Mercury from Coal Combustion

A 1997 study in Minnesota indicated a relationship between mercury deposition trends and regional coal combustion (as well as other possible sources) spanning a period of two centuries (Engstrom and Swain 1997). Sediment core sampling from rural lakes in eastern Minnesota indicated a growth in mercury deposited from the air since the first half of the 1800s, with the amount peaking in the 1960s and 1970s at 4.0 to 5.2 times higher than pre-industrial levels. Deposition then declined during the 1980s to levels 3.0 to 3.4 times greater than pre-industrial levels. By comparison, sediment core samples from remote coastal lakes in southeastern Alaska showed a general increase in mercury deposition since 1850 that continued increasing after the 1960s with no downward trend. If mercury deposition trends are largely driven by a global pool, one would expect to see the same trend profiles in the remote Alaska lakes as in the rural Minnesota lakes. Because the remote Alaska lakes showed no general decrease in mercury deposition, the downward trend in the Minnesota lakes likely occurred due to changes in regional rather than global mercury emissions. The Minnesota study identified two changes related to regional coal combustion that occurred at the same time as the decrease in mercury deposition to the lakes. These were the increase in use of technologies at coal-fired power plants (as well as metal smelters) that incidentally reduced mercury air emissions and a shift away from coal to natural gas and oil for commercial and residential heating. (Other changes that may have contributed to the decline in mercury emissions include the reduced industrial use of mercury and a decrease in emissions from waste incineration.)