Limestone layers reveal aqueduct's timeline

The limestone layers inside the Nîmes aqueduct date back to around 270 AD, marking when the city's water supply was cut off. However, water continued to flow upstream toward the surrounding fields for another 140 years, as shown in a study published in Geoarchaeology on September 15, 2026, by researchers from Johannes Gutenberg University of Mainz.

Limestone acts as a historical record

The limestone deposits on the aqueduct's canal walls functioned like a natural archive, recording the passage of time through annual layers. These layers are similar to the growth rings of a tree, and the team collected samples from ten points along the 50 km route to analyze the isotopic composition of each layer, covering about three centuries of human activity.

Aqueduct's construction and modifications

The Nîmes aqueduct began operating around 50 AD, with its route descending only about a dozen meters over 50 km. Eleven years later, engineers added a second water source, likely due to a drought mentioned by the historian Tacitus. They then reinforced the structure, and by around 150 AD, half of the intended water flow to the city was diverted to irrigate large estates.

Shift in water use and maintenance

By the middle of the 3rd century, the limestone deposits on the aqueduct walls changed in texture, becoming more porous and mixed with clay and sand. This suggests that maintenance had been neglected, likely due to a lack of labor. Around 270 AD, the city's water supply ceased abruptly, and the water began flowing exclusively toward the farms along the route.

Aqueduct continued to function after the city's decline

While Nîmes lost its water supply, the aqueduct upstream continued to operate for about 140 years, until the 5th century. The farms that had previously received half the water flow now received the full amount. This suggests that the aqueduct's failure was not due to structural collapse, as the system remained intact and functional.

Population decline as a possible cause

The researchers suggest that the city may have lost its population, possibly due to the Plague of Cyprian, which occurred between 250 and 270 AD. This disease affected the Roman Empire and may have caused a crisis in nearby Cahors. However, there is no direct evidence such as bones or ancient DNA to confirm this hypothesis.

Limestone analysis extends to other aqueducts

The same team has also studied the carbonates in aqueducts in Arles and Constantinople. This method is gaining interest among archaeologists because it can date events that are not visible through traditional excavations. The Nîmes aqueduct still runs underground through the garrigue of the Gard region, over fifty kilometers in length.