Supply became contaminated as it passed through giant network of lead pipes that distributed water around city, scientists believe
Tap water in ancient Rome, provided by its famous aqueducts, was contaminated with up to 100 times more lead than local spring water, researchers say.
Huge volumes of fresh water flowed along aqueducts to the heart of the Roman empire but the supply was contaminated as it passed through the giant network of lead pipes that distributed water around the city.
Researchers in France said levels of lead in Roman drinking water were a concern, but were probably insufficient to cause widespread mental problems, or potentially drive up crime rates, through lead poisoning.
"It's marginal. You would start being worried about drinking that water all your life," said Francis Albarède, who led the study at Claude Bernard University in Lyon. "Even though they probably did not get degenerate, as some people say, or even get more violent, lead pollution might have been something to be concerned about."
The scientists inferred lead pollution levels by analysing sediment cores taken from the Trajanic harbour basin at Portus, a major port of imperial Rome, and a canal that connected the port to the Tiber river.
The Trajanic harbour, a hexagonal inland basin, was built in the early years of the second century AD to give safe mooring to merchant ships as the population of Rome expanded.
Albarède's team studied lead isotopes in a nine-metre-deep core drilled from the harbour and a 13-metre core taken from the canal, which carried a record of contaminants from the Tiber. Sediments trap contaminants as they form, so studies of sediment cores drilled from ocean floors and river beds can reveal levels of environmental pollution dating back hundreds or thousands of years.
The tests on the Tiber sediments were striking. They showed that two kinds of water mixed in the river. The first was natural river water, which carried lead isotopes originally from the Apennines and volcanic rock in the Alban hills south-east of Rome.
The second type was much cleaner drinking water, that had drained into the river, and was contaminated with isotopes of lead not found in Italy. The researchers believe the lead was mined elsewhere, perhaps in Eifel in Germany, or even the English Pennines, and then brought back in ingots to make lead piping.
Further tests on the sediments showed that levels of lead in the Roman tap water varied over time from 14 to 105 times higher than those found in natural spring water. The findings are reported in the Proceedings of the National Academy of Sciences.
Though Rome's complex plumbing system gave officials control over the distribution of water around the city, it was not unknown for locals to punch holes in the pipes to draw water off, increasing the number of people exposed to the lead. Even so, Albarède believes that any health problems caused by lead piping could not have brought the civilisation to its knees.
"Can you really poison an entire civilisation with lead? I think it would take more than lead piping in Rome to do that," he said.