|
Posted 11/09/01
People build houses, plant fields and construct cities on the top layers of the planet's surface. These layers, however, are far from solid. They are flexible and mobile, some parts more than others. Arjun Heimsath, Assistant Professor of Earth Sciences at Dartmouth, measures this dynamic land movement by calculating erosion rates in different parts of the world.
Scientists know that the earth's processes have a profound effect on human lives, and research to study and predict erosion will help people adapt to our changing planet.
In northern Australia, for example, Heimsath's work involves uranium mining. "When you extract uranium during the mining process, you produce radioactive waste. It is piled into an earthen dump that looks like a giant loaf of bread, acres across. That mound of tailings is susceptible to erosion," Heimsath said. With his colleagues, he works to determine how the waste piles will erode over time, and how that eroded material will move through the environment. Their calculations, using one of the most accurate procedures to date, quantify erosion, computing the speed at which land melts into itself, lakes, streams and eventually the ocean.
To conduct his research, Heimsath extracts cosmic isotopes from rock and sediment samples. "Cosmic rays are high energy particles coming in from both galactic and solar sources. I'm primarily interested in the galactic ones that have higher energies. When they hit minerals in a specific way, they knock electrons and neutrons off atoms and create numerous different isotopes," he said. "Concentrations of these isotopes build over time at rates dependent on their location." From those isotope concentrations, also called radionuclides, he can determine how long that material has been there and how fast it's eroding or breaking down.
Heimsath's procedure for determining the age and erosion rate of rocks located up to a few feet beneath the soil emerged from a similar method for rocks on the surface. He collects the samples using a hammer and chisel, a rock drill or a shovel. The sample size ranges from a few grams of gravel and dirt to larger rocks about the size of a gallon of milk. Back in the lab, Heimsath works to extract the chemical data found in the quartz in his samples.
"A solution of acid dissolves everything but the quartz, and then we further break down the quartz to get the radionuclides," Heimsath said. The gallon-container-size original sample is reduced to a pinhead amount of powder. It's placed in a nuclear accelerator where a beam of ions hits the sample, resulting in another beam of electrons. It's then accelerated to close to the speed of light, and the precise atomic makeup of the sample can be measured.
Once he's obtained those figures, he plugs them into a mathematical landscape simulation - where every location or point has a number representing the elevation - to predict how a given set of environmental parameters will move sediments and eroded material around the system.
"My measurements will help explain how sediment moves off the tailings dump, down the hillslope, across grassy flood plains into a stream, picked up by flowing water, moved through rapids and across tree stumps and settles in sandbars. Tree roots could take up some nutrients, while more sediment moves further into natural settling ponds. Finally it gets to the ocean, which is about 40 to 50 kilometers away," he said. The sediment's journey is known as "source to sink," and every step along the way indicates a potential for toxic infiltration.
According to Heimsath, this erosion research is critical to understanding and preserving northern Australian economic and environmental health. An important export product for Australia and a vital source of income for some aborigines, uranium is used to generate nuclear power, and it's also valuable for producing radioisotopes, which have medical uses, metallurgical and engineering applications, and are used as food preservatives.
Heimsath's fieldwork in Nepal links local farming endeavors with his more modern research techniques. In the Himalayan region, known for its striking hills and mountains, landslides plague farmers. It was once thought that thousands of years of farming damaged the countryside.
"Now we can say with relative certainty that humans play a minor role in Himalayan erosion, and it's the natural background processes that are more significant. So, if we get a measurement of long-term erosion rates and compare them to short-term erosion rates from agricultural watersheds, then we can answer the question of 'What is the role of humans?' more definitively," said Heimsath.
These studies of erosion rates contribute to the "source to sink" initiative of the National Science Foundation. Heimsath's work in Australia is supported by an Australian Research Council Grant, and in Nepal, the National Science Foundation funds the research.
The following is from a conversation in August 2001. Professor Heimsath had just returned from his first field season in Australia.
My job is about how things are eroding blended with land management. In Australia, the setting is controversial because it concerns uranium mining from aboriginal land, as well as environmentally sensitive because we're concerned about contamination of the watershed. This area is gorgeous. It's stunningly beautiful. It's a sparsely populated part of Australia in the northern part of the Northern Territories. It has a tropical climate - monsoon season is roughly November though February; dry season is the rest of the year.
The landscape feels ancient. You walk on it, and you get the sense that this place has been around for awhile. The cliffs are stunning - sandstone cliffs that dramatically rise up out of the plain. These flood plains are dotted with palm trees and date trees and swamp grass; home to the notorious saltwater crocodile, which grows to 25-feet long and will chomp you out of your boat. The introduced water buffalo that came in from Indonesia with the old colonist have gone feral and into the swamp. This part of Australia has a very interesting tale of natural history.
On top of the cliffs and on the highlands up out of the swamp area is land dotted by scrub brush. It looks completely empty, but during the wet season, these exquisite waterfalls come off the dry brush land. So you've got monsoon season and dry season. During the rain, the place turns brilliant green, almost fluorescent, water spills out these cliffs and the red in the rock is dramatically brought out, and it's completely inaccessible because it's pretty much all underwater. All the dirt roads flood in the rain. So the only way to get in is by helicopter. During the dry, everything turns brown and parched, and the dirt roads are completely negotiable and accessible.
Also found here is one of the richest and largest uranium deposits in the world. This combination of ultra-high-tech modern uranium mines plomped down in this ancient landscape that's inhabited and owned and managed by the aboriginal people, who do still live in relatively non-modern conditions, is a fascinating backdrop to the science. Fortunately, land managers, the mine managers, and the government regulatory agencies are very conscience of the dangers of the radioactive tailings, which is the waste associated with the mine, and how it might get into the ecosystem. They are enthusiastic about my research. I'm enchanted and humbled by this region where I work.
|
