Minnesota Statewide Mercury Total Maximum Daily Load

Mercury is a neurotoxin, meaning it damages the central nervous system. The developing nervous system
is at the greatest risk for damage. Mercury is also a global pollutant; it is transmitted around the world
and accumulates to levels in fish that are potentially toxic to humans and wildlife. This report sets a
target for fish tissue concentration of mercury that is generally safe for human consumption, and
translates the target to reduction goals for mercury sources. Environmental contaminants are usually treated as media-specific—an air, water, or soil contaminant. Mercury is a multimedia pollutant: transported by air, stored in soil, and chemically transformed and bioaccumulated in water. Mercury reductions needed to achieve the target for safe fish consumption are translated to mercury emissions reductions, because 99 percent of mercury load to Minnesota’s lakes and streams is from atmospheric deposition. Because this report includes jargon from the Clean Water Act and the Clean Air Act, the following point of clarification is needed: this report will refer to water releases of mercury as “discharges” and air releases as “emissions.” While stacks from air emission sources may be referred to as point sources in the air-permitting arena, the Total Maximum Daily Load (TMDL) concept arises from the Clean Water Act, where “point sources” refer to identifiable pipe conveyances and include wastewater and stormwater, which have National Pollutant Discharge Elimination System (NPDES) permits. Therefore, air sources (i.e., stacks) will be referred to as “point source emissions” and water sources (i.e., pipes) will be referred to as “point source discharges.”