Most of the septic tanks issues detected in Donegal since 2013 have now been resolved.
The tests were carried out by Donegal County Council on behalf of the Environmental Protection Agency.
The report shows that there was an 18% failure rate in the county last year, much lower than the national average of 51%.
Figures also show that 84% of fault identified since 2013 have been fixed.
EPA inspector Noel Byrne says it's anticipated that a new grant scheme should result in a lower failure rate going forward: