Environmental groups have been urging the lawmakers to force Duke Energy to dig up its ash currently in unlined pits close to rivers and water supplies and move it to licensed landfill.

Members of a state Senate committee who reviewed Governor Pat McCrory’s clean-up plan questioned why no time trajectory was included.

Lawmakers and environmentalists have since asked for a specific schedule to be included into the legislation.

The schedule would set out exactly how long the company has to remove its dumps at four power plants, including the site of February’s Dan River spill, the effects of which can still be seen along the river.

The current proposal requires Duke to submit closure plans for the four sites but leaves it up to state environmental officials to decide how long the company has to complete the task.

McCrory’s plan has been heavily scrutinised by environmentalists due to the governor’s strong connection to the company.

Before heading into politics, he retired from Duke after working at the company for nearly 30 years.

State officials acknowledge that all of Duke’s unlined dumps are contaminating groundwater.

A Duke executive previously told legislators it would take decades and could cost up to $US10 billion ($A10.7 billion) for the company to move all of its ash to lined landfills.

The costs would almost certainly be passed down to consumers.