Leitrim and Roscommon are bottom of the table in the report on septic tanks published today by the Environmental Protection Agency.
Almost 90% of tanks inspected in Leitrim failed EPA tests, while just under 80% of tanks inspected in Roscommon failed.
The average failure rate nationwide for septic tanks is 50%.
Longford had a low failure rate in comparison to its neighbouring counties, at 45% , but it carried out just 11 inspections last year, compared to 27 carried out in Leitrim, and 47 in Roscommon.
The main reasons for the failures include structural defects and a lack of maintenance.
The report showed that 47% of issues identified in tanks in Leitrim were rectified by the end of last year, 45% had been addressed in Roscommon, and 94% had been resolved in Longford.