Fly ash produced by the Marsa power station when it was still coal-fired, which was dumped on the cliff-edge in Bengħaisa, contained high levels of radioactivity, according to a University study.
The pulverised fuel ash had "very high levels of all the radionuclides (radioactive contaminants) under test, namely K40, PB212 and PB214", the research found. The three elements are derivatives of potassium and lead.
Winds and rain occasionally spill the fly ash, which was covered with soil off the cliff-edge and into the sea below. The mound is situated at the back of the Freeport on the south eastern cliff face that borders Ħal Far industrial estate.
Scientific tests on the mound of fly ash were conducted five years ago by Josette Camilleri and Franco Montesin from the University's Faculty of Architecture and Civil Engineering and Michael Sammut from the hospital's Pathology Department. The study was published in the American journal Waste Management.
"I was surprised by the reaction when it was recently revealed that fly ash from the power stations was dumped in a quarry, because everybody seems to have forgotten that radioactive fly ash produced when coal was burned at Marsa was dumped in a disused quarry at Bengħisa," Dr Camilleri said.
Coal was last used in the Marsa power station in 1995, when it was modified to run on heavy fuel oil. This also produces fly ash which, though not radioactive, contains traces of hazardous heavy metals.
Dr Camilleri contacted The Sunday Times after this newspaper revealed that between 2004 and 2008 Enemalta had dumped fuel oil fly ash produced by the Marsa power station in a quarry belonging to the Polidano Group when the contractor was not licensed to handle and store such hazardous waste.
Polidano Brothers used this fly ash as 'bulking material' for concrete used to construct retaining walls in buildings until the Malta Environment and Planning Authority issued an enforcement order to stop the storage and use of the ash 2008.
The 2005 research, which also studied the use of fuel ash as a replacement for cement and aggregate in concrete mixes, established it was safer to use the fly ash produced from coal burning in concrete mixes rather than dump it.
The study showed that replacing between 10 and 20 per cent of cement in concrete mixes with fly ash produced radioactive levels that were lower than that of limestone from which stone is derived. The resulting concrete was of good quality.
"The health risk of such modified concrete should not be substantially different from the utilisation of limestone, which is the main building material in Malta," the researchers said.
"From this research it would seem that although pulverised fuel ash was disposed in a landfill as it was deemed to be the easiest solution, it would have been more economical to utilise it for construction purposes."
The researchers said land filling of fly ash was problematic as leaching of toxic waste substances took a long time to form and would persist for hundreds of years.
"Consequently it is not practical to design waste disposal sites with bottom liners and leachate collectors since such systems are unlikely to remain intact during the entire leaching period and treatment of the leachate is problematic and costly," they concluded.
In 1991, the Marsa power station used 250,000 tonnes of coal producing around 50,000 tonnes of pulverised fuel ash. The issue of radioactivity and fly ash was raised in Parliament last week by Labour MP George Vella who asked Finance Minister Tonio Fenech whether government was considering radioactivity testing.
Mr Fenech said fly ash from the power stations could not be radioactive because it was a by-product of oil combustion and as a result there was no need for radioactivity testing.