Almost half of household septic tanks inspected under water services legislation last year failed to meet required standards, according to new figures from the Environmental Protection Agency (EPA).
A total of 1,110 domestic wastewater treatment systems were inspected by local authorities during 2016, exceeding the target of 1,000 set by the EPA.
Of these, 545 septic tanks (49pc) failed the inspection, requiring homeowners to upgrade their wastewater systems or carry out remediation work.
The remaining 565 septic tanks were found to be compliant.
The national inspection plan has been developed by the EPA, and is delivered by local authorities. The regime of septic tank inspections began in 2013 and focuses on areas where the potential risk to public health and valuable water resources is greatest.
Each local authority is set a target number of inspections to carry out based on environmental risk. Longford County Council was the only local authority that failed to meet or exceed the target number of inspections last year. Only five septic tanks were inspected in the county during 2016 - some way short of its target of 14.
The highest number of inspections was conducted by Wexford County Council, which inspected 129 septic tanks. This is almost double the 67 wastewater systems that the council was required to inspect by the EPA.
The figures, which were released by the EPA under the Freedom of Information Act, are provisional and subject to verification.
A spokesperson for the EPA said that any possible sanctions against local authorities that failed to meet inspection targets would not be considered until the data was finalised.
Longford County Council did not respond to a request for comment.
The number of inspections carried out last year represents a small increase in the number that was conducted in 2015, when nearly 45pc of the 1,097 septic tanks inspected failed to meet the required standards.
During that year, two local authorities - Mayo County Council and Donegal County Council - failed to carry out the target number of inspections set by the EPA.
The highest number of inspections was again carried out by Wexford County Council, which exceeded its target of 67 septic tank inspections by 38.
Householders whose septic tanks fail local authority inspections are issued with an advisory notice outlining the steps required to bring the wastewater system up to the required standard.
In 2015, the most common reasons for septic tanks failing the inspections were problems with the operation and maintenance of the system, followed by a need for the units to be desludged. The EPA has a supervisory role in relation to each local authority's performance of its functions.