July 2007

The Mystery in the Clark Fork River

Why has pollution from mining operations sped down river much more quickly than expected?

Michael Hochella collecting water samples in the Clark Fork River.
A cluster of nanoparticles that contains a considerable amount of lead, one of the most toxic metals in the Clark Fork River system.
Montana’s Clark Fork River

Why has pollution from mining operations sped down river much more quickly than expected? Geochemist Michael Hochella suspects nanoparticles, incredibly small bits of matter.

The Clark Fork River, located in western Montana, runs through areas that have been heavily mined. "About a century-and-a-half ago mining started in this area, mining first for silver and later for copper and zinc. The problem with that was that it created a huge amount of toxic metal contamination. Those metals are toxic to life as you come down the river.�? When this heavy metal pollution is left on flood plains abutting the river, it creates areas called "slickens," barren fields that are too toxic for even the hardiest of plant life."

But while the effects of the pollution can be clearly seen in these "slickens," what is happening in the water carrying these metals is still a mystery. These heavy metals should be settling on the bottom of the riverbed, but instead some metal contamination has been found further downstream than expected. Michael Hochella has found nanoparticles containing some of these heavy metals in the water of the Clark Fork. These miniscule particles are so small and light that they never settle and could explain the movement of the pollution.

But finding these small particles is no easy feat. To give an idea of the scale of nanoparticles, according to Michael Hochella, a human is dramatically closer to the size of Mt. Everest than to the size of a nanoparticle.

Complicating matters is the nature of nanoparticles. Scientists are finding that particles at this size do not share the properties of the same substance at any other size. "When these same minerals get as small as a nanoparticle, we really don't know how these things behave. It's the same mineral, the same crystal structure and the same chemistry, but they're so small they behave differently. In fact in many cases, they behave dramatically differently. And if they're carrying the toxic material that you're interested in, the change in their properties as they get very small become the key to what you're trying to find out." For example, gold melts at about 2000 degrees Fahrenheit. But a nanoparticle of gold melts at 800 degrees Fahrenheit. These fundamental differences in properties could have a large impact on the way the substance interacts in an ecosystem. Figuring out exactly what these differences are and how that changes the metal’s effect on living creatures is the next step in Michael Hochella’s research.

Links to:
On Pulse: Michael Hochella’s blog

Related programs:
Science Diary: How Toxins Move - Clark Fork River
Science Diary: How Toxins Move - Hours in the Library
Science Diary: How Toxins Move - Headwaters
Science Diary: How Toxins Move - Restoration
Science Diary: How Toxins Move - Comparison
Science Diary: How Toxins Move - Shelved
Science Diary: How Toxins Move - Samples
Science Diary: How Toxins Move - Found It!