possible to investigate the complex history of these ancient infrastructure. The results were recently published in the journal Geoarchaeology. 

“We took high-resolution isotope measurements of the limestone crusts that formed in the aqueduct, which are the key for tracking the use of these water supply systems over time—like a tape recorder that has been recording for centuries,” explains Christoph Spötl from the Institute of Geology at the University of Innsbruck. “Without the carbonate archives of the aqueduct, it would be impossible to reconstruct these connections,” says geoarchaeologist Cees Passchier from the University of Mainz. “However, since the deposits are heavily contaminated with clay, they cannot be dated using conventional dating methods. Instead, we analyzed stable oxygen and carbon isotopes from the carbonates and correlated the isotope profiles to determine the times of their simultaneous deposition,” adds Gül Sürmelihindi, also from the University of Mainz. “This enabled us to identify the same annual layers in the carbonates and thus determine their relative deposition periods and, consequently, the historical timing of the alterations and changes to the water supply system in Arles.”

Carbonates provide the complete picture

“This study clearly shows how a Roman aqueduct functioned over several centuries and was rebuilt, efficiently maintained, and modified by the Romans at various times. This makes it one of the clearest examples of a sustainable water management system in antiquity,” says Sürmelihindi. “In contrast to earlier studies, in which we usually dealt with a single aqueduct, here we examined the complex water supply system of ancient Arles, consisting of several aqueducts, a basin, and connected water structures in the city,” adds Passchier, also from the University of Mainz. While some connections between different elements of the urban water supply system had long been suspected, the researchers have now succeeded in confirming this picture using archive material from the Arles water system and proving the long service life of the Roman aqueduct in Arles.

The researchers also discovered that the basin originally served as a collection basin in front of an aqueduct arch bridge: such basins were built so that sand and other suspended solids could settle. The northern aqueduct was added later as an improvisation. The researchers recognized this because the architectural remains flow into the basin at a higher point. Another piece of the puzzle was provided by collapsed ceiling fragments from the Constantine Baths in Arles, whose water source was previously unknown. “We also found aqueduct carbonates in these collapsed ceiling fragments of the northern aqueduct. Apparently, the aqueduct was restored on the orders of Emperor Constantine during the construction of the baths in the early 4th century AD, and the flaked-off carbonates were used as building material for the roof of the baths,” said Sürmelihindi. In this way, the researchers were able to answer the previously unanswered question of how the baths were supplied with water and until when the Roman aqueduct was in operation – at least until the baths were built, because the carbonate extracted from the aqueduct was used for their construction. The aqueduct was most likely in operation well into the 5th century AD, i.e., until the arrival of the invading Franks and Burgundians.

The role of the large lead pipes from Roman times, discovered in the 19th century, which ran across the riverbed of the Rhone, was also long disputed. Where did these pipes transport the water? Researchers were able to answer this question with the help of carbonates: deposits with a similar isotopic composition to those found in the aqueducts of the north and south arms were found in the lead pipes. This confirmed that a reverse siphon of the Arles aqueduct supplied the opposite district of Trinquetaille.