History of Mercury Loading in the Upper Mississippi River Reconstructed from the Sediments of Lake Pepin

Abstract

An array of sediment cores was analyzed to determine historical trends in mercury (Hg) accumulation in Lake Pepin, a natural lake on the Upper Mississippi River. Whole-basin Hg accumulation rates increased from 3 kg/yr before European settlement (ca. 1830) to a maximum of 357 kg/yr in the 1960s. The recent Hg accumulation rate (110 kg/yr, 1990−1996) is experimentally indistinguishable from measured Hg loadings in the river entering the lake, indicating that accumulation rates in Lake Pepin correspond quantitatively to river loadings. The modern accumulation rate represents a decline of almost 70% from the peak value, reflecting large decreases in Hg inputs to the Mississippi River from industrial and municipal point sources in the metropolitan Minneapolis/St.Paul area upstream. A total of 18.1 t of Hg has been deposited in Lake Pepin since 1800; half of that (9.0 t) was deposited between 1940 and 1970, when regional growth accelerated rapidly but pollution control mechanisms were inadequate. Point sources accounted for approximately 60% of the Hg accumulating in Lake Pepin in the 1960s, but these inputs have been virtually eliminated since then.

Mercury trends in fish from rivers and lakes in the United States, 1969–2005

“…Major sources of anthropogenic Hg include mining, emissions from fossil fuel combustion, industrial processes (metal smelting and refining, chemical production, cement, and pulp and paper), and waste incineration (USEPA 1997). From about 1940 to 1970, a period of rapid industrial growth and inadequate wastewater treatment, local point-sources were the dominant source of Hg in many urbanized areas (Pirrone et al 1998;Balogh et al 1999). Improved wastewater treatment, stricter regulation of discharges to air and surface waters (USEPA Clean Water Act, Clean Air Act, Safe Drinking Water Act, and Toxic Substance Control Act), and reductions in industrial use of Hg since the 1960s have eliminated most Hg point-sources in urban areas (Balogh et al 1999).…”

“…From about 1940 to 1970, a period of rapid industrial growth and inadequate wastewater treatment, local point-sources were the dominant source of Hg in many urbanized areas (Pirrone et al 1998;Balogh et al 1999). Improved wastewater treatment, stricter regulation of discharges to air and surface waters (USEPA Clean Water Act, Clean Air Act, Safe Drinking Water Act, and Toxic Substance Control Act), and reductions in industrial use of Hg since the 1960s have eliminated most Hg point-sources in urban areas (Balogh et al 1999). The rapid decrease in Hg concentrations in fish collected by the NCBP in the 1970s likely results from this overall reduction of Hg in direct releases to the atmosphere and surface waters ( Fig.…”

“…Atmospheric deposition is the dominant source of inorganic Hg in many aquatic ecosystems (Fitzgerald et al 1998;Downs et al 1998;Wiener et al 2006;Peterson et al 2007). Industrial and municipal point-sources of mercury to rivers and lakes were very important historically, but have been greatly reduced since the implementation of stricter regulatory controls in the 1970s (Balogh et al 1999;Krabbenhoft et al 2005). Widespread Hg contamination of fish in remote areas with low rates of Hg deposition suggests Hg loading in an ecosystem alone does not define Hg concentration in fish (Krabbenhoft et al 2005;Wiener et al 2007).…”

“…Transport of atmospheric Hg to aquatic ecosystems and Hg cycling within aquatic systems are complex and not fully understood. Catchment size, geochemistry, hydrology, climate, soil, land-use, land cover, and topography all influence delivery of Hg to aquatic ecosystems (Balogh et al 1999). Human activities contribute atmospheric emissions and direct point source Hg contamination, and create land disturbances that enhance Hg delivery to aquatic ecosystems (Balogh et al 1999).…”

Mercury as a global pollutant

“…Then, a reconstruction of local and regional trends in Hg deposition to water bodies is possible. For instance, Hg concentration profiles in sediment cores from a natural lake on the Upper Mississippi River record the history of the lake contamination that began with European settlement in the early 1800 's , peaked after the second world war and declined to current levels [68]. Lake-sediment records have also shown that atmospheric Hg deposition could have increased by a factor 3-4 since pre-industrial times [69].…”