The Environmental Protection Agency (EPA) has found quality in Irish waters has deteriorated between 2015 and 2017.
In its water quality report for last year, it says that 197 river water bodies have improved in quality - but 269 water bodies have deteriorated compared with the last full assessment.
This means a net overall decline of 3% - or 72 water bodies.
It says the long-term loss of high quality river sites is continuing, with a further 0.6% decline since 2015.
Most pollution is caused by too much nitrogen and phosphorus entering waters.
These come from human activities, predominantly farms and urban areas.
"Despite a long-term reduction, recent data indicates that levels of nitrogen and phosphorus are beginning to rise again", the EPA says.
"Unless addressed, this is likely to lead to further declines in water quality in the future."
Source: EPA
However, there have been some positive changes, as serious pollution continues to decrease.
It says only two river water bodies were 'seriously polluted' in the latest reporting period compared to five in 2013-2015.
"Historically there were 91 seriously polluted water bodies in the late 1980s.
"Fish kills are at an all-time low with only 14 reported in 2017 compared to 31 in 2016", it says.
Source: EPA
Dr Matt Crowe is director of the EPA's Office of Evidence and Assessment.
"The signals in this report are not good and tell us that water quality is still getting worse in some areas despite improvements in others.
"This is simply unacceptable.
"We must do more to halt deterioration in water quality so that we protect this most precious public resource.
"Substantial additional resources have recently been put in place by the State with the creation of the Local Authority Waters Programme and the Agricultural Sustainability Support and Advice Programme.
"These programmes will support action at local level to address the issues causing water pollution.
"We now need to start seeing visible improvements in water quality through the work of these new programmes."
