U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting Charleston South Carolina March 8-12,1999--Volume 1 of 3--Contamination From Hard-Rock Mining, Water-Resources Investigation Report 99-4018A

Geomorphological Context of Metal-Laden Sediments in the Animas River Floodplain, Colorado

By Kirk R. Vincent, Stanley E. Church, and David L. Fey

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ABSTRACT

The watershed of the upper Animas River in the San Juan Mountains of Colorado was the site of extensive mining and ore milling during the late 19th and early 20th centuries. Using geologic mapping, stratigraphic and sedimentological studies of floodplain sediments, geochronology, historical records, and geochemical analysis of sediments, we conclude the following. Prior to mining, the river valley below the town and ore mill site of Eureka was composed of small, multi-thread, gravel bedded channels. These were located within a silty floodplain consisting of willow thickets and possibly intermittent and localized beaver ponds. A radical change in the stream and floodplain environment started sometime around the turn of the century and concluded with aggradation and burial of older sediments with sheets of gravel. This was caused by ore milling, not mining or other activities. Mills in and near Eureka supplied huge quantities of tailings to the river, at rates 50 to 4,700 times greater than the natural (pre-mining) production of sediment from hillslopes. Floodplain sediments have naturally high zinc concentrations of about 1000 parts per million, but ore milling resulted in an increase of zinc concentrations by as much as an order of magnitude. Using vanadium as a lithologic tracer for sediment derived from natural erosion of the watershed, we estimate that the fine fraction of streambed and floodplain sediments deposited after 1900 A.D. contain, in general, two-thirds tailings and one-third natural sediments.