South Dakota study finds mercury levels in lakes increase during flooding
PIERRE, S.D. — A recent study by the South Dakota School of Mines & Technology found that mercury levels can increase during times of flooding and lake expansion — exactly what’s happening this soggy spring and summer.
The SDSMT study, published this year in the “Journal of Soils and Sediments,” found that 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 U.S. during the 1970s.
Mercury concentrations have gone down with emission reductions and pollution control in recent decades. Historic mining and industrial operations are less common sources of mercury pollution.
The focus of the study was to determine the main source of mercury and to understand the history of mercury deposition in South Dakota.
Three South Dakota state agencies — Game, Fish and Parks; Department of Environment and Natural Resources; and the Board of Health — run tests on state waters that include levels of mercury in fish.
GF&P, for example, samples at least 10 lakes each year for a panel of 25 contaminants, testing fish for metals; pesticides; polychlorinated biphenyls, an organic chlorine compound once widely used in coolant fluids; and mercury.
John Lott, GF&P chief of aquatic research, said that mercury enters the food chain from
many sources, either natural or man-caused.
Natural sources include the creation of methylmercury. Methylmercury is formed from inorganic mercury by the action of microbes that live in aquatic systems including lakes, rivers, wetlands, sediments, soils and the open ocean. The production of methylmercury has been primarily attributed to anaerobic bacteria in the sediment.
The Department of Health’s website explains mercury contamination this way: “In lakes and other bodies of freshwater, small organisms convert naturally occurring inorganic mercury into its organic form, methylmercury. Methylmercury binds with particles and sediments eaten by smaller fish. Larger game fish prey on these smaller, mercury contaminated fish. Because fish eliminate mercury at a very slow rate, it accumulates in their tissues and organs where it cannot be removed by filleting or cooking, unlike organic contaminants that concentrate in the skin and fat.”
According to United States Geological Survey, “The toxic effects of mercury depend on its chemical form and the route of exposure. Methylmercury is the most toxic form. It affects the immune system, alters genetic and enzyme systems, and damages the nervous system, including coordination and the senses of touch, taste and sight... Exposure to methylmercury is usually by ingestion, and it is absorbed more readily and excreted more slowly than other forms of mercury. Elemental mercury, the form released from broken thermometers, causes tremors, gingivitis and excitability when vapors are inhaled over a long period of time. If elemental mercury is ingested, it is absorbed relatively slowly and may pass through the digestive system without causing damage.”
Robert Hanten, GF&P fisheries biologist, said that elemental mercury is different than methyl-mercury.
As larger and larger organisms consume smaller ones, mercury builds up each step up the aquatic food chain.
Hanten said that testing of fish in various bodies of water – lakes and streams – was begun in South Dakota in the early 1990s.
Of the numerous lakes in the state, 23 have received fish consumption advisories from the Board of Health and GF&P. These consumption advisories say certain types and sizes of fish caught in specific South Dakota waters are not healthy to eat under certain circumstances
“These are case-by-case water bodies,” Hanten said, each can be affected by contaminates according to “exposed shoreline and atmospheric disposition of the water surface.”
“What we are trying to do is provide the public with information so they can make an informed decision,” Lott added. People can choose to eat the fish they catch, but only in recommended amounts. He added that the suggested mercury threshold, 1 part per million, which is 10 times lower than the amount of mercury needed to display any effects.
Mercury concentration is usually not a concern, until the concentration of mercury is magnified as it accumulates up through the food chain.
Once a lake or dam has been placed on the consumption advisory list, it will be tested again in approximately five years.
Hanten said that any earlier testing would probably not show any different results. Upon questioning, he did say that no South Dakota body of water placed on the consumption advisory list has ever been removed from the list.
Lott stressed that not all waters in the state have been sampled.
The New Wall Lake near Wall was tested in September 2016. Certain size and species of fish were found to have warning concentrations of mercury.
Lott concluded, “It is great that people are out there using our natural resources. This simply informs them so they can chose how to harvest it.”
If fishing a consumption advisory body of water, fishermen should either keep caught fish for limited consumption over time, or release the fish back into the water.
Maps provided on the GF&P and DENR websites show which lakes and dams have been tested, and whether they have needed to be posted or not. The red triangles indicate bodies of water with consumption advisories, and green fish indicate a body of water where fish tested low in mercury and other contaminants.
Current mercury fish consumption advisories, by county and lake
South Dakota’s main fish of concern are walleye. Other fish of concern, depending on the body of water, include: northern pike, black crappie, largemouth bass, smallmouth bass, and white crappie. As far as the mercury concern, sizes of these fish range from all sizes to specific sizes, such as, say, 13 — 16 inches.
Brookings/Kingsbury counties — Twin Lakes
Brown — Elm Lake
Butte — Newell Lake
Clark — Reid Lake
Clark — Swan Lake
Codington — Long Lake
Corson — Pudwell Dam
Dewey — Lake Isabel
Kingsbury/Brookings — Twin Lakes
Marshall — South Buffalo Lake
McCook/Minnehaha — North Island Lake
Minnehaha — Twin Lakes
Pennington — New Wall Lake
Perkins — Coal Springs Reservoir
Potter — Lake Hurley
Tripp — Lake Roosevelt
Day — Bitter Lake, Hazeldon Lake, Lake Minnewasta, Lardy Lake, Lynn Lake, Middle Lynn Lake, Opitz