A study by researchers at the South Dakota School of Mines & Technology on
the concentration of mercury in the sediments of seven South Dakota lakes found
that the pollutant generally enters the water bodies through runoff and can
increase during times of flooding and lake expansion.
The paper, titled “Historical
sediment mercury deposition for select South Dakota, USA, lakes: implications
for watershed transport and flooding,” was published this year in the Journal
of Soils and Sediments.
Emissions from coal fired power
plants are the main source of mercury pollution around the world. Mercury is distributed
through the atmosphere until it settles on the land. Global deposition of mercury
began with the start of the industrial revolution and peaked in the United
States in the 1970s. The concentrations have gone down with emission reductions
and pollution control in more recent decades. Historic mining and industrial operations
are less common sources of mercury pollution.
The objective of the study was to
determine the main source of mercury and to understand the history of mercury
deposition on a diverse range of lakes in the state. The study included Wall
Lake, West 81 Lake, Lynn Lake, Island Lake, Lake Hurley and Lake Sinai in the
eastern part of the state, and Lake Isabel in the western part of the state.
The study was undertaken during the winter months when researchers drilled
through the ice on the frozen lakes and took core samples of the mud on the
lake bottom. By carbon dating the layers that were deposited in years past, they
were able to look at the history of mercury deposition over the past century.
Mercury accumulation from the
atmosphere on the land is eventually washed into waterways, wetlands and lakes
where it can build up in sediment and/or be chemically changed in form to enter
the aquatic food chain and then accumulate in fish. The research concludes
that, “Mercury in the sampled South Dakota lakes appears to emanate from
watershed transport due to erosion from agricultural land use common to the
Northern Great Plains.” The research also shows that “For sampled South Dakota
lakes, watershed inputs are more significant sources of Hg (mercury) than
atmospheric deposition.”
Two sites, Lake Sinai and West 81
Lake near Brookings, have undergone cycles of flooding and drought in recent
decades. “These lakes showed a tremendous increase in mercury following times
of lake expansion and flooding,” says Dr. Jim Stone, one of the researchers on
the study. The ultimate source of the pollution is the atmospheric deposition
of mercury. “But once it falls to the ground the way the watershed is managed can
impact how it ends up in these lakes,” says Stone.
The study was undertaken by Maria
K. Squillace, Heidi L. Sieverding and Jim Stone, Ph.D., in the Department of
Civil and Environmental Engineering, and Hailemelekot H. Betemariam in the
Department of Geology and Geological Engineering at SD Mines. Others at
University of Wisconsin-Platteville, North Dakota State University and Michigan
Technological University also contributed to the work.
The South Dakota Department of
Heath maintains a list of
lakes in the state with mercury advisories.