EIGHTy notices were issued to householders in County Wexford about the parlous state of their septic tanks in 2016, the highest number of any county.
The EPA, advising homeowners to protect themselves, their familes and their neighbours from the risks posed by septic tanks, said 544 failure advisory notices were issued in the year, an 11.5 per cent increase on 2015.
Local authorities which carry out the inspections can prosecute a homeowner if they fail to comply with the advisory notice.
Wexford's fail rate of 80 tanks was followed by Cork, where 68 failure notices were issued.
* Half of septic tanks failed local authority inspection in 2016, posing a potential health risk to homeowners and their neighbours.
* Half of sites with a septic tank and a private drinking water well onsite failed inspection and more than a quarter (29 per cent) failed due to the risk to human health and the environment.
* A quarter of septic tanks failed due to owners not removing sludge build-up from their tanks, an issue that can be easily rectified by homeowners.
The EPA said septic tank failures were mainly due to a lack of proper operation and maintenance.
It said the failure by homeowners to maintain and operate a septic tank system adequately can pose a health and environmental risk through the pollution of private drinking water wells or water courses.
Darragh Page, Programme Manager of the EPA's Office of Environmental Enforcement said homeowners are putting themselves, their families and their neighbours at risk of ill health if they do not maintain their septic tank system adequately.
'There are simple steps that homeowners can take to ensure their system is managed properly and will pass an inspection.
'These include: having the sludge emptied from the tank on a regular basis, using a permitted contractor and retaining the receipt and, if the homeowner has a package treatment system, having it regularly serviced and keeping a record of servicing.
'By taking these simple steps homeowners can protect themselves and their environment from contamination,' said Mr Page. The EPA said more than half of the sites that failed inspections in 2016 were now compliant. It said 473 advisory notices issued between 2013 and 2016 are still open, 18 of them since 2013.
The National Inspection Plan 2015-2017 requires local authorities to undertake a minimum of 1,000 inspections of domestic waste water treament systems each year. A greater proportion of inspections are required in areas of higher contamination risk, the focus being on the risk to public health and protecting water resources.