More than half of domestic waste water treatment systems inspected last year were not up to standard, with almost a quarter posing a risk to human health or the environment, an Environmental Protection Agency report has found.
There are almost 500,000 of these systems in the country and most of them are septic tanks.
The EPA said 433 of the 809 systems tested in 2020 failed, with 23% found to be a risk to human health or the environment.
A national inspection plan has been in place for the last eight years. It charges councils with carrying out 1,000 checks annually. Last year, inspections were down slightly as a result of the pandemic.
Maintenance issues or structural defects that cause leaks are the main reasons behind defective tanks. Faults can contaminate wells or cause pollution to other water sources.
There was a wide variation in the number of tests carried out by different local authorities. Just one inspection took place in Co Kerry, while 111 happened in Co Wexford.
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Councils issue advisory notices to homeowners whose systems fail, requiring repairs to be carried out.
The EPA said that since 2013, three quarters of defective tanks have been remedied but it is concerned about a failure to resolve older cases.
It has called on local authorities to focus efforts on dealing with these cases.
Householders are advised to ensure systems are properly maintained and to investigate if effluent is ponding in gardens or leaking into ditches or streams.
While the number of inspections in 2020 was down on previous years, the EPA said its target to complete 4,000 site visits will be met.