Naturally occurring radiogenic isotopes make effective groundwater flow tracers

December 13, 2000

Researchers can detect groundwater flow paths by using naturally occuring isotopes in the water as flow tracers.

A collaboration of researchers successfully identified regional groundwater flow paths in the Eastern Snake River Plain aquifer by analyzing radiogenic isotope ratios. The research team studied changes in the naturally occurring isotopes strontium-87 and strontium-86--variations in the mass of the element strontium. The team included researchers from the University of Illinois at Urbana-Champaign, Idaho National Engineering and Environmental Laboratory, Los Alamos National Laboratory, Lawrence Berkeley National Laboratory, and the U.S. Geological Survey. They report their results in the October issue of Geology.

Groundwater is more than just H20--it also contains trace amounts of many other elements such as potassium and sodium, chlorine and iodine, and even naturally occurring radiogenic isotopes such as strontium-87. "Radiogenic isotopes are the non-radioactive products of natural radioactive decay and are found in all rocks and water," said University of Illinois geologist Tom Johnson. The combinations and ratios of such isotopes found in groundwater can be as distinctive and traceable as a person's fingerprint.

Researchers can use that fingerprint to figure out where water originated and where it has traveled, but it's complicated because groundwater is constantly evolving. For years, researchers have analyzed the overall abundance of dissolved elements in the water to learn this information. However, radiogenic isotopes may provide a simpler story to read because their evolution is less complex. As water flows through rock underground, it dissolves the rock over time. New minerals are precipitated and the ratio of strontium isotopes in the groundwater evolves towards the ratio in surrounding rock, giving researchers a surprisingly clear picture of how quickly water moves in a specific area.

Lead researcher Tom Johnson describes this work as a proof of this concept. "Researchers have been measuring radiogenic isotopes in groundwater for the last ten years or so, but this is one of the first studies using that information to define fast and slow flow paths," he says.

Using the concentrations of dissolved elements in water for this purpose is much more difficult. They are dissolved, precipitated, or adsorbed as the water interacts with the surrounding rock, soil, and microbial populations. The resulting changes do give researchers information about flow direction and speed; but the water fingerprint can get so smeared and distorted that the meaning of the data is difficult to decipher.

Scientists already know a great deal about the Eastern Snake River Plain aquifer flowing under the INEEL. The aquifer rocks are primarily basalts with a limited range of radiogenic isotope ratios--making the area an ideal location for this research. Some of the source recharge water--water that replenishes the aquifer--in part comes from Birch Creek and the Little Lost River. The recharge water has a higher strontium-87/86 isotope ratio than the aquifer itself. This contrast allows researchers to track the flow of recharge waters through the main aquifer.

"This is quite similar to tests commonly used in aquifer studies where artificial tracers are injected and their behavior observed," said LANL geochemist Robert Roback. "The main difference, of course, is that we observe nature's tracers. Using these natural tracers is a great advantage because they allow us to evaluate much larger geographic regions over longer time periods than are possible using artificial tracer tests."

The research team sampled wells over a three-year period and compared their data with known groundwater flow directions, looking for spatial patterns that would indicate faster or slower areas of flow. They found that faster traveling water retains the higher ratio of strontium of the source water for longer flow distances. Water that moves slowly has prolonged contact with surrounding rocks, and the strontium ratios are about the same as those found in the aquifer rocks.

The fast flow zones are aligned with the sediment filled valleys north of the INEEL. "Groundwater resulting from infiltration snow and rainfall in the mountains north of the Eastern Snake River Plain flows down these valleys providing significant recharge to the aquifer beneath the INEEL," explains INEEL geochemist Travis McLing. "Think of the faster flow paths in the aquifer as extensions of those valleys north of the plain." Slow flow zones are located at the toes of the Lost River and Lemhi mountain ranges.

The ability to locate different flow zones is important to the Department of Energy's environmental remediation mission. Fast flow paths are a big concern for researchers trying to create models that predict underground fluid flow. The features that control flow rates--fractures in basalt, differences in sediment properties between basalt flows, aquifer thickness--are subtle and difficult to detect. "This aquifer is literally a maze of fractures," said University of Illlinois' Johnson.

In their paper, the research team contends that radiogenic element analyses could simplify groundwater flow interpretations and should be routinely performed in groundwater studies.
-end-
This work was supported by the DOE's Office of Basic Energy Sciences through the Wolf Broido Program. Funding for some of the collaborative work was supplied by the DOE's Environmental Management Science Program.

The INEEL is a science-based, applied engineering national laboratory dedicated to supporting the U.S. Department of Energy's missions in environmental, energy, science and national defense. The INEEL is operated for the DOE by Bechtel BWXT Idaho, LLC, in partnership with the Inland Northwest Research Alliance.

Technical contact: Tom Johnson at the University of Illinois, (217) 244-2002, or tmjohnso@hercules.geology.uiuc.edu, or Travis McLing at the INEEL, (208) 526-7269, or tm1@inel.gov.

Media contact: Deborah Hill, (208) 526-4723 or dahill@inel.gov; or Mary Beckman, (208) 526-0061, or beckmt@inel.gov; or James Kloeppel, University of Illinois, (217) 244-1073, or kloeppel@uiuc.edu.

Visit our web site: http://www.inel.gov

Note to editors: The research paper titled Groundwater "fast paths" in the Eastern Snake River Plain aquifer: Radiogenic isotope ratios as natural groundwater tracers, was published in the journal Geology, October 2000, volume 28, no. 10, Pages 871-874.

DOE/Idaho National Laboratory

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