Half the septic tanks examined by officials in Cork failed inspections.
Across Cork county, 172 inspections of septic tanks were carried out in 2017 and 2018. However, 49% of these were found to be defective because they were not built or maintained properly.
The Environmental Protection Agency (EPA) said faulty systems can contaminate household wells and pollute rivers.
The report from the EPA did show that of the 289 failing septic tanks inspected in Cork between 2013 and 2018, that 89% had since been fixed, one of the highest repair rates in the country.
The EPA has urged householders with faulty septic tanks to avail of the proposed expanded grant scheme when it becomes available.
Dr Tom Ryan, Director of the EPA’s Office of Environmental Enforcement said: “If you do not maintain your septic tank, it can contaminate your own or your neighbour’s well or your local stream, putting your health at risk and that of your family and neighbours."
"You can take simple steps to maintain your septic tank by making sure it is not leaking, ponding or discharging to ditches and by cleaning it out regularly.”
The report also found that nearly one-third of systems that failed inspections during 2013-2018 are still not fixed.
Noel Byrne, Senior Scientist in the EPA’s Office of Environmental Enforcement said: “It is important that householders fix systems where problems are detected. To improve water quality, the government’s proposed expanded septic tank grant scheme, due to be launched later this year, will increase the maximum grant aid available to €5,000 and remove the means test requirements.”
Domestic wastewater treatment systems are used by rural householders to treat sewage. There are nearly half a million systems in Ireland and most (90%) are septic tanks.