Following a series of alarming reports, regional governor Gianni Chiodi said that water was "no longer poisoned" and was of "excellent quality".
Tests published this week showed toxic waste had infected the water supply of some 700,000 people in the Abruzzo region - more than half the territory's population - between 2004 and 2007, after industrial waste buried for decades leaked into the water supply.
Montedison, now known as Edison, is accused of burying some 250,000 tonnes of toxic and industrial waste at a facility in Abruzzo, which has been open since the 1960s.
Some 19 Montedison executives are currently on trial as a result of the investigation into the leaks.
Forest rangers and environmentalists from the WWF first reported finding traces of toxic chemicals in the water supply in 2007.
These initial tests led to a broader investigation, the results of which were released Wednesday by Italy's state public health institute.
The Corriere della Sera daily said the chemicals found in the water supply could affect the nervous system, the liver and kidneys and are suspected of being carcinogenic, although no cases of illnesses linked to the pollution have yet been reported.
"Knowing 700,000 people might have drunk polluted water for years is terrifying," Environment Minister Gian Luca Galletti told the Il Mattino daily.