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The Cheyenne River Sioux Tribe asked the Agency for Toxic Substances and Disease Registry (ATSDR) to assess the public health impact of environmental contaminants detected in fish from the Cheyenne River Basin Site. The site includes about 125 miles of river basin that extend from the Whitewood Creek National Priorities Site to the Oahe Reservoir. Whitewood Creek has been impacted by mine tailings and other hazardous materials released during gold mining activities near Lead and Deadwood, South Dakota. These wastes have spread into river bed sediments in the Belle Fourche and Cheyenne River.

Mercury, which was used in gold processing until 1970, is of particular concern because of its ability to bioaccumulate in aquatic biota. In the early 1970s, elevated levels of mercury(> 0.5 parts per million [ppm]) were detected in fish collected from Lake Oahe and its tailwaters. This finding prompted the South Dakota State Health Officer to post a fishing advisory for the Cheyenne Arm of the Oahe Reservoir.

On April 21 to 23, 1998, a contractor for the Environmental Protection Agency - Region 8 (EPA) collected 41 fish from three locations in the Cheyenne River Basin. An additional 39 fish were collected from the Moreau River, a reference area north of the Cheyenne River Basin and not subject to contamination from Whitewood Creek.

Fillets from the fish were analyzed for total metals (Target Analytical List), methyl mercury, and dimethyl mercury. In some of the fish samples, mercury recovery was below Quality Control limits, so the results were qualified as being estimated. ATSDR assumes the data to be accurate; however, our conclusions are subject to change if there are significant revisions in the data.

In fish, methyl mercury accounts for 80 to 99 percent of the mercury content of fish tissues. Therefore, the following analysis conservatively assumes that all mercury detected in the TAL mercury analyses is in the form of methyl mercury, which is more toxic than inorganic mercury.

Mercury concentrations in fish fillets ranged from 0.08 to 0.85 parts per million (ppm) on a wet weight basis. The average mercury concentrations detected in each species of fish from the four sampling locations are summarized in the following table:

Table 1.

Average mercury concentrations (in ppm wet weight) in fish from the Moreau River and Cheyenne River Basin
Northern Pike Channel Catfish Walleye White Bass
Moreau River 0.35 (10) 0.34 (10) 0.36 (10) 0.32 (9)
Foster Bay 0.39 (10) 0.20 (10) 0.41 (10) 0.54 (5)
Cherry Creek - 0.08 (1) 0.26 (4) -
Bridger - 0.29 (1) - -

(The number of fish in the sample is in parentheses)


Using Student's t Test, the mercury concentrations in fish from Foster Bay were compared with those from the Moreau River. There were no statistically significant differences between the two locations in mercury concentrations in northern pike, walleye, and white bass; in channel catfish, the mercury concentration in fish from Foster Bay was significantly lower than in fish from the Moreau River (p < 0.002). Therefore, these data indicate that mercury concentrations in fish from Foster Bay are comparable to or less than mercury concentrations in fish from the Moreau River, an area that has not been impacted by mining wastes from the Whitewood NPL site.

The Food and Drug Administration (FDA) has jurisdiction over the levels of contaminants in fish sold in interstate commerce. The FDA Action Level for mercury in fish (edible parts) is 1 ppm. The mercury concentrations detected in all fish in this study were below the FDA Action Level. However, it may not be appropriate to use the FDA Action Level to assess the health hazard from eating fish from the Cheyenne River Basin because: (1) FDA Action Levels apply only to fish sold in interstate commerce. (2) FDA Action Levels factor in economic considerations. (3) Fish consumption rates for active fishermen could be higher than those assumed for the consumption of commercially bought fish.

The health risks associated with eating fish with chemical contamination depends on the contaminant levels as well as the amount, frequency, and length of fish consumption. In this consultation, ATSDR conservatively assumes that a fisherman will eat one meal of fish (8 ounces or 227 grams) per week throughout the year. This equates to a fish ingestion rate of 32 grams per day. For lower fish consumption rates, the resulting hazard would be proportionally less.

In Foster Bay, the average concentration of mercury detected in the 35 fish that were analyzed was 0.363 ppm. Therefore, a fisherman who eats, on average, 32 grams of fish per day would ingest a mercury dose of:


This dose is less than ATSDR's chronic Minimum Risk Level (MRL) for methyl mercury of 5 x 10-4 mg/kg/day. Therefore, eating fish from the Cheyenne River Basin at the assumed rate does not pose a health hazard. An MRL is an estimate of daily human exposure to a chemical that is likely to be without appreciable risk of deleterious effects (non-carcinogenic) over a specified period of exposure. The MRL is based on protecting the most sensitive population, which, for methyl mercury, is fetuses that are exposed in utero. For other individuals, the hazard associated with eating fish from the Cheyenne River Basin would be even less.


  1. The concentrations of mercury detected in fish from the Cheyenne River Basin were not significantly elevated as compared to a reference area (Moreau River).

  2. The concentrations of mercury detected in fish from the Cheyenne River Basin do not pose a health hazard for sport fishermen who eat fish from the basin.

Kenneth Orloff, Ph.D., DABT
Cheyenne River Basin

Chief, EICB, DHAC (E32) Date

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