The news is causing concern to environmental pressure groups who suspect a possible link between aluminium and Alzheimer's Disease.

BBC NI's Health Correspondent Dot Kirby: "Concern growing over aluminium levels"

Aluminum is routinely added to the Province's drinking water to remove suspended matter.

DoE Water Service official George Alexander says the higher than permitted levels found in some samples do not pose a health threat.

"What we are talking about are the water quality regulations.

"The Water Service has the statutory responsiblity to comply with them.

George Alexander: Action under way to limprove water supplies

"Now we admit that at times we don't always comply. We have a major capital works programme in progress to rectify that situation," he added.

The government says that the bulk of the aluminum people ingest comes from their food and the envioronment, not their water supply.

But ever since the 1960s environmentalists have been concerned that there is a link between aluminium in the water and Alzheimer's Disease.

Margery Magee: Evidence against aluminium "circumstantial"

Margery Magee from the Alzheimer's Disease Society says: "There is only a circumstantial evidence in the medical research that has been done.

"But what we would say is that the brain is a very delicate organism and it was never designed to ingest large quantities of toxic substances."

The government points out that a substantial amount of the aluminum in water occurs naturally.

One of the reservoirs with the highest levels is Silent Valley in the Mourne Mountains. The DoE says tests on mud indicate there have been high aluminum levels there for more than 100 years.

Morris Dunlop: Acid rain to blame

Morris Dunlop from Environmental Assistance disputes this view.

He says that aluminium was not present in the Mournes in the current concentration when the Silent Valley was developed in the 1920s.

"The aluminium which the government says is natural is the result of acidification. Acid rain leaches the aluminium from the rocks and so it produces aluminium in the water".

Whatever the cause, the government plans to spend £500m in upgrading Northern Ireland water treatment facilites over the next ten years.