Democrats Jon Richards and Chris Larson called on the US Environmental Protection Agency to reject a request by owner Lake Michigan Carferry to allow the 4,200-gt Badger (built 1953) to continue the practice.
They say the coal-fired steamship should not be given a permit extension beyond 19 December.
The Milwaukee Journal Sentinel reported the ferry dumps at least 500 tons of coal ash into the lake each sailing season.
