EXECUTIVE SUMMARY

• Reconnaissance geochemical data for water, colloid, and bed sediment samples, collected primarily during low flow in 1995, provide a synoptic view of the sources, movement and partitioning of metals in the Animas River watershed. The source of most of the iron, aluminum, copper, lead, zinc, arsenic, and cadmium in the Animas River is in the Animas River drainage basin upstream from site A-72 at Silverton, Colo.
• In the Animas River watershed upstream from Silverton, Cement Creek has the lowest pH (3.89) at low flow and carries most of its metal load in the aqueous phase. Mineral Creek has a pH of 6.35 at low flow and carries most of its metal load in the colloidal phase. Most of the metals in the Animas River above the confluence with Cement Creek reside in the colloidal component of the bed sediments. Downstream from the Animas River-Cement Creek confluence in the mixing zone, most metals from the dissolved load of Cement Creek are partitioned to the colloidal phase. Along upper Mineral Creek, and upstream from the Animas River-Cement Creek confluence, colloids aggregate, settle, and become an integral component of the bed sediments where they are stored until high-flow (snowmelt) runoff.
• At site A-72 during low flow, dissolved zinc and aluminum concentrations exceed the acute aquatic-life standard whereas the concentrations of the other metals do not exceed the chronic aquatic-life standard. Dissolved manganese concentrations are just below the chronic aquatic-life standard.
• Colloidal concentrations of copper, iron, zinc, and aluminum, if bioavailable upon ingestion as suggested by recent research, could exceed one or both of the aquatic-life standards at site A-72. There is a substantial loss of iron and aluminum colloids and their sorbed metal loads to the bed sediments in the Animas Canyon reach between Silverton and Bakers Bridge during low flow.
• More than 50 percent of the zinc in the bed sediments from Mineral Creek, 75 percent of the zinc in the bed sediments from Cement Creek, and 50 percent of the zinc in the bed sediments above the Animas River-Cement Creek confluence is in the form of sphalerite. Zinc concentrations in sphalerite from the bed sediments of the Animas River above the confluence with Cement Creek are about 1,000 ppm, from Cement Creek about 800 ppm, and from Mineral Creek about 400 ppm.
• Three water and colloid samples collected at high flow during snowmelt runoff (high flow) indicate that dissolved zinc concentrations far exceed the acute aquatic-life standard at site A-72 and are still twice that level as far downstream as Bondad. Metals stored in the colloidal phase of the bed load are moved downstream during high flow as is indicated by the high colloidal iron, aluminum, copper, lead, and zinc concentrations. Any remedial action that removes acidic metal-bearing discharges into Mineral and Cement Creeks and the upper Animas River or its tributaries will likely lessen the cumulative effect of metal toxicity on aquatic life.
• Lead-isotopic data from bed-sediment samples collected at low flow indicate that Mineral Creek contributes five to 10 percent of the total mass of metals in the bed sediments, Cement Creek contributes up to 40 percent of the metals in the bed sediments, and the bed sediments upstream from the Animas River-Cement Creek confluence contribute 50 to 60 percent of the metals at site A-72.
• Lead-isotopic data from bed-sediment samples collected downstream from site A-72 document the dilution of the metals derived from the Animas River Basin above Silverton by metal contributions from the tributaries. At Durango 75 km downstream, about 80 percent of the metals in the colloidal component of the bed sediments were derived from the Animas River watershed above Silverton whereas at Aztec, New Mexico, 57 percent of the metals were derived from this upstream source.
• No measurable contamination from the La Plata mining district at the headwaters of Junction Creek was found in the Animas River.
• Selenium concentrations in the bed sediments of the Animas River are elevated near the confluence of Lightner Creek in Durango, Colo. These elevated concentrations are related directly to the outcrop pattern of the Mancos Shale.
• Modeling of the sediment transport using the lead-isotopic data and the normalized labile copper, lead, and zinc data from bed sediments of the Animas River indicates some differential transport of the colloidal phase of the bed sediments. The chemical data from the waters and colloids indicate sorption of dissolved metals as a function of pH which changes with downstream distance where it is stored until remobilized during high-flow events. This colloidal component of the bed sediments dominates the bed sediment chemistry.
• Aggregation and transport of the colloidal component to the bed sediments reduces pore space between detrital grains in the bed, introduces toxic levels of metals into the food chain habitat for aquatic invertebrates, and if bioavailable to fish when ingested as suggested by recent research, may be a major cause of the deterioration of fish habitats in the stream bed.
• From lead-isotopic data, we have identified five or more discrete mineralization styles in the Animas River watershed above Silverton. Using these data, we can calculate the relative contribution of metals from each of these mineralization styles and the mines that exploited them to Mineral and Cement Creeks and the Animas River above its confluence with Cement Creek. The dominant lead-isotopic signature in the bed sediments of both Cement Creek and the upper Animas River is from the vein-type ore mineralization characteristic of the Eureka graben.
• Lead-isotopic data from stream sediments from Browns Gulch on the west side of the Mineral Creek drainage matches thevein-type ore in the Eureka district. These data suggest that vein-type ore from the Eureka district extends to the west of Cement Creek. The surface expression of this structure may be obscured by the hydrothermal alteration associated with Red Mountain No. 3.

INTRODUCTION

          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.

Summary of Acid-Mine Drainage Studies in other Areas

          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.

Study Objectives

          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.