Water quality in more than one- third of Ireland's rivers and lakes fails to comply with the 1998 Phosphorus Regulations - and farmers are among the prime culprits, according to the Environmental Protection Agency (EPA).
Overall, it said, water quality complied with the regulations at just 63.4 per cent of river and lake monitoring stations - up 3.4 per cent since the mid-1990s. The percentage of river stations of highest biological water quality had also declined.
However, the EPA said "significant improvements" in water quality were apparent in Dublin city, Kildare, Fingal, Westmeath, south Dublin and Meath, though a "significant decline" in compliance levels was recorded in Donegal and Wicklow.
Of 496 lakes assessed, 401 complied with the targets set in the regulations. Those counties with a relatively large number of non-compliant lakes include Monaghan (31), Cavan (14), Clare (14), Leitrim (9), Cork (7) and Longford (6).
Agricultural activities and sewage discharges were the main sources of nutrient enrichment of waters. This finding will add further fuel to the heated debate over measures to implement the EU nitrates directive, which has been opposed by the farming lobby.
Dara Lynott, director of the EPA's office of environmental enforcement, said tackling pollution from agricultural sources "remains the greatest challenge", and efforts needed to be stepped up if Ireland was to meet the stringent targets of the EU water framework directive.
New powers under the nitrates regulations, which are being redrafted under pressure from farmers, would "enhance the enforcement tools available to local authorities and we would strongly advise them to ensure that these regulations are fully enforced".
Though progress had been made by a number of local authorities in tackling water pollution, Mr Lynott said more of them should "act on the wide array of information already available to them on pollution black spots and suspected causes to ensure compliance".
Measures being taken by the more active local authorities include the implementation of agricultural bylaws, carrying out farm surveys, reviewing discharge licences and upgrading and constructing sewage treatment plants.
Eleven local authorities carried out significant farm survey work in the 2002 to 2004 period, the EPA noted. Mr Lynott welcomed the development of teams within some local authorities to tackle agricultural pollution.
