The U.S. Geological Survey Abandoned Mine Lands Initiative (USGS-AML) is focused on the evaluation of the effect of past mining practices on the water quality and the riparian and aquatic habitats of impacted stream reaches downstream from historic mining districts located primarily on federal lands. This problem is manifest in the eleven western states (i.e., west of 1020) where the majority of hardrock mines having past production are located on federal lands. Ferderer (1996) has developed a prioritization ranking of watersheds affected by historic mining activities on the basis of the number of past-producing metal mines per watershed. Using this mine census data as a first filter on the density of past-producing hardrock mines, he found that 18 watersheds contain more than 300 hardrock mines having past production, 20 watersheds contain 201-300 past producing hardrock mines, and 74 watersheds contain 101-200 past producing hardrock mines. In areas of temperate climate and moderate to heavy precipitation, the effect of rapid chemical and physical weathering of sulfides exposed on mine waste dumps and drainage from mines has resulted in elevated metal concentrations in the streams and bed sediments. The result of these processes is an unquantified impact on the quality of the water and the aquatic and riparian habitats that may limit their recreational resource value. One of the confounding factors in these studies is the determination of the component of metals derived from the hydrothermally altered but unmined portions of these drainage basins (Runnells and others, 1992). Several of these watersheds have been or are being actively studied to evaluate the effects of acid mine drainage (AMD) and acid rock drainage (ARD) on the near surface environment. The Animas River watershed in southwestern Colorado (fig. 1) contains a large number (>300) of past-producing metal mines that have affected the watershed. Beginning in October 1996, the USGS began a collaborative study of these impacts under the USGS-AML Initiative (Buxton and others, 1997).
The initial environmental characterization of the main tributary drainages in the Animas River watershed upstream from Silverton began in 1991 under the auspices of the Colorado Dept. of Public Health and Environment (J.R. Owen, written commun., 1997). These initial studies showed both seasonal and regional variability in surface water chemistry, and noted that, upstream from Silverton, many streams are generally acidic although pH is quite variable, depending on the geologic setting, precipitation, and amount of material that mining has exposed. Rocks in many tributary streams are coated with aluminum- and iron-hydroxides indicating the movement of metals under acidic conditions. Aquatic habitats in the Animas River above Bakers Bridge are known to be impaired.
The present study was funded under a cooperative program between the U.S. Geological Survey, the Bureau of Reclamation, the City of Durango, the Southern Ute Indian Tribe, and the Southwest Water Conservation District in October 1995. The Animas River Stakeholders Group (ARSG), a group of local concerned citizens in the basin from Silverton and Durango, the Colorado Dept. of Public Health and Environment, and the U.S. Environmental Protection Agency (EPA) have cooperated in providing data, consultation, and advice on local conditions within the watershed. Sunnyside Gold allowed access to their property to collect samples as did several property owners downstream from the Animas Canyon. The Southern Ute Indian Tribe provided guided access to our sampling sites on the Southern Ute Indian Reservation. This work was greatly enhanced by consultations with our colleagues on the project and individual members of the ARSG. W.G. Wright was particularly helpful in arranging field access and assisting S.E. Church in the field during October 1995.
Acidic metal-rich waters from mine drainage, waste rock, and mill tailings from inactive and abandoned mines, and naturally acidic waters from hydrothermally altered areas often enter stream reaches in upland watersheds and impact water quality. When these waters mix with less-acidic water and bed sediments from unmineralized areas, the physical and chemical conditions change during downstream transport. Metals are partitioned between water, colloids, and bed sediments through processes such as sorption, mineral precipitation, photoreduction, and biological interaction. Through these processes, all or portions of the metals are separated from water and are carried as suspended colloids or sediments, which ultimately settle to become a part of the bed sediment or become entrapped by microbes (algae). Thus, these metals may become available to aquatic organisms and larger forms of aquatic life in the food chain during transport downstream.
One of the most important geochemical processes as mine drainage waters mix with water downstream is the dilution and neutralization of the acid, and subsequent formation of iron colloids (Runnells and Rampe, 1989, Kimball and others, 1995). The iron colloids precipitate in the water where they form larger aggregates and settle from the water to the bed sediment, coating the rocks in the stream. Ochre-colored stream beds are characteristic of many mine-affected streams as well as mineralized areas that have not been disturbed by mining (Runnells and others, 1992). These iron colloids are solids with effective diameters from less than one nanometer (10-9 meters) when they first precipitate to greater than a few microns (10-6 meters) after they aggregate (Ranville and others, 1989 van Olphen, 1977). Hydrous iron-oxide colloids have extensive surface areas that sorb and enhance the partitioning of toxic metals (Jenne, 1977 Morel and Gschwend, 1987 Stumm and Morgan, 1996). The sub-micron particles have a strong tendency to aggregate which creates a continuum of particle sizes from less than 0.001 micrometers (mm) to greater than one mm (Buffle and Leppard, 1995a 1995b). These particles may be carried downstream as suspended colloidal particles. They also may precipitate on or bind to the surfaces of rocks and sand grains in the stream bed where they form a chemical sediment component of the bed sediments.
The objectives of this study in the Animas River watershed are:
1) to identify the source areas of large metal loads in the Animas River watershed,
2) to determine the extent and time frames of metal movement downstream in the Animas River basin,
3) to measure the dispersion of metals in the river sediments downstream from these major metal sources,
4) to determine the partitioning of metals among water, colloids, and bed sediments, and
5) to document the fractionation of the metals between the dissolved phase, a suspended colloidal phase, and bed sediment and coatings.