Almost 8 in 10 septic tank systems in Mayo failed inspection in 2017 and 2018 - the highest failure rate in the country - posing a risk to human health and the environment.
The Environmental Protection Agency has published a review of over 2,000 inspections of septic tanks in 2017 and 2018, which showed that - across the country- nearly half of the systems failed inspection because they were not built or maintained properly.
In Mayo, the failure rate rose to 78% - compared to 54% in Sligo, 39% in Co Galway and 0% in Galway City.
Faulty septic tank systems can contaminate household wells and pollute rivers and streams.
The EPA is urging householders to fix septic tanks that are not working properly, and says the Government's proposed increase in the septic tank grant scheme will help address the issue.