Analyses of the carbonate deposits in the aqueduct featuring the famous Pont du Gard shed light on the water supply of the ancient city of Nîmes

Researchers at Johannes Gutenberg University Mainz (JGU) have gleaned new insights into ancient water management in the French region around Nîmes by analyzing carbonate deposits within the Roman aqueduct that features the famous Pont du Gard bridge. Among other findings, the aqueduct – built around 50 AD – was expanded to include an additional water source after eleven years of operation; this event coincided with a historically documented dry spell. A century after the aqueduct's construction, the water supply to the city was deliberately throttled; it is likely that some of the water was diverted to agricultural land in the surrounding area instead. Another 120 years later, the urban water supply ceased completely, coinciding with an empire-wide economic crisis and the Plague of Cyprian (250-270 AD). Nîmes was presumably depopulated by the plague, rendering the water supply to the city unnecessary. "The carbonate deposits document human interventions spanning roughly 300 years during the Pax Romana and also offer insights into social unrest," summarized Dr. Gül Sürmelihindi of JGU’s Institute of Geosciences. The team led by Professor Cees Passchier published the results of the carbonate analysis in the journal Geoarchaeology on September 15.

After eleven years, the aqueduct was expanded to include a second water source

For their study, Sürmelihindi and her colleague Passchier extracted samples of carbonate rock – formed over time by the flowing water – from ten locations along the 50-kilometer-long aqueduct and analyzed the rock layers using isotope analysis. Variations in water inflow and temperature had caused distinct patterns to form within the layers, comparable to tree rings. "At the base of the samples, there is a carbonate layer about two centimeters thick, covered by a thin layer of clay," said Passchier. According to him, the composition of this layer – and especially the clay layer, both of which were deposited during the aqueduct's first eleven years of operation – points to a drought in the western Mediterranean region, an event also documented by the Roman historian Tacitus. "The nature of the carbonate changes in the subsequent layers: after eleven years, a different type of water was introduced. An additional source was tapped to ensure the aqueduct had a sufficient water supply," said Passchier. This correlates with structural modifications to the aqueduct – such as the raising, reinforcing, and partial buttressing of the sidewalls to accommodate the higher water level – that are still visible today on the Pont du Gard bridge.

Powerful figures diverted water

Around 150 AD, the deposited carbonate layers became significantly thinner: the water level in the aqueduct dropped. "About half of the water supplying the city was diverted to irrigate fields – given the large volume, this appears to have been an authorized withdrawal," said Sürmelihindi. Such an arrangement would likely have been possible only for highly influential individuals; furthermore, the city does not seem to have had a critical need for the water. For the next hundred years or so, the aqueduct functioned flawlessly – albeit with a reduced volume of water reaching the city.

Water supply to the city ceases around 270 AD: The Plague of Cyprian?

From the mid-third century onwards, the color and composition of the carbonate changed: it became more porous and incorporated clay and sand – signs that the aqueduct was no longer being well maintained. "Great economic and political turmoil prevailed across many parts of the Roman Empire. The carbonate deposits reveal that the city's condition was deteriorating," said Sürmelihindi. Around 270 AD, carbonate deposition came to an abrupt halt in samples taken near the city: the water supply to Nîmes was completely cut off. Instead, owners of vast estates were diverting the water upstream, as evidenced by the continuous deposition of carbonate in the rural sections of the aqueduct.

"The results suggest that something happened to the population of Nîmes," said Sürmelihindi. "It is possible that the Plague of Cyprian – which is primarily documented in North Africa and even in Rome, where it claimed many lives – also posed a problem in southern France, as suggested by our study’s co-author, Professor Andrew Wilson of the University of Oxford. Not much is known about this plague in Western Europe." Did this epidemic depopulate Nîmes, causing survivors to move from the city to the countryside? Archaeological findings support the hypothesis derived from the carbonate analyses: many cities in southern France contracted at this time, some by barbarian invasions, but some by unexplained reasons. The city of Cahors – located about 300 kilometers west of Nîmes – was also in crisis, a fact revealed by Passchier and Sürmelihindi through their study of local carbonate deposits; this further reinforces the theory regarding the Plague of Cyprian.

In rural areas, however, water from the aqueduct continued to be used for another 140 years, well into the 5th century. People drilled holes into the sidewalls to extract water for agriculture – a practice evidenced by massive limescale deposits on the aqueduct's exterior caused by the resulting leaks. "The city could easily have repaired the aqueduct during those 140 years," says Passchier. "Apparently, however, the decline in population density and urban wealth was permanent."