Uranium-Series Isotopes as an Environmental Tracer

Laurie Wirt

Sadly, Laurie passed away on June 26, 2006 following a kayaking accident. Her project work to that date is briefly discussed on this web page. For information concerning this work, please contact Ray Johnson, rhjohnso@usgs.gov.

Introduction

Until recently, ²³⁴U/²³⁸U activity ratios could only be used as an environmental tracer in areas with high-level activities, such as uranium mines and radioactive waste sites. Improved resolution in the analytical precision of uranium-series isotopes by ICP-MS and TIMS has tremendous potential to greatly expand applications where low-concentrations of uranium-series isotopes (²³⁴U/²³⁸U) can be used as environmental tracers and as a geochronology tool. New applications for uranium-series isotopes as an environmental tracer in a variety of geologic settings were investigated. This work was done in collaboration with Mike Ketterer (Northern Arizona University) and Jim Paces (USGS).

Ketterer, M.E., and Khourey, C.J., 1998, High precision determination of ²³⁴U/²³⁸U activity ratios in natural waters and carbonates by ICPMS, in Morrow, R.W., and Crain, J.S., eds., Applications of inductively coupled plasma-mass spectrometry to radionuclide determinations, Vol. 2, ASTM STP 1344: American Society for Testing and Materials.

Paces, J.B., Ludwig, K.R., Peterman, Z.E., Neymark, L.A., and Keneally, J.M., 1998, Anomalous groundwater ²³⁴U/²³⁸U beneath Yucca Mountain-evidence for local recharge?, in High-level radioactive waste management, Proceedings of the Eighth International Conference, Las Vegas, Nevada, May 11-14, 1998: La Grange Park, Illinois, American Nuclear Society, p. 185-188.

Objectives

The objectives of this investigation were to (1) measure U-series isotopes in a variety of natural settings and mine sites with a wide range of acid to neutral conditions, and (2) use ²³⁴U/²³⁸U to determine geologic sources for ground water traveling through a variety of different rock types. This effort complemented work on other aqueous geochemical investigations.

Study Sites

Leavenworth Gulch, North Fork Clear Creek watershed, Colorado, an area affected by pitchblende deposits. Monthly (collected from May through October, 2003) stream-water samples and stream-sediment leachates (Denver West Quad) were collected and analyzed to examine seasonal variability of stream flow down gradient from mined uranium deposit.
Hopi Indian Reservation, Hopi Buttes (Colorado Plateau), northeastern Arizona, where uranium deposits are present as breccia-pipe deposits. Archived ground-water samples were analyzed (collected by George Breit, USGS).
Verde River headwaters in north central Arizona (transition zone between Basin and Range and Colorado Plateau), where uranium tends to be elevated along deep flow paths associated with basin-bounding faults. Archived ground-water samples were examined. The ground water is in contact with limestone, igneous, and metamorphic rocks, and basin-fill sediments that are a composite of surrounding mountains. This work aided in determining sources of ground water to alluvial aquifers in the Verde River headwaters, Arizona.
USGS Denver West Quad Project, Colorado. Archived water and stream sediments (from Bob Zielinski, USGS) were analyzed.
Ground waters from Eastern Sardinia (Italy) were examined in collaboration with Mario Lorrai, University of Cagliari, Italy. These waters are in contact with uranium-bearing igneous and metamorphic rocks.

Highlights

Distinctive ²³⁴U/²³⁸U activity ratios (ARs) are a useful environmental tracer of sources of ground water to discharge springs from two basin-fill aquifers and the regional Paleozoic limestone aquifer at the headwaters of the Verde River. The marine carbonate (Paleozoic aquifer) had the lowest uranium concentrations (0.6 to 0.8 µg/L) and highest ARs (4.0 to 9.2), suggestive of slow leaching of uranium over long time periods and preferential dissolution of ²³⁴U by decay-related alpha recoil. In contrast, the basin-fill aquifers are characterized by relatively constant ARs of 3.0 plus-or-minus 0.3 and uranium concentrations greater than 1.0 µg/L (Wirt and Ketterer, 2003).

Related Work

In a collaborative project with the National Park Service, the USGS Crustal Imaging and Characterization Team is using expertise in geology, geochemistry, and geophysics to help understand the source of ground water to Montezuma Well in Central Arizona. Montezuma Well is a natural spring which is part of the Montezuma Castle National Monument. The geology in the area indicates ground water flow through a series of lithologies, including basalts, sandstones, and limestone. Details on the structural features and locations of basalt flows and dikes are provided by well logs and geophysics (gravity anomalies). In addition, rock/water interaction with the basalts provide a unique signature of high strontium concentrations with low strontium isotope ratios. The strontium isotope ratios are being measured using the USGS Mineral Resources Program multicollector ICP-MS. Results to date indicate shallow and deep ground-water flow paths. Ground water in Montezuma Well follows a deeper flow path which results in high concentrations of carbon dioxide and arsenic with strontium isotope ratios that indicate contact with both the basalt and the underlying sandstones and limestones. The shallow ground water does not have elevated concentrations of carbon dioxide and arsenic, but does have high strontium concentrations and low strontium isotope ratios that are typical of shallow ground-water flow through the near surface basalt flows. This information is being provided to the National Park Service as part of their water rights efforts in protecting the flow to Montezuma Well for future generations.

Products

USGS Reports

Wirt, Laurie, DeWitt, Ed, and Langenheim, V.E., eds., 2004, Geologic framework of aquifer units and ground-water flow paths, Verde River headwaters, north-central Arizona: USGS Open-File Report 2004-1411. Available online at http://pubs.usgs.gov/of/2004/1411/.

Abstracts, Presentations, and Posters for Professional Technical Meetings

Wirt, Laurie, and Ketterer, M.E., 2003, ²³⁴U/²³⁸U disequilibrium as an environmental tracer of ground-water discharge to springs in the upper Verde River headwaters region, North Central Arizona [abs.]: Geological Society of America 2003 Annual Meeting, Seattle, Washington, November 2-5, 2003, Abstracts with Programs, v. 35, no. 6, p. 200. (Abstract and presentation) Available online at http://gsa.confex.com/gsa/2003AM/finalprogram/abstract_67352.htm.

Contact Information

Ray Johnson
Box 25046 Denver Federal Center MS 973
Denver, CO 80225-0046
Phone: (303) 236-1885
Email: rhjohnso

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