This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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 A.D.—was expanded to include an additional water source after 11 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 A.D.). 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," said 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.
For their study, Sürmelihindi and her colleague Passchier extracted samples of carbonate rock—formed over time by the flowing water—from 10 locations along the 50-kilometer-long (31-mile-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 11 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 11 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 raising, reinforcing and partially buttressing the sidewalls to accommodate the higher water level—that are still visible today on the Pont du Gard bridge. Around 150 A.D., 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.
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