Over a fifth of septic tanks inspected last year were a risk to human health or the environment.
The Environmental Protection Agency says over 800 tests were carried out by local authorities in 2020.
54 per cent of the tanks failed, because they weren't built or maintained properly.
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Six local authority areas had failure rates of above 75 per cent, with 85 per cent below the required standard in Roscommon.
59% were below the stndard in Leitrim, with 505 of the tanks inspected in Longford failing the EPA test.
