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ST. CHARLES — City officials have dropped their court challenge of Ameren’s plan to drill a new extraction well as part of the company’s efforts to clean up contamination of the wellfield that used to supply much of St. Charles’ drinking water.

Mayor Dan Borgmeyer said Friday that the city has been assured that Ameren will dig the well only with approval of the federal Environmental Protection Agency.

U.S. District Judge John Ross on Thursday agreed to dismiss the city’s request for a temporary restraining order, which had been filed Sept. 14 in St. Charles County Circuit Court. Ameren had asked that the case be moved to federal court.

A spokesman for Ameren said the company is pleased that the city has backed off its “improper� restraining order action and urged St. Charles to refrain from interfering with Ameren’s remediation efforts.

Ameren has been doing groundwater cleanup in the city for the past decade because of contamination from cleaning solvents used earlier at a substation there.

In May, the city and St. Charles County filed a lawsuit seeking millions of dollars from Ameren in damages and to reimburse the city for its expenses in dealing with the contamination. That case is pending.

The EPA says the city’s water remains safe and that the wellfield contamination hasn’t exceeded allowable levels.