Tens of tonnes of phosphorus continue to seep into Lough Leane, the largest of the Lakes of Killarney and the jewel of local tourism.
The pollution continues despite warnings from scientists and environmentalists and appeals to farmers, local authorities, householders and the tourist industry to clean up their act.
The pollution means that the lough is at serious risk of an algal bloom this summer, given the warm conditions, the senior executive chemist with Kerry County Council warned yesterday.
A potentially toxic bloom last occurred in 1997, when warning signs went up along the shores and the public was asked to keep away from the water.
The main source of pollution in the lake is phosphorus, a three-year study has found, with poor practice in the storage and spreading of slurry by farmers in the catchment area accounting for the single greatest phosphorus loading.
Bed-and-breakfasts and hotels also contribute, with the disposal of fats and oils into the sewage system a particular problem.
Septic tanks and forestry also contribute to the overloading of phosphorus.
"If the level of phosphorus in the lake increases above a critical limit and if weather conditions are also favourable - that is, bursts of rain interspersed with warm, calm, sunny conditions - a serious risk of algal blooms will exist," Mr David Lenihan, senior executive chemist with Kerry County Council, said.
Some 30 tonnes of phosphorus is going into the lake each year. Scientists have warned that 20 tonnes is enough to guarantee an algal bloom.
Mr Fergus Dillon, senior environmental engineer with Kerry County Council, has appealed to householders and the hotel industry to use phosphate-free detergents and has asked farmers to be more aware of the need to tidy up their farmyard management practices.
Lough Leane is one of the main tourist attractions in Killarney. Many of the town's top hotels and guesthouses are situated along its shores.
The cost of cleaning up the lake has been estimated at around €15 million. Millions will also have to be spent in changes in farming practices to assist recovery.
There has been a standoff in introducing agricultural bylaws for Lough Leane which would limit slurry-spreading and enforce rigorous standards on storage.
Councillors have refused to implement them until farmers are assisted financially.
Cllr Brian O'Leary (FF) said he was disappointed there was still no indication that funding was coming from the Department of Agriculture to enable farmers to carry out such work.
There have also been calls on the Government to reduce VAT on phosphate-free detergents. Such a measure would encourage people to buy these detergents, Cllr Michael Gleeson, of the South Kerry Independent Alliance, said.
The final report from a task force set up four years ago on the future management of Lough Leane is due to be published in September.