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How Do Contaminants Mix at the Confluence of Streams

Acid mine drainage from Cement Creek (left) mixes with Animas River, Colorado (right)
Acid mine drainage from Cement Creek (left) mixes with the waters of the Animas River, Colorado (right) in 1996. Chemical reactions in this mixing zone affect the transport of contaminants down the Animas River.
Acid mine drainage from California Gulch (right) mixes with the Arkansas River, Colorado (left)
Acid mine drainage from California Gulch (right) mixes with the waters of the Arkansas River, Colorado (left). The chemical reactions that occurred as water from the Gulch mixed with the River's water controlled the quality of water downstream of the confluence.

Although the visual picture of mixing zones at stream confluences appears straightforward (see photos), the chemical reactions that occur make them extremely complex from a water-quality perspective. For example, chemical changes occur when a stream contaminated with acid mine drainage combines with a stream with near-neutral pH water; these reactions happen very rapidly and influence the subsequent transport of metals downstream of the mixing zone. U.S. Geological Survey scientists reported in the journal Hydrological Processes on a new method they developed to study the transport of contaminants, such as toxic trace metals, as they pass through stream mixing zones. The method involves injecting conservative chemical tracers in the stream and monitoring the concentrations of the injected tracers along with the concentrations of other dissolved chemicals downstream of the injection point. Monitoring data are then analyzed with a solute-transport model that allows the calculation of mixing ratios of the two streams, contaminant loading along the streams, and the extent that natural attenuation processes reduce the concentration of contaminants. The results generated by this method can be used to determine the fate of contaminants downstream of areas affected by acid mine drainage and other similar types of contamination.

Cement Creek, Colorado
Cement Creek, Colorado, is the site of USGS investigations of the fate of acid mine drainage.


Cox, M.H., and Schemel, L.E., 2007, Chemical and hydrologic data from the Cement Creek and upper Animas River confluence and mixing zone, Silverton, Colorado, September 1997: U.S. Geological Survey Open-File Report 2007-1048, 4 p (includes a data file).

Schemel, L.E., and Cox, M.H., 2005, Descriptions of the Animas River-Cement Creek confluence and mixing zone near Silverton, Colorado, during the late summers of 1996 and 1997: U.S. Geological Survey Open-File Report 2005-1064, 4 p (Online only).

Schemel, L.E., Cox, M.H., Runkel, R.L., and Kimball, B.A., 2006, Multiple injected and natural conservative tracers quantify mixing in a stream confluence affected by acid mine drainage near Silverton, Colorado: Hydrological Processes, v. 20, no. 13, p. 2727-2743, doi:10.1002/hyp.6081.

Schemel, L.E., Kimball, B.A., Runkel, R.L., and Cox, M.H., 2007, Formation of mixed Al-Fe colloidal sorbent and dissolved-colloidal partitioning of Cu and Zn in the Cement Creek-Animas River confluence, Silverton, Colorado: Applied Geochemistry, v. 22, no. 7, p. 1467-1484, doi:10.1016/j.apgeochem.2007.02.010.

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