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Science for Watershed Decisions on Abandoned Mine Lands: Review of Preliminary Results, Denver, Colorado, February 4-5, 1998

Watershed Characterization from the Air: Application of Geophysical Techniques to Watershed Characterization in the Boulder River Watershed, Montana

By A.E. McCafferty¹ and B.D. Smith²

During December 1996 and October 1997, two airborne geophysical surveys were flown over parts of the High Ore, Cataract, and Basin Creek drainages. Magnetic and five frequencies of electromagnetic data were collected from a helicopter along closely spaced flight lines [200 meters (m)] at low altitude (60 m) above the ground surface. It is important to note that the first survey was flown at the beginning stages of the project so that geophysical maps and their derivative products could be used by other project scientists to place priorities for ground investigations during the first field season.

The primary objectives of the surveys were to (1) provide subsurface information on ground-water flow and physical-property patterns to aid in prediction of possible contaminant pathways to and from proposed repository sites; (2) characterize the conductivity and magnetization of shallow geologic units to aid in identification of metal-rich rocks contributing to the metal loads in the High Ore, Cataract, and Basin Creek drainages; and (3) provide site-specific information for remediation and/or risk-assessment issues.

Geologic units rich in metal-sulfide minerals can have distinct geophysical signatures. The magnetic and conductivity data were mathematically combined to infer locations of metal-bearing plutons and zones of hydrothermal alteration, both of which may be host to metal sulfides. The various combinations of magnetization and conductivity were tentatively ranked in terms of the potential to generate acid drainage. Areas are mapped where metals and acid runoff due to mining activity and natural processes may negatively affect surface water. For example, areas with magnetization domains with extremely low values associated with relatively high conductivity levels can indicate structures and point sources likely to produce contamination. Additionally, this model identifies areas that are covered by apparently nonacid-generating rock units and may reveal areas where metal-rich rocks are at depths shallow enough to possibly allow alteration, subsequent leaching and, therefore, transport of heavy metals to streams. Follow-up studies will involve field checking of anomalous areas and the integration of these results with geologic and hydrologic information to provide a clearer understanding of the possible environmental effects within the identified areas.

¹U.S. Geological Survey, MS 964, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (anne@usgs.gov)

²U.S. Geological Survey, MS 964, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (bsmith@usgs.gov)

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