The Little Blue Run coal ash site is the largest in the US, spanning across the borders of West Virginia and Pennsylvania.

The plan requires FirstEnergy to have closed the ash pond by the end of 2028, three years before the company’s proposed date.

The company must stop disposing waste there in 2016 with the company expected to begin capping activities next year.

The permit requires the company to increase the groundwater and surface water monitoring points from 74 monitoring points to more than 300.

It must implement controls for noise, odours and particulate emissions while also conducting quarterly seep reconnaissance, institute corrective actions when contaminated seeps are found, pursue groundwater remediation measures and conduct post-closure monitoring for as long as environmental problems remain at the site.

FirstEnergy has posted a financial assurance bond of more than $US169 million ($A182million), the largest bond for a waste management facility ever required by DEP.

“The process to close the largest coal combustion waste disposal impoundment in the country was strenuous and thorough, involving citizens, who provided numerous comments, DEP staff and FirstEnergy,” DEP’s waste management program manager Mike Forbeck said.