The US Environmental Protection Agency claims coal ash from the plant was poorly disposed at Rotary Drilling Supply’s site in Jefferson County, fouling wetlands, a tributary to a creek and part of a lake.

The EPA alleges that 140,000 tonnes of coal ash were delivered from the plant to the site between 2004 and 2008 and tests showed they polluted the water sources.

In a statement released on Thursday the EPA said the proposal would require Rotary Drilling Supply to place a protective cap over still-existing ash piles to prevent run-off and take other short and long-term measures to prevent further migration of pollutants.

Rotary Drilling must purchase “wetland mitigation bank credits” to ensure the restoration or enhancement of local conservation areas to offset the damage it allegedly caused.

The company must submit plans within a month of the settlement being finalized following a 30-day public comment period.

The settlement announced on Thursday named only Rotary Drilling but according to the Post Dispatch there is a separate agreement between Rotary Drilling, Ameren and two other involved parties to share responsibility for the site clean-up.

Terms of the agreement aren't publicly available.

In a statement, Ameren Missouri said it had no part in the remediation project and "no obligations" under the EPA settlement.

The utility also said Rotary Drilling "was responsible for applying for all necessary permits and authorizations relating to the project, including the use of coal ash as fill material".

An attorney for Rotary Drilling told the Post Dispatch his client was using the ash to level the property for possible future development but made no comment on the settlement.