Six Pathways at the Midnite Mine

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Multiple Exposure Pathways

Let’s face it: toxic substances are all around us. They are part of the natural and man-made world. That can be pretty frightening. But to cause harm, toxic substances must actually get into your body through your skin, eyes, digestive system, or lungs. And even if you do touch, swallow, or breathe these substances, they must reach certain levels in your body to cause damage.

ATSDR scientists sometimes investigate sites where toxic substances can get into people’s bodies in more than one way—or by multiple exposure pathways. They found just such a site on an Indian reservation in the Northwest.

Exposure at an Abandoned Mine

In 2008–2009, an ATSDR team investigated exposure pathways at the Midnite Mine uranium site on the Spokane Indian Nation Reservation in Washington state. The mine operated between 1954 and 1981, but ATSDR found that toxic chemicals still present in 2009 were a source of exposure for people who used the land and water nearby.

exposure pathways

The mining company left behind more than 33 million tons of waste rock (unprocessed and low-grade ore) and large open pits partially filled with water and waste rock. In addition, the site included areas of sediment, surface water, soil, and groundwater affected by the mine. Contaminated surface and ground water flowed into a nearby creek and then into the Spokane River.

Six Exposure Pathways

Past investigations showed that metals, including arsenic, cadmium, manganese, and uranium (and radioactive isotopes and decay products of uranium) had moved from sources on the site into local ground water and surface water. Both mining activities and environmental processes had moved the toxic substances into water and sediment in the area.

Some members of the Spokane Indian Nation still used the area impacted by the mine site for tribal cultural practices. They harvested plant and animal foods like roots, berries, and fish and used the water in traditional sweat lodge ceremonies.

ATSDR explored ways people could come into contact with the toxic substances at the site and concluded that levels of the contaminants could possibly be harmful. The site team found that metals in the water draining or seeping from the mine could get into people’s bodies by drinking the water or by breathing water vapor heated during sweat lodge ceremonies.
In addition, ATSDR scientists concluded that people using the property could be exposed by accidentally or intentionally swallowing

  • sediments along drainage spots in the mining-affected area,
  • plants and roots in mining-affected area,
  • aquatic plants from drainage areas at the site or from Blue Creek, or
  • fish from Blue Creek.

Not all hazardous waste sites have the same exposure pathways, but the diagram at the right from the ATSDR public health assessment guidance manual provides a picture of how complex and varied exposure pathways can be.

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Page last reviewed: July 9, 2015
Page last updated: July 9, 2015