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Importance and Causes of Daily Variation in Metal Concentrations in Streams

Fisher Creek in the New World mining district, Montana, near Yellowstone National Park. Fisher Creek is the site of a study of how chemistry changes as water moves downstream
Fisher Creek in the New World mining district, Montana, near Yellowstone National Park. Fisher Creek is the site of a study of how chemistry changes as water moves downstream
(Click on photo for larger version)

In 2003, USGS scientists found that concentrations of some metals dissolved in stream waters fluctuate significantly on a daily cycle, and that this phenomenon occurs consistently across the northern Rocky Mountains. This finding had profound implications for studies evaluating metals contamination in mountain streams, particularly contamination associated with metals mining, which is extensive across the Rocky Mountains. Since 2003, further research has explored the extent and possible causes of these concentration variations. For example:

  • All previous work on diurnal (daily) metal variations had been conducted during the summer, when working conditions are less difficult. The question then arose as to whether diurnal metal variations occur at other times. Repeated sampling at a site on Prickly Pear Creek, Montana, during a variety of seasonal conditions demonstrated that diurnal variations in metal concentrations occur year round and irrespective of the amount of flow in the stream.
  • Pioneering research in the laboratory and field by a colleague and a University of Wyoming Ph.D. candidate demonstrated that photosynthesizing organisms living on the streambed of High Ore Creek, Montana, can create diurnal metal variations very quickly over a short reach of stream.
  • Seasonal variation in dissolved Zn concentrations in Prickly Pear Creek near Helena, Montana, 2000-03. Data shown by blue squares were collected during winter low flow. Data in red are for high flows during spring runoff. Data in green are for summer low flow
    Seasonal variation in dissolved Zn concentrations in Prickly Pear Creek near Helena, Montana, 2000-03. Data shown by blue squares were collected during winter low flow. Data in red are for high flows during spring runoff. Data in green are for summer low flow
    (Click on photo for larger version)
    Sampling of three sites with very different pH conditions along Fisher Creek, Montana, demonstrated the downstream change in the nature of diurnal metal variations. In the headwaters, the stream is acidic (pH = 3.3) because it drains a partially mined mineral deposit. In this reach, iron is the only metal that exhibits diurnal variation. At the downstream site, the water is neutral (pH = 6.8), and metals such as copper, zinc, and manganese (but not iron) display diurnal variation.

These findings are important for designing field studies of metals contamination in mountain streams and for interpreting the processes that affect metals cycling in the environment.

References

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Created on October 28, 2005