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Scientific Investigations Report 2007-5246

Uranium in Surface Waters and Sediments Affected by Historical Mining in the Denver West 1:100,000 Quadrangle, Colorado

By Robert A. Zielinski, James K. Otton, R. Randall Schumann, and Laurie Wirt

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Scientific Investigations Report
2007-5246 PDF (9.91 MB)

Geochemical sampling of 82 stream waters and 87 stream sediments within mountainous areas immediately west of Denver, Colorado, was conducted by the U.S. Geological Survey in October 1994. The primary purpose was to evaluate regionally the effects of geology and past mining on the concentration and distribution of uranium. The study area contains uranium- and thorium-rich bedrock, numerous noneconomic occurrences of uranium minerals, and several uranium deposits of variable size and production history. During the sampling period, local streams had low discharge and were more susceptible to uranium-bearing acid drainage originating from historical mines of base- and precious-metal sulfides.

Results indicated that the spatial distribution of Precambrian granites and metamorphic rocks strongly influences the concentration of uranium in stream sediments. Within-stream transport increases the dispersion of uranium- and thorium rich mineral grains derived primarily from granitic source rocks. Dissolved uranium occurs predominantly as uranyl carbonate complexes, and concentrations ranged from less than 1 to 65 micrograms per liter. Most values were less than 5 micrograms per liter, which is less than the current drinking water standard of 30 micrograms per liter and much less than locally applied aquatic-life toxicity standards of several hundred micrograms per liter.

In local streams that are affected by uranium-bearing acid mine drainage, dissolved uranium is moderated by dilution and sorptive uptake by stream sediments. Sorbents include mineral alteration products and chemical precipitates of iron- and aluminum-oxyhydroxides, which form where acid drainage enters streams and is neutralized. Suspended uranium is relatively abundant in some stream segments affected by nearby acid drainage, which likely represents mobilization of these chemical precipitates. The 234U/238U activity ratio of acid drainage (0.95–1.0) is distinct from that of local surface waters (more than 1.05), and this distinctive isotopic composition may be preserved in iron-oxyhydroxide precipitates of acid drainage origin.

The study area includes a particularly large vein-type uranium deposit (Schwartzwalder mine) with past uranium production. Stream water and sediment collected downstream from the mine’s surface operations have locally anomalous concentrations of uranium. Fine-grained sediments downstream from the mine contain rare minute particles (10–20 micrometers) of uraninite, which is unstable in a stream environment and thus probably of recent origin related to mining. Additional rare particles of very fine grained (less than 5 micrometer) barite likely entered the stream as discharge from settling ponds in which barite precipitation was formerly used to scavenge dissolved radium from mine effluent.

Version 1.0

Posted May 2008

Suggested citation:

Zielinski, R.A., Otton, J.K., Schumann, R.R., and Wirt, Laurie, 2008, Uranium in surface waters and sediments affected by historical mining in the Denver West 1:100,000 quadrangle, Colorado: U.S. Geological Survey Scientific Investigations Report 2007–5246, 54 p.




Description of the Study Area

Uranium Mines and Mining History


Analytical Methods



Surface Water

General Chemical Character of Filtered Surface Water

Stream Discharge and Uranium Load

Aqueous Geochemistry of Uranium

Dissolved or Particulate Uranium

Spatial Distribution of Dissolved Uranium

Uranium Isotopes

Comparison of Stream-Water Concentrations with Water-Quality Standards

Stream Sediments

General Summary

Sediment Composition Variations Within the Main Stem of Clear Creek

Spatial Distribution of Uranium in Sediment

Sediment Concentrations and Sediment Quality Guidelines for Ecotoxicity

Fission-Track Radiography of Stream Sediments

Scanning Electron Microscope Observations

The Schwartzwalder Uranium Deposit

Description and Mining History

Previous Studies of Mining Effects

Mining Effects Determined in this Study



References Cited

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