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

Colloid Formation and Transport of Aluminum and Iron in the Animas River near Silverton, Colorado

By L.E. Schemel,¹ B.A. Kimball,² and K.E. Bencala³

Effects of inflow from two creeks on the transport of aluminum and iron in the Animas River near Silverton, Colorado, were studied during September 1996. Chemical compositions of both tributary creeks were influenced by remains of past mining activities and natural mineral deposits. Inflow from Cement Creek was low in pH (<4) with high (total) concentrations of aluminum and iron (5.3 and 7.5 milligrams per liter, respectively). Aluminum was dissolved in Cement Creek, but the increase in pH to about 7 upon mixing with the Animas River transformed the aluminum into colloidal particles. Iron was present both in dissolved and colloidal forms in the creek and in the river. Although discharge from Cement Creek accounted for only 21 percent of the flow of the Animas River downstream of the confluence, it contributed 95 percent of both the aluminum and iron. Inflow from Mineral Creek further increased transports of colloidal aluminum and dissolved and colloidal iron in the Animas River; however, the difference in pH between the two streams was small (<0.5 units) and no additional colloid formation was apparent. Inflow from Mineral Creek accounted for approximately 43 percent of the flow, 57 percent of the aluminum, and 55 percent of the iron in the Animas River below the confluence. Mass-balance calculations indicated that colloidal aluminum and iron were transported in this reach of the river without large losses by settling.

¹U.S. Geological Survey, MS 439, 345 Middlefield Road, Menlo Park, CA 94025 (lschemel@usgs.gov)

²U.S. Geological Survey, 1745 West 1700 South, Salt Lake City, UT 84104 (bkimball@usgs.gov)

³U.S. Geological Survey, MS 439, 345 Middlefield Road, Menlo Park, CA 94025 (kbencala@usgs.gov)

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