Slick ice has covered roads and sidewalks across much of the Northwoods in recent days. This also means more salt is being used to melt that ice.That salt has been making its way into lakes in southern Wisconsin for decades. The City of Madison says the chloride levels in Lake Mendota have been increasing by about one milligram per liter a year since 1962.While chloride level data is lacking on many Northwoods lakes, data collected from one local lake makes it clear that water bodies here are not immune.