Two former employees of chemical company Freedom Industries have pleaded guilty to charges related to last year's chemical spill in West Virginia, which has been linked to holes in a corroded tank.

As many as 300,000 local residents were unable to use their tap water for up to 10 days after thousands of gallons of a coal-cleaning agent known as MCHM were released into the Elk River in Charleston, West Virginia, on January 9, 2014.

West Virginia American Water, which has a water supply intake about two miles downstream from the chemical spill, had to flush out its system before the tap water was safe to drink.

After the release, more than 400 people received hospital treatment for symptoms linked to exposure to the chemical.

At separate hearings on Wednesday, former plant manager Michael Burdette and environmental consultant Robert Reynolds admitted negligent discharge of a pollutant. Earlier in the week, former Freedom owners Charles Herzing and William Tis also pleaded guilty to pollution charges.

All four men are scheduled to be sentenced in June.

Meanwhile, former Freedom owner Dennis Farrell and former president of the company, Gary Southern, are scheduled to go on trial later in the year on charges related to the spill.

Freedom Industries filed for bankruptcy protection just eight days after the incident. Southern has also been charged with bankruptcy fraud, wire fraud and lying under oath in connection with the bankruptcy case, the Associated Press reported.