Muddy Creek is nondescript, a narrow stream trickling through the sagebrush steppe of southern Wyoming. But like many Western waterways, it carries selenium, a natural poison that seeps from rocks and dirt and accumulates in the food chain much as mercury does.
Both humans and animals need tiny amounts for good health, but too much is dangerous. In areas with a lot of selenium in the soil, including Utah's Middle Green River Basin and Nevada's Stillwater National Wildlife Refuge, activities that bring soil into contact with water -- irrigating, mining, drilling and road building -- boost natural concentrations, and can cause illness and deformities in people, livestock and wildlife.
Now, researchers are studying the Muddy Creek watershed, trying to determine how much of the element occurs naturally, and how much is being released by human activity. Tracking selenium sources in this way is tricky, but essential; few studies have examined where selenium comes from and where it ends up.