Rainwater in Western Lake Superior Sanitary District (WLSSD) is higher in mercury contamination than the treated sewage water leaving the district. Like all large Great Lakes wastewater plants, the WLSSD is under federal order to reduce mercury in its discharge to 1.8 parts per trillion. This is an effort to keep Lake Superior free from mercury, which builds up in fish making them unsafe for humans to eat.
But, many experts say it’s impossible to meet the wastewater standard as it is far below the level of mercury in rainwater that falls on the Northland, which is now about 12 parts per trillion. The WLSSD receives untreated wastewater averaging about 100 parts per trillion mercury. Before the mercury flows into the St. Louis River, its treatment process reduces that to about 2.6 parts per trillion, on average.