Volume 2, Issue 14 September 2005
Geochemistry Underfoot
For most of us, the effects of a rainstorm fade shortly after the last drop falls. The sun shines through the clouds and the streets begin to dry. Dig a little deeper though and you'd realize that the end of a shower is really just the beginning of the Earth's interaction with the fallen water. Slowly, the rain seeps into the watershed and makes its way into nearby streams. UC Berkeley professor James Kirchner wants to know what happens along the way. His research on this and other earth and planetary science questions could someday aid environmental remediation efforts and inform decisions about sustainable land use.
James Kirchner at the San Rafael Reef in Utah
"If it happens between 10 meters above the surface and 10 meters below, I'm probably interested in it," says Kirchner, who also directs Berkeley's Central Sierra Field Research Stations.
These days, Kirchner has three major research thrusts that all fall within that relatively thin slice of the planet. Geomorphology is the study of how Earth's topography evolves to form mountains, river valleys, and other surface formations. Meanwhile, the rainwater project spans two fields of study: hydrology and aqueous geochemistry, the chemical reactions between water and rocks.
"If it rained in Berkeley today, maybe a trillion drops would fall on campus," he says. "Some fraction of those would reach the stream today, some fraction tomorrow, and more the day after. We'd like to measure that distribution of transit times and the reactions that occur as the water moves underground."
Kirchner and his colleagues are developing techniques to study the factors controlling rainfall and runoff both physically and chemically. Their approach is to measure environmental "tracer chemicals" such as chloride that occur naturally in rainwater. Then, they look for the same tracers in samples from the stream flow and compare the two measurements. The fluctuations in those measurements provide the researchers with a good sense of how long it takes the rainfall to get to the stream.
A view of a watershed at Stony Creek Fan near Orland, California
"Looking at the movement of rainfall can also tell us how long contaminants like pesticides, fertilizers, or industrial compounds might be retained underground in the watershed before they're released into the stream," Kirchner says.
In one experiment, the researchers analyzed the amount of chloride in rainfall and runoff from a headwater stream in Plynlimon, Wales. They showed that contaminants would initially flush out of the watershed quickly but that low-level contamination could continue to steep into streams for years. The next step is to determine how contaminated water chemically reacts as it moves through the ground. Kirchner and his colleagues are using chemical tracers and soil samples to identify possible reactions.
"The question is whether the water moves through highly channelized flowpaths or through the soil like it's a sandbox, contacting the surface of every grain of every mineral," Kirchner says. "It'd be wonderful to give the subsurface of the Earth an MRI, but it just isn't feasible. So all we can work from are the chemical tracers in the water to tell us where it's been."
Kirchner's second large research project is a bit closer to the ground we stand on. With collaborators from Lawrence Livermore National Laboratory's Center for Accelerator Mass Spectrometry and other institutions, Kirchner is honing new techniques to measure long-term rates of physical erosion and weathering at the Earth's surface. Various processes convert rock to soil that either dissolves or makes its way down hill slopes where it becomes sediment in streams. Scientists would like to understand how those processes are linked to variations in topography, climate, and ground cover.
"It's important to know how the Earth's surface evolves but it's also relevant to long-term sustainability questions of forest management, logging, and agricultural practices," Kirchner says.
Until several years ago, it was extremely difficult to approximate the speed at which the various processes occur, he says. Then, the researchers developed a novel method for calculating how fast the Earth's surface erodes.
Wide angle view of the Center for Accelerator Mass Spectrometry FN tandem accelerator and mass spectrometer that Kirchner used to measure cosmogenic particles in mineral samples. (courtesy LLNL)
The Earth is constantly being bombarded with particles such as gamma ray neutrons that stream down from the upper atmosphere. These particles can transform certain elements into rare isotopes. For example, when a gamma ray neutron hits the nucleus of a silicon atom, it becomes a particular aluminum isotope. That isotope only exists as a product of this cosmic ray reaction.
Minerals in the Earth's surface are usually shielded from these gamma rays by the mass above them. But as that shielding is naturally eroded away, the minerals are subjected to a stronger flux of the gamma rays. As a result, the concentration of the aluminum isotope, for example, increases.
"We can measure that concentration and know how fast or slowly the sample has ridden the erosional elevator to the surface," Kirchner explains. "Once you've got these measurements from many places, you can ask questions about whether steep slopes erode faster than gentle slopes or how erosion rates change when you go from dry climates to wet climates."
The next step is to combine those measurements with techniques to chemically analyze rock and soil. Then, Kirchner says, the researchers can learn how fast soil minerals are made from rock and the rate at which natural processes boost the fertility of soil. Chemical weathering of rock is also tied to the amount of carbon dioxide in the atmosphere and the greenhouse effect, albeit on timescales of millions of years.
"All of this work makes it more interesting to look out the window everyday," Kirchner says. "You don't just see the scene in front of you but you also have an idea of what's happening below the surface."