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| Sufficient water quality | Good water quality | |
| 2000 | 98.4615384615385 | 91.5384615384615 |
| 2001 | 97.7099236641221 | 87.7862595419847 |
| 2002 | 97.7099236641221 | 84.7328244274809 |
| 2003 | 96.9465648854962 | 84.7328244274809 |
| 2004 | 97.7099236641221 | 87.7862595419847 |
| 2005 | 96.1832061068702 | 90.8396946564885 |
| 2006 | 96.9465648854962 | 90.0763358778626 |
| 2007 | 96.9465648854962 | 80.1526717557252 |
| 2008 | 93.1297709923664 | 77.8625954198473 |
| 2009 | 93.1297709923664 | 82.4427480916031 |
| 2010 | 96.9 | 90.1 |
| 2011 | 98.5 | 83 |
| 2012 | 97 | 66.9 |
| 2013 | 96.8 | 84.4 |
| 2014 | 94.1 | 86.7 |
| 2015 | 93.4 | 83.2 |
| 2016 | 92.9 | 85.8 |
| 2017 | 93 | 84.5 |
| 2018 | 94.5 | 86.2 |
During the 2000-2018 period more than 80% of bathing sites surveyed in Ireland had good water quality, except for 2008 and 2012. Over 90% of sites surveyed had sufficient water quality over this period.
In 2018, 86.2% of bathing water sites surveyed had good water quality and 94.5% sufficient water quality.
| Compliance of public water supplies with THM standard | Compliance of public group water supplies with THM standard | Compliance of private group water supplies with THM standard | |
| 2010 | 89.3 | 76.2 | 85.5 |
| 2011 | 91 | 87.1 | 93.6 |
| 2012 | 87 | 60.3 | 93.8 |
| 2013 | 92.1 | 79.8 | 94.4 |
| 2014 | 93 | 93.1 | 93.3 |
| 2015 | 91.5 | 87.2 | 95.7 |
| 2016 | 93.1 | 95.7 | 90.9 |
| 2017 | 94.3 | 96.6 | 96 |
The quality of public water supplies as measured by compliance with E.Coli standards has increased from an average annual 99.4% in 2005-2009 to 99.9% in 2017. Compliance of Public Group Water Supplies has increased from 99.1% to 99.9%, while compliance of Private Group Water Supplies has increased from 85.6% to 98.9% over the same period.
The compliance rate of Public Water Supply for trihalomethanes (THM) has increased from an average annual 90.5% in 2010-2014 to 94.3% in 2017. The compliance rate of Public Group Water Supplies increased from 79.3% to 96.6%, while the compliance of Private Group Water Supplies increased from 92.1% to 96.0% over the same period.
| No treatment/preliminary treatment | Primary treatment | Secondary treatment | Secondary treatment with nutrient reduction | |
| 1997 | 46 | 28 | 22 | 4 |
| 1999 | 36 | 38 | 21 | 5 |
| 2001 | 30 | 41 | 21 | 8 |
| 2003 | 31 | 2 | 58 | 9 |
| 2005 | 16 | 2 | 70 | 12 |
| 2007 | 9 | 1 | 75 | 15 |
| 2009 | 6 | 1 | 78 | 15 |
| 2010 | 5.9 | 1.6 | 72.6 | 19.9 |
| 2011 | 5.7 | 1.6 | 72.1 | 20.6 |
| 2012 | 4.6 | 1.5 | 71.1 | 22.8 |
| 2013 | 4.6 | 1.7 | 67.8 | 26 |
| 2014 | 4.6 | 1.2 | 68.6 | 25.7 |
| 2015 | 4.2 | 1 | 68.3 | 26.5 |
| 2016 | 2.7 | 1.3 | 69.4 | 26.6 |
| 2017 | 2.2 | 1.2 | 66.6 | 30 |
There has been a significant improvement in the level of treatment of urban waste water in agglomerations of over 500 persons. In 2017, 30% of urban waste water received secondary treatment with nutrient reduction compared with only 6% in 1997-2001. Only 2% of urban waste water received either no treatment or preliminary treatment in 2017, compared with 37% in 1997-2001.
River water quality in Ireland has deteriorated over the 1987-2015 period.
The proportion of unpolluted river water in Ireland fell from 77.3% in 1987-1990 to 68.9% in 2013-2015.
The level of seriously polluted river water has remained at less than 1% over the 1987-2015 period.
There was an improvement in groundwater quality in Ireland as measured by the amount of nitrates detected by monitoring stations over the 1995-2018 period.
The number of samples with less than 5 mg/l NO3 (milligrams per litre of nitrates) in groundwater increased from an average annual 15.6% in 1995-1997 to 26.2% in 2013-2014, before falling to 23.8% in 2017- 2018.
The proportion of samples between 5 and 10 mg/l also increased from an average annual 15.6% in 1995-1997 to 21.6% in 2017-2018.
In contrast, the number of samples with more than 25 mg/l fell from an average annual 19.5% in 1995-1997 to 16.7% in 2017-2018.
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