Two former executives of a West Virginia chemical company entered guilty pleas Monday in connection with the January 2014 spill that contaminated a river used for drinking water for 300,000 people.

In separate hearings, William Tis, 60, and Charles Herzing, 64, each told US District Judge Thomas Johnston in Charleston, West Virginia, that they were guilty of one count of causing unlawful discharge into the Elk River of the chemical MCHM, a cleansing agent used in processing coal to reduce power-plant emissions.

The spill from a storage tank at a Freedom Industries terminal on the Elk River took place about 2,400 metres upstream from a water intake used by a water company serving several counties around Charleston, the state capital.

Residents were told to not drink or bathe with their running water for up to 10 days after the spill, while the toxic, licorice-smelling chemical dissipated. Local authorities trucked in water and distributed bottled water for days after, and concerned citizens organized activists groups in response to the incident.

Tis was secretary and Herzing vice president of Freedom Industries, a Charleston-based producer of specialty chemicals for the mining, cement and steel industries. Both were members of the company's board of directors.

Under their plea agreements, they admitted sharing responsibility with other executives for the company's violation of permits that required protections to prevent and mitigate chemical leaks. Both had been charged with three criminal counts under federal law including the Clean Water Act.

Sentencing for both men was scheduled for June 22. Under their plea agreements, both face at least 30 days and up to one year in prison, are subject to fines and restitution and must cooperate with ongoing government investigations.

The case was investigated by the FBI and the federal Environmental Protection Agency, US Attorney Booth Goodwin said.

Freedom Industries and two other ex-officials have agreed to plead guilty to spill-related charges, the Charleston Gazette reported.

Two other former Freedom executives still face charges in the case.

Coal mining is a pillar of the economy in rural, mountainous West Virginia.