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Coal emissions and flooding increase mercury levels in South Dakota lakes

South Dakota Game, Fish & Parks
A walleye.

South Dakotans hitting the water this summer should be aware of new mercury advisories on certain lakes.

Game, Fish and Parks reports from mid-May have added two more lakes to the Mercury Fish Consumptions Advisories list from the state health department.

Kiesz Lake in McPherson County and Dry Lake #2 in Clark County now have mercury advisories on Walleye over 16 inches and 22 inches, respectively. The department tested 150 lakes total.

The list now includes 26 lakes across 17 counties. Fish with high mercury levels include Walleye, Northern Pike, Largemouth and Smallmouth Bass, and White and Black Crappie in sizes ranging from 12 inches to 30 inches and over.

These mercury advisories are nothing new in South Dakota.

Jim Stone and Heidi Sieverding, faculty in the Civil and Environmental Engineering department of the South Dakota School of Mines and Technology, published a paper on the causes of high mercury levels in South Dakota along with their colleagues in 2018.

Their research found coal emissions in the atmosphere most often lead to high mercury levels.

“But it’s not just here in South Dakota, it’s everywhere, and as it falls into the watersheds here in South Dakota, it can either stay in place or move as sediments as it moves into the lakes,” Stone said.

The other leading cause of mercury accumulation is water runoff from flooding and watersheds. Because mercury is one of the basic elements, it does not break down naturally. As a result, mercury can stay in a lake for years after being deposited through erosion or precipitation.

“Most of the lakes in South Dakota are closed-basin lakes, especially in the Prairie Pothole Region, where what’s coming into the system with the precipitation is not leaving the system,” Sieverding said.

Lake expansion also leads to higher mercury levels, as the growing lakes and wetlands draw in more mercury over time.

The accumulated mercury eventually makes its way into the food chain, which leads to mercury advisories for certain fish. Larger, older fish have higher levels of mercury, so state advisories include certain sizes that consumers should avoid.

Stone says these high mercury levels will likely not disappear any time soon.

“If we get into a drought-phase, that might lessen the watershed impacts related to that, but then you’re going to get more extreme rain events that might occur that might wash whatever’s deposited over time into there,” he said.

Sieverding adds that cutting down on fossil fuel consumption and soil erosion could help limit additional mercury coming into the water system.

“It’s one of those things that if you cut off the source, over time nature will, ultimately, heal itself,” she said.

Anglers should follow posted guidelines before consuming fish caught from lakes on the advisory list. Mercury poisoning can lead to neurological changes, especially in young children and babies.

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