It has taken years of planning and legal battles, but dredging operations have finally started on the bottom of Ashland harbor. Cancer-causing pollutants, wood from closed lumber mills and the debris of old docks litter Lake Superior’s bed.  Northern State Power Company of Wisconsin may have to cover the 97-million dollar cost of the project and it is asking its natural gas customers to dig deeper.  Most of the pollution has been tied to a plant which discharged a tar-like substance created when it used coal to produce electricity for Ashland customers.