Offaly has been slow to carry out their required inspections.
Half of the septic tanks in the country failed inspection last year with 20 per cent identified as a risk to human health and the environment.
The findings are contained in an Environmental Protection Agency report on Domestic Waste Water Treatment Systems.
Laois had the highest failure rate in the midlands at 56%, while the stats are much more favourable in Offaly and Westmeath with failure rates of 17% and 14% respectively.
It was Offaly that had the most inspections required at 20, with a number carried forward from 2021. However, only six of the required 20 were done.
In Laois, all nine required inspections were carried out and in Westmeath seven out seven took place.
Noel Byrne of the EPA said the delay in fixing faulty septic tanks is posing a risk to public health.

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