Summary and Conclusions from 

Investigation of the Effects of Historical 

Mining in the Animas River Watershed, 

San Juan County, Colorado

By U.S. Geological Survey

Chapter A of

Integrated Investigations of Environmental Effects of Historical 

Mining in the Animas River Watershed, San Juan County, Colorado

Edited by Stanley E. Church, Paul von Guerard, and Susan E. Finger

Professional Paper 1651

U.S. Department of the Interior

U.S. Geological Survey
Contents

Introduction.....................................................................................................................................................3

Study Area.......................................................................................................................................................4

Environmental Effects ...................................................................................................................................6

Sources of Trace Elements ..........................................................................................................................6

Premining Geochemical Studies .................................................................................................................7

Watershed Conditions (1996–2000) ...........................................................................................................10

Potential for Restoration of Functional Aquatic Habitat .......................................................................13

References Cited..........................................................................................................................................16

Figures

 1. Index map of western United States showing the two watersheds studied 

during the U.S. Geological Survey Abandoned Mine Lands Initiative .................................4

 2. Photograph of Animas River downstream of Silverton ..........................................................5

 3. Map showing distribution of zinc in surface water, low flow ...............................................8

 4. Photograph of mixing zone looking downstream from confluence of Cement 

Creek with Animas River .............................................................................................................9

 5. Photograph of upper Cement Creek just south of Gladstone showing iron 

colloids coating streambed, looking south across Cement Creek .....................................10

 6. Maps showing zinc concentrations in premining sediment and in modern 

sediment .......................................................................................................................................11

7. Zinc toxicity map showing categories of risk to aquatic life ...............................................12

 8. Map showing sites where remediation work has been done in the study area ..............14

 9. Photograph of Silver Wing mine, a privately held, inactive mine site on east 

side of Animas River ...................................................................................................................15
Introduction

The Animas River watershed is one of many watersheds 

in the western United States where historical mining has left 

a legacy of acid mine drainage and elevated concentrations of 

potentially toxic trace elements. Many abandoned mine lands 

are located on or directly affect Federal land. Cleaning up 

these Federal lands and restoring these watersheds will require 

a substantial investment of resources and many years of work. 

As part of a cooperative effort with Federal land-management 

agencies, the U.S. Geological Survey implemented an 

Abandoned Mine Lands Initiative in 1997. The two watersheds 

studied under the initiative are the Animas River watershed in 

Colorado and the Boulder River watershed in Montana (fig. 1). 

The goal of the initiative was to use the watershed approach 

to develop a strategy for gathering and communicating the 

scientific information needed to formulate effective and costefficient remediation of affected lands in a watershed. The 

major premise of the watershed approach is that contaminated 

sites having the most profound effect on water and ecosystem quality within an entire watershed should be identified, 

characterized, and ranked for remediation (Buxton and others, 

1997). Remediation of watersheds affected by historical mining 

should be done in phases and should be followed by sufficient 

monitoring to document the improvement of the aquatic and 

riparian habitat.

The watershed approach provides an effective means 

to evaluate the overall status of the Nation’s natural resources 

and to communicate these results to the public. Land- and 

resource-management agencies are faced with evaluating 

risks associated with thousands of historical mine sites in 

many watersheds on Federal lands, particularly in the western United States. The level of scientific study conducted 

for the Animas River watershed will not be feasible for 

every watershed affected by historical mining; however, 

the detailed scientific studies described herein can help 

Chapter A

Summary and Conclusions from Investigation 

of the Effects of Historical Mining in the Animas 

River Watershed, San Juan County, Colorado

By U.S. Geological Survey

Federal land-management agencies decide which characterization efforts would be most useful in evaluating other 

such watersheds. The watershed approach helps to focus 

remediation activities at sites where restoration will have 

the most overall benefit. We implemented the following in 

this investigation:

• During this study, the watershed approach resulted in 

the collection of extensive information on the geology 

and geochemistry of rock and sediment, the hydrology and water chemistry of streams and ground water, 

and the diversity and health of aquatic and terrestrial 

organisms.

• We inventoried historical mines and characterized 

draining adits and mine and mill wastes.

• We defined geologic conditions that control acidity 

and release of potentially toxic trace elements.

• We collected and chemically analyzed hundreds 

of water, rock, sediment, and mine- and mill-waste 

samples.

• We conducted toxicity tests, assessed fish distribution 

and habitat, and analyzed fish tissue and biofilm.

• We examined benthic macroinvertebrates and mapped 

their distribution to evaluate ecosystem health.

• We defined hydrological regimes, measured chemical 

and physical parameters to define conditions of streamflow, and evaluated plausible sources of trace elements 

to streams.

• We provide these data, along with the GIS coverages developed during the study, in the database on 

the enclosed CD-ROM (Sole and others, this volume, 

Chapter G).
Study Area

The Animas River watershed study area is the northernmost headwaters of the Animas River watershed in San Juan 

County, Colo. (fig. 1). It encompasses the drainage basins of 

the Animas River at and upstream from Silverton, Colo., its 

two main tributaries, Cement and Mineral Creeks, and a short 

reach of the Animas River downstream from the confluence 

with Mineral Creek (fig. 2). Gold was first discovered in the 

study area at the site of the Little Giant mine in Arrastra Creek 

in 1871, although the presence of an arrastre in Arrastra Creek 

indicates that Spanish explorers found some free-milling gold 

at this site more than a century earlier. Mineral exploration did 

not begin until after the signing of the Bernot Treaty with the 

Ute Tribe in 1873. Franklin Rhonda, a topographer with the 

1874 Hayden Survey, worked in the study area within a year 

after the establishment of the town of Silverton and described 

the water in both Cement and Mineral Creeks as iron sulfate 

waters that were undrinkable (Rhonda, 1876). Large quartz 

veins containing sulfide minerals are exposed at the surface in 

the study area. Studies of the complex geology and subsequent 

mineralization have been conducted in the area for more than a 

century (for example, Ransome, 1901; Burbank, 1933; Varnes, 

1963; Burbank and Luedke, 1968, 1969; Lipman, 1976; 

Lipman and others, 1976; Casadevall and Ohmoto, 1977; 

Bove and others, 2001). The terrain is rugged: the elevation 

of the town of Silverton is 9,305 ft, and some of the mountain 

peaks in the headwaters rise to more than 13,800 ft (fig. 2). 

Elevations of the three Red Mountains shown on the cover 

photograph range between 12,219 and 12,747 ft and form the 

northwest boundary of the study area at the headwaters of 

Mineral Creek.

The findings from studies that characterized the geologic 

setting, defined the structural, mineralogical, and hydrothermal characteristics of the mineral deposits, and summarized 

the mining history of the watershed are as follows:

• Much of the study area is in a late Oligocene volcanic 

center that erupted many cubic miles of lava and volcanic tuff and formed the Silverton caldera. The study 

area was subsequently extensively mineralized (Lipman, 

1976; Lipman and others, 1976; Bove and others, 2001; 

Yager and Bove, this volume, Chapter E1, pl. 1).

• Multiple episodes of hydrothermal activity produced 

widespread areas of alteration and caused extensive mineralization following the formation of the 

Silverton caldera (Bove and others, 2001). Previous 

geologic mapping of the study area, compiled by 

Yager and Bove (this volume, pl. 1), provides a geologic framework for subsequent interpretation of study 

results.

• Bove and others (this volume, Chapter E3) mapped 

zones of hydrothermal alteration using the data 

acquired by the Airborne Visible and Infrared Imaging 

Spectrometry (AVIRIS) instrument (Dalton and others, 

this volume, Chapter E2). Field testing of AVIRIS 

results and verification of the hydrothermal alteration 

suites by detailed field mapping and X-ray defraction 

results were used to develop the final maps. These field 

tests were used to refine the identification of hydrothermal mineral suites identified using the computer 

algorithms developed for AVIRIS data reduction 

(Clark and others, 2003; Dalton and others, 2004). As 

a result, AVIRIS spectroscopy has been shown to be a 

cost-effective tool for mapping hydrothermal mineral 

assemblages at the watershed scale.

Figure 1. Index map of the western United States showing 

the two watersheds studied during the U.S. Geological Survey 

Abandoned Mine Lands Initiative. Enlarged view shows Animas 

River watershed in southwestern Colorado.

0 5 10 15 20 25 MILES

Durango

Silverton

Ignacio

Animas River

watershed

study area

SAN JUAN

LA PLATA

Boulder

River

watershed

Animas

River

watershed

in Colorado

WA

OR

ID

WY

MT

NV

CA

UT CO

AZ

NM

4 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Summary and Conclusions 5

• Five different hydrothermal alteration assemblages 

were identified and the different water chemistry from 

each was characterized (Bove and others, this volume). A substantial component of the elevated metal 

concentrations and acidity in water can be attributed 

to weathering of hydrothermally altered rock. For 

example, the Red Mountains area (cover photograph) 

was heavily altered and mineralized, and the soil 

and surface water are so acidic that vegetation in the 

hydrothermally altered zones is sparse below tree line 

(Dalton and others, this volume; Mast and others, this 

volume, Chapter E7).

• Airborne magnetic and electromagnetic surveys 

show the crustal structure at three different depths 

(Smith and others, this volume, Chapter E4, pls. 3 

and 4). Buried plutons, mineralized structures, and 

fractures carrying conductive ground water were 

identified in the subsurface by use of geophysical data. 

These fractured aquifers have important implications 

for future remediation activities in the Animas River 

watershed.

• Magnetic boundaries and changes in electrical conductivity are associated with faults, veins, and buried 

plutons. Detailed analysis of the geophysical signatures 

of geologic structures by McDougal and others (this 

volume, Chapter E13) shows that a significant number 

of the veins and structures identified by the geophysical 

data (36 percent) have not been evaluated for mineral 

resources.

• Predictive models (McDougal and others, this volume) 

utilize the geophysical data to infer plausible groundwater flow paths or barriers that need to be incorporated into any proposed remediation plans.

• Silver, lead, free-milling gold, and later zinc and copper were produced from more than 300 mines in the 

study area (Church, Mast, and others, this volume, 

Chapter E5). Beginning about 1890, mineral production in the study area averaged about 200,000 short 

tons of ore per year for a century (Jones, this volume, 

Chapter C). The Sunnyside mine, the last operating 

mine in San Juan County, closed in 1991. Total production of mines in the Animas River watershed study area 

between 1871 and 1991 is estimated at 18.1 million 

short tons of ore.

Many of these historical mines are on public land, administered by both the U.S. Bureau of Land Management (BLM) 

in the Department of the Interior and the U.S. Department of 

Agriculture (USDA) Forest Service, and they have been abandoned by those who originally worked the properties. Others 

are on patented claims and are owned by private citizens or are 

orphaned properties held by San Juan County for delinquent 

taxes. Remediation objectives for the watershed face a number 

Figure 2. Animas River downstream of Silverton taken from U.S. Highway 550 looking north-northeast, 

Sept. 26, 1997. Confluence of Mineral Creek (left foreground) with Animas River (far right) is in foreground. 

Town of Silverton sits on flood plain area known as Bakers Park downstream from Cement Creek, which 

flows through Silverton from left background and enters Animas River upstream of Mineral Creek 

confluence. Rugged mountainous terrain in background typifies majority of the Animas River watershed 

study area.
of difficult challenges: issues of land ownership and financial 

liability, technical approaches needed to mitigate acid mine 

drainage without resorting to costly treatment facilities, and 

the need for adequate space to build new repositories to move 

those mine wastes that cannot be mitigated in place are but a 

few. These issues must be successfully addressed by consensus 

in the Silverton community, where much of the local economy 

for decades has been built around tourism in a historical mining district.

Environmental Effects

Historical mining began in an era of little concern for 

or understanding of the effects of mining on the environment. 

Development of our abundant mineral resources was one of 

the public policies used by Federal Government to populate 

the West following the Civil War. The lure of adventure and the 

promise of wealth were the motivating factors to entice people 

to emigrate west and settle the frontier. Following the initial 

phase of development and the completion of the Silverton 

narrow-gauge railroad in July 1882, mining in the basin 

progressively became more mechanized as milling technology rapidly improved. Mill waste was dumped in the lakes, 

rivers, and streams until about 1935, when the waste from the 

Mayflower Mill was retained. Once milled, most sulfide concentrates were shipped to Durango, Colo., for smelting (Sloan 

and Skowronski, 1975). Of the 18.1 million short tons of ore 

produced in the basin, an estimated 8.6 million short tons of 

mill waste (about 48 percent) was discharged directly into surface streams prior to about 1935 (Jones, this volume).

Determining the effects of historical mining in the Animas 

River watershed study area necessarily involved many detailed 

and specific scientific studies to quantify aspects of the environmental effects attributable to historical mining and milling. 

Individual chapters in this volume are briefly summarized in 

von Guerard and others (this volume, Chapter B) so that the 

reader may select those studies that address the problem of 

concern or specific area of interest. The basic findings in this 

report are listed under four broad headings.

Sources of Trace Elements

Several sources of trace elements and acidity affect 

surface streams in the watershed study area. Of the greatest 

concern are those that can be attributed directly to historical 

mining and that can and perhaps should be addressed through 

some remedial action.

• Studies of various aspects of water quality in the 

watershed (1996–2000) showed that historical mining has resulted in water-quality degradation (Kimball 

and others, this volume, Chapter E9; Wright, Simon, 

and others, this volume, Chapter E10; Walton-Day 

and others, this volume, Chapter E24).

• Sources of contaminants, both anthropogenic and 

natural or undisturbed, were detected by the numerous 

tracer studies completed along many of the reaches 

of the major surface streams and some tributaries. 

Much of the metal loading, which is summarized in 

Kimball and others (this volume), can be attributed 

to 24 specific areas in the watershed, although numerous other sources were identified. Major structures, 

hydrothermally altered areas, and major draining adits 

are among these sources.

• Numerous mine sites, both on public and on private 

land, have been identified as sources of metals and 

acidity from these tracer studies (Kimball and others, 

this volume). Metal loading from mine sites is summarized by Mast and others (this volume).

• Mine sites and mill-tailings piles are obvious candidates 

for sources of potentially toxic trace elements and acidity. Nash and Fey (this volume, Chapter E6) studied 

and ranked more than 100 mine and mill sites located 

on public lands. Of these, about 40 mine and mill sites 

were found to be of sufficient size, to have enough 

adit discharge, and (or) to have sufficient mine waste 

to be considered significant sources of trace elements 

and acidity at the watershed scale. Mines located on 

private land were not considered or ranked in the study 

by Nash and Fey (this volume) because access to these 

sites generally was not granted by the property owners.

• Most large mine sites in the study area are located on 

private land or on land with mixed private/Federal ownership. Data from mine and mill sites located on private 

land were provided by State agencies and a local citizens group, the Animas River Stakeholders Group, and 

are discussed and separately ranked in Wright, Simon, 

and others (this volume).

• Water-quality measurements from 75 inactive historical 

mines indicated that most mine water was dominated 

by calcium sulfate, had variable pH ranging from 2.35 

to 7.8, and had highly variable chemistry (Mast and 

others, this volume; Nash and Fey, this volume).

• The fact that water chemistry from flowing adits has an 

essentially constant chemical makeup throughout the 

years of our study demonstrated that adit-water chemistry is not diluted by ground water coming into a mine 

pool during spring runoff, but rather that a large supply 

of water-soluble salts exists in the mines, and that these 

salts dissolve and saturate the fresh water added by 

infiltration by snowmelt each year. Thus, mine-adit flow 

is a constant source of contaminants and acidity to surface streams (Church, Mast, and others, this volume).

• Isotopic studies of sulfate in adit water demonstrated 

that both oxidation of pyrite and dissolution of gypsum, anhydrite, and calcite control the sulfur and oxygen systematics of mine water (Nordstrom and others, 

this volume, Chapter E8).

6 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Summary and Conclusions 7

• Accelerated weathering of the large volumes of freshly 

crushed rock present in mine-waste and mill-tailings 

sites (Church, Mast, and others, this volume; Fey and 

others, 2000) has resulted in the release of elevated 

concentrations of cadmium, copper, and zinc into surface streams (Kimball and others, this volume; Wright, 

Simon, and others, this volume; Mast and others, this 

volume).

• Not all sources of metals and acidity in the study 

area are anthropogenic. Weathering of hydrothermally altered rock not associated with historical mine 

sites also results in the release of trace elements and 

acidity that degrade water quality (Bove and others, 

this volume). The annual freeze-thaw cycle each 

winter exposes fresh mineral surfaces to the weathering process. Weathering of pyrite, the ubiquitous iron 

sulfide present in hydrothermally altered and mineralized rock, results in the release of sulfuric acid and trace 

elements that potentially are toxic to aquatic life. This 

weathering process takes place whether or not a mineral 

deposit has been mined or the ground disturbed.

• Values of pH in surface streams today at low flow range 

from less than 3.5 to near neutral. Trace elements are in 

solution at low pH values, but as pH increases, different 

trace elements precipitate from solution and are sequestered by the iron-oxyhydroxide colloidal fraction. For 

example, a map of the concentrations of zinc in surface 

water at low flow (fig. 3) shows high dissolved zinc 

concentrations in some stream reaches.

• Trace elements are mobilized and acidity introduced 

into streams by water flowing through mine workings (Bove and others, this volume; Mast and others, 

this volume; Church, Mast, and others, this volume), 

across mine-waste dumps (Nash and Fey, this volume; 

Kimball and others, this volume), and over and through 

mill tailings disposed of on the flood plain (Vincent 

and Elliott, this volume, Chapter E22) and in stream 

reaches (Kimball and others, this volume; Wright, 

Simon, and others, this volume).

• Water most affected by historical mining will have a pH 

<3 with elevated concentrations of iron, aluminum, cadmium, copper, lead, zinc, arsenic, and nickel. Using the 

dissolved sum-of-metals as a measure of trace-element 

contamination from historical mining sites, Wirt and 

others (this volume, Chapter E17) showed that the 

dissolved sum-of-metals exceeded 1,200 μg/L in the 

Prospect Gulch subbasin on upper Cement Creek 

(fig. 3).

• Trace elements are partitioned between the dissolved 

and the colloidal phase in surface streams. Transport 

of colloids (fig. 4) results in variable concentrations of 

metals in the dissolved and suspended phases during 

different seasons of the year (Church and others, 1997; 

Fey and others, 2002). Settling of colloids onto the 

streambed is one mechanism of natural attenuation that 

caused changes in the distribution and bioavailability 

of some, but not all metals to aquatic biota (Kimball 

and others, this volume; Besser and Brumbaugh, this 

volume, Chapter E18, Besser and others, this volume, 

Chapter D).

Premining Geochemical Studies

One of the fundamental questions in the study of the 

effects of historical mines on the environment is always: What 

were conditions prior to mining? We specifically focused efforts 

on this question to provide data that could be used to establish 

reasonable remediation target concentrations for potentially 

toxic trace elements.

• Water quality in undisturbed background sites in the 

study area is controlled by the hydrothermal alteration 

assemblages (fig. 3; Bove and others, this volume; 

Mast and others, this volume; Kimball and others, this 

volume).

• Detailed statistical, mass-balance, and inverse modeling approaches were used to determine the percentage 

of potentially toxic trace-element loads that could be 

attributed to weathering of undisturbed areas within 

discrete subbasins (Bove and others, this volume). 

These studies had mixed results, and interpretation 

of the data was complicated by the geologic complexity of the area, density of historical mines and mills, 

distribution of mill tailings, and uncertainty about the 

ground-water contributions.

• Detailed studies of ferricrete distribution (Yager and 

Bove, this volume, pl. 2), classification, and age 

constraints (Verplanck and others, this volume, 

Chapter E15) show that ferricrete deposits mark areas 

of emergence of the paleo ground-water table (Wirt 

and others, this volume). Ferricrete deposits are spatially distributed down slope from areas of extensive 

pyrite-rich hydrothermal alteration. Active ferricrete 

deposition occurs down gradient of changes in slope 

that force low-pH ground water to the surface in sedge 

grass marshes and iron bogs (fig. 5).

• The presence of schwertmannite (a hydrous iron 

sulfate) in iron deposits in iron bogs and ferricrete 

deposits can be used to constrain the water chemistry 

of paleo ground water to a pH range of about 3.5 to >4 

in acid-sulfate water (Wirt and others, this volume; 

Stanton, Yager, and others, this volume, Chapter E14). 

Ferricrete deposits therefore provide a record of the 

type and chemistry of ground water being supplied to 

the surface drainage basins prior to mining.
Figure 3. Distribution of zinc in surface water during low flow (Wright, Simon, and others, this volume, fig. 17).

Silverton

Howardsville

Middleton

Chattanooga

Gladstone

Animas

Forks

x Storm Peak Ohio x Peak

SAN MIGUEL CO

HINSDALE 

CO

SAN JUAN CO

CONT ENT NI

AL DIVIDE

OURAY CO

Vermilion Peak

elev. 4,235 m

>1,000

Range of dissolved zinc concentrations,  in micrograms per liter

>500 – 1,000

>250 – 500

>100 – 250

<100

No data

EXPLANATION

Alteration Types

Acid-sulfate (AS)

Quartz-sericite-pyrite (QSP)

Vein-related quartz-sericite-pyrite (V-QSP)Weak sericite-pyrite (WSP)Propylitic (PROP)

High-ranking draining mine

High-ranking waste-rock pile

Permitted mine site under remediation

Permitted waste-rock pile or mill tailings site under remediation

Base from U.S. Geological Survey1:100,000 Digital Line GraphsAlbers Equal-Area Conic projectionStandard parallels 37o30' and 40o30'

Central meridian 37o00'

0

0

1

1

2

2

3

3

4 MILES

4 KILOMETERS 

107o37'30" 107o45'

37o

52'30"

x

37o45'

 River

Cinnamon Cr

West Fork

South Fork

Animas

North Fork

Arrastra

Creek

SilverLake

HighlandMary

South 

Fork

Creek

Bear

Creek

Mineral 

Clear Cr Clear Lake

Fuller Lake

IceLake

Columbine Lake

Mill Cr

Lakes

Cunningham Cr

Cement Creek

Fo South kr

Illinois Gulch

M

di dle Fk M ni eral Cr

Gulch

Eureka

Maggie

Minnie

Boulder

hc Gul

Gulch

Gulch

x

California

Mountain

3,792 m

Animas River

watershed

study area

8 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Summary and Conclusions 9

• Oxidation of ferrous iron to ferric iron by atmospheric and biogenic molecular oxygen lowered pH 

and resulted in the precipitation of both amorphous 

iron oxyhydroxides and sulfates. The primary difference between these deposits and ferricrete deposits 

is largely the site of deposition. Both iron bogs and 

wetlands represent both a source of dissolved trace 

elements to surface streams and a transient sink for 

metals that were precipitated and trapped in these 

deposits. However, once these iron-rich deposits 

become inactive (that is, they become bog iron or 

ferricrete deposits), they are subject to erosion (fig. 5). 

They then become a source of trace-element-rich iron 

sediment that contributes to the active streambed sediment budget. Erosion rates of these ferricrete deposits 

are slow, as they also contain substantial amounts of 

amorphous silica (Wirt and others, this volume).

• Geomorphological studies of the Cement Creek valley 

(Vincent and others, this volume, Chapter E16, pl. 5) 

clearly showed how the last glacial retreat has significantly influenced landforms, how those landforms 

controlled surface and ground-water flow and stream 

gradient, and how they affected the distribution of 

sedge marshes and ferricrete deposits in the Cement 

Creek valley.

• Wood found in ferricrete deposits was always postglacial in age. Wood preserved in peat deposits and 

in stumps in growth position in ferricrete was dated 

using 14C methods (Verplanck and others, this volume; 

Vincent and others, this volume), and these ages were 

used to calculate rates of accumulation of peat and 

estimate rates of erosion in the Cement Creek valley 

(Vincent and others, this volume).

• Geochemical studies of sediment from premining terrace deposits (Church, Fey, and Unruh, this volume, 

Chapter E12), constrained by the 14C age data, indicate 

that premining geochemical conditions in Cement 

Creek were not very different than they are today. The 

observations of Rhonda (1876) indicating that, in 1874, 

the water in Cement Creek was “so strongly impregnated with mineral ingredients as to be quite unfit for 

drinking” confirm our conclusions.

• Studies of both paleo and active iron bogs (Stanton, 

Yager, and others, this volume) and of wetlands 

(Stanton, Fey, and others, this volume, Chapter E25) 

indicate that these sites are also sources of acidic water 

and elevated trace-element concentrations that affect 

water quality (fig. 5).

• Church, Fey, and Unruh (this volume) present data 

from both modern and premining terraces that clearly 

show that historical mining has had an effect on the 

distribution and concentration of trace elements in 

streambed sediment (for example, zinc, fig. 6). The 

source of these metal anomalies can be correlated 

directly to mining and milling through studies of the 

isotopic signature of the lead from mill waste and in 

sediment. Since water-quality data are not available 

from 1871 when mining began, the data from stream 

sediment preserved in premining terraces provide the 

best evidence of the state of the aquatic habitat when 

mining began. Anecdotal data indicate that native trout 

could be caught in the Animas River prior to mining 

(von Guerard and others, this volume), and the report 

by Rhonda (1876) indicates that water quality in the 

Animas River was good prior to mining.

• No newspaper accounts exist that document fish living 

in either Cement Creek or Mineral Creek upstream of 

the confluence with South Fork Mineral Creek. Data 

from springs in unmined areas (Mast and others, this 

volume), stream-sediment data from terraces (Church, 

Fey, and Unruh, this volume), and the absence of 

paleontological evidence for a viable aquatic ecosystem 

at the time the terrace deposits formed suggest that a 

viable macroinvertebrate community probably did not 

exist in either Mineral Creek upstream from the confluence with South Fork Mineral Creek or in Cement 

Creek prior to mining (Church and others, 1999).

Figure 4. View of mixing zone looking downstream 

(southwest) from confluence of Cement Creek (right) with 

Animas River (left). Iron colloids precipitate from more 

acidic surface water in Cement Creek (pH ≈3.5) upon mixing 

with more neutral water of Animas River. Photograph taken 

May 1998, by Briant A. Kimball.
Watershed Conditions (1996–2000)

The following is a summary of current conditions in the 

Animas River watershed study area as shown by our studies.

• Investigations of the aquatic habitat throughout the 

study area indicate that brook trout are surviving in 

some stream reaches of the Animas River (Besser and 

Brumbaugh, this volume; Besser and Leib, this volume, Chapter E19).

• Besser and others (this volume) provide an ecological 

risk assessment focused on fish and aquatic invertebrates of the Animas River watershed on the basis of 

their exposure to aluminum, cadmium, copper, and 

zinc. This study was based on water-quality measurements made during low flow in the years 1996–2000. 

For evaluating ecological risks, Besser and others 

focused on aquatic resources (that is, fish and benthic 

macroinvertebrates), and the effects of metals on the 

food web. In particular, they conclude:

1. Dissolved and colloidal-bound metals pose the 

greatest risk to benthic macroinvertebrates and 

trout.

2. The risk to stream biota associated with zinc 

toxicity in surface streams (fig. 7) and of 

the other trace elements studied occurred at 

potentially toxic levels in all acidic stream 

reaches (pH <4.5). However, risks to stream 

biota in near-neutral stream reaches (pH >6.5) 

varied widely.

3. Copper posed the greatest risk to aquatic life, 

but that risk diminished downstream from the 

source in near-neutral stream reaches.

4. Sources of risk to stream biota were ranked 

in terms of severity and indicate where 

remediation efforts would produce the most 

immediate and lasting results.

• Food-web studies indicate different risks of toxicity 

to stream biota. Concentrations of copper and cadindicating dietary exposure of toxic trace elements. 

Copper concentrations in invertebrate diets and in 

trout liver suggested chronic copper toxicity in stream 

reaches where brook trout survived.

Figure 5. Upper Cement Creek just south of Gladstone (fig. 3), showing iron colloids covering 

streambed; looking south across Cement Creek. Ferricrete deposits form the banks of the stream; older 

inactive deposits are dry and dark brown. Acidic water drains from the sedge grass marsh, through a 

small iron bog and into the creek. Note pronounced change in vegetation at break in slope in background. 

Shallow pools in the creek accumulate iron-rich colloidal sediment that sequesters trace elements.

10 Environmental Effects of Historical Mining, Animas River Watershed, Colorado

mium remain constant or increase across trophic levels 
Summary and Conclusions 11

Zinc, in parts per million

 EXPLANATION

> 1,500750–1,500375–750225–375< 225

> 1,500750–1,500375–750225–375< 225 

Zinc, in parts per million

 EXPLANATION

Mine

Mill

Tailings

00 1

1 2 MILES

2 KILOMETERS

107o50'30" 107o45' 107o37'30" 107o30'

37o

58'

37o

52'30"

37o

45'

Chattanooga Eureka

Animas

Forks

Howardsville

Silverton

Gladstone

Animas River

watershed

study area former Lake

Emma

Elk Park

Ani mas R vi er

Cunningham Creek

Arrastra Creek

Deer Park Creek

Molas C reek

Sultan Creek

eB ar Creek

Mill Creek

M di dle Fork

ni M

keer C l are

South Fork

ne me C C t er ke

Elk Creek

C ni n ma on Creek

P ci ayune Gulch

Maggie Gulch

Eureka Gulch

Minnie Gulch

BrendelGulch

C ali of nr ai Gulch

Browns

Gulch

Topeka

Gulch

P al ec Gr hcl u

Deadwood Gulch

Prospect Gulch

A

B

Animas River

watershed

study area

00 1

1 2 MILES

2 KILOMETERS

107o50'30" 107o45' 107o37'30" 107o30'

37o

58'

37o

52'30"

37o

45'

Chattanooga

Eureka

Animas

Forks

Howardsville

Silverton

Gladstone

former

Lake

Emma

Elk Park

An mi as

R vi er

Elk Creek

Cunningham Creek

Arrastra Creek

Deer Park Creek

Molas C reek

Sultan Creek

e B ar Creek

Mill Creek

Middle Fork

ni M

eer C l are

k

South Fork

t ne me C er C ke

P al ec c ul Gr h

Deadwood Gulch

P ci ayune Gulch

 aC liforn ai Gulch

BrendelGulch

Browns

Gulch

Topeka

Gulch

C ni n ma on Creek Prospect Gulch

Maggie Gulch Eureka Gulch

Minnie Gulch

Figure 6. Zinc concentrations in 

A, premining sediment and 

B, modern sediment (from Church, Fey, and Unruh, this volume, fig. 28). Dots on stream segments show sample 

localities.
Figure 7. Zinc toxicity map showing categories of risk to aquatic life (Besser and others, this volume, fig. 4).

Animas River

watershed 

study area

C O L O R A D O

Durango

Chattanooga Eureka

Animas

Forks

Howardsville

Silverton

Gladstone

No toxicity

Acute toxicity to sensitive taxa

Chronic toxicity to brook trout

Acute toxicity to brook trout

Zinc

 EXPLANATION

Mine

Mill

Mill tailings

Stream gauge 0

0 1

1 2 MILES

2 KILOMETERS

107°50'30" 107°45' 107°37'30" 107°30'

37°

58'

37°

52'

30"

37°

45'

former

Lake

Emma

Elk Park

Animas

River

Cunningham Creek

Arrastra 

 Creek

Deer Park Creek

Molas Creek

Sultan Creek

Bear Creek

Mill Creek

Middle Fork

 Sout h Fork

Cement Creek

Elk Creek

Mineral Creek

Brendel

Gulch

 California Gulch

Cinnamon Creek

Picay une Gulch

Maggie Gulch

Eureka Gulch

Minnie Gulch

Browns

Gulch

Topeka

Gulch

Deadwood Gulch

ProspectGulch

M34

A72

A68

C48

A72

A53

Bou dl er Gu cl h

South Fork

Placer Gulch West

Fork

North

Fork

• Larison and others (2000) documented the toxic effects 

of cadmium accumulation in white-tailed ptarmigan 

(Lagopus leucrus) in the study area in conjunction with 

our work, showing that elevated cadmium levels in 

bone in the ptarmigan resulted in physiological effects.

• Studies of the variations in flow and surface-water 

chemistry measured at the gauging stations on each 

of the tributaries and downstream of the confluence 

of Mineral Creek on the Animas River indicate that 

the poorest water quality occurs during low flow in 

January and February. Toxicity tests of water and 

sediment using sensitive aquatic test organisms 

showed that this was the critical period for highest 

toxicity (Besser and Leib, this volume). Elevated 

concentrations of zinc during this period were highly 

toxic to amphipods and early life stages of brook trout, 

less toxic to fathead minnows, and least toxic to adult 

brook trout. Both fathead minnows and brook trout 

12 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Summary and Conclusions 13

showed significant toxicity effects due to the elevated 

concentrations of copper and toxicity, which approach 

the LC50 value for fathead minnows. These toxicity 

tests were used to establish the chronic toxicity thresholds for early life stages of brook trout.

• Documentation of changes of instream loads of cadmium, copper, and zinc throughout the year indicated 

that different stream reaches contributed different 

portions of the trace-element loads throughout the year 

(Besser and Leib, this volume; Leib and others, this 

volume, Chapter E11).

• The physical habitat in the streams also has been 

degraded as a result of historical mining. Those changes 

currently limit over-winter survival of trout because 

of the aggradation of sediment, infiltration of voids in 

gravel-bottomed streams by fine-grained iron-rich sediment, loss of willow thickets that stabilize the riverbanks, and subsequent loss of deep pools (Milhous, this 

volume, Chapter E21; Vincent and Elliott, this volume).

• As a direct result of milling at Eureka, sheets of gravel 

accumulated downstream in the braided reach at rates 

estimated to be 50 to several thousand times that prior 

to mining. Historical photographs of the riparian zone 

(Vincent and Elliott, this volume), along with analysis 

of the recovery of this braided reach downstream from 

the large Sunnyside Eureka Mill documented using 

aerial photographs of this reach taken in each decade 

since the 1950s, show that some recovery of the riparian habitat has resulted from natural erosion processes. 

Substantial amounts of the mill tailings originally 

disposed of in this reach (70–80 percent, Vincent and 

Elliott, this volume) have been transported downstream 

and dispersed in historical stream deposits (Church, Fey, 

and Unruh, this volume). An additional 10 percent of the 

mill tailings in this reach were removed by Sunnyside 

Gold, Inc., during remediation in 1996, and an estimated 

10 percent of the mill tailings remain as discrete deposits 

in the braided reach downstream of Eureka.

• Studies of the premining geomorphology indicate that 

the Eureka reach was not a meandering single-thread 

channel prior to mining. Riparian zone recovery has 

taken place at the lower end of the braided reach 

immediately upstream of Minnie Gulch, and surface 

water quality is improved there (fig. 3). However, complete recovery of the riparian zone likely would take 

many centuries if no remediation were done (Vincent 

and Elliott, this volume).

• Studies of premining sediment quality in this reach 

clearly show elevated zinc concentrations prior to mining downstream from Eureka (fig. 6A), but the effects 

of milling at Eureka are very evident from the contrast 

of zinc concentrations in modern streambed sediment 

(fig. 6B).

Potential for Restoration of Functional 

Aquatic Habitat

Finger and others (this volume, Chapter F) address 

the effects of remediation and monitoring of recovery of 

stream biota in the streams as that process continues. A 

large number of remediation projects and activities have 

been undertaken, largely by Sunnyside Gold, Inc., at their 

own properties. Sunnyside Gold, Inc. also has done remediation at other sites in the watershed to reduce metal loading, 

under an agreement with the Colorado Department of Public 

Health and Environment. Additional remediation projects 

have been done by the U.S. Bureau of Land Management 

and the USDA Forest Service where they have clear ownership and liability under the Federal Water Pollution Control 

Act, revised by Congress in 1972 into the Clean Water Act 

(P.L. 92–500). The goals of the Clean Water Act include “the 

discharge of pollution into navigable waters be eliminated by 

1985,***the discharge of toxic pollutants in toxic amounts 

be prohibited, and an interim goal of water quality, which 

provides for the protection and propagation of fish, shellfish, and wildlife, and***creation in and on the water***by 

July 1, 1983” [P.L. 92–500, §101(a), 33 U.S.C. §1251(a)]. 

The local community stakeholders group and individual 

property owners in Silverton, with funding from Non Point 

Source (NPS) 319 grants (EPA) and severance tax funds 

provided through the State of Colorado, have also completed remediation projects (Finger and others, this volume). 

Remediation projects completed through 2003 are summarized in figure 8. Remediation work done at the Silver Wing 

mine (fig. 9) is an excellent example of the public/private 

collaboration that will be required to achieve lasting success 

in reducing the metal loads to the watershed. The property 

owner has secured a 319 NPS grant from EPA for a passive 

treatment system to treat water draining from the mine, and 

he has diverted the adit flow away from the mine-waste pile 

in front of the mine adit. Similar types of activities have 

been conducted by Sunnyside Gold, Inc., and by the Federal 

land-management agencies in cooperation with the local 

stakeholders, to reduce and divert acid mine drainage away 

from mine-waste piles, remove highly reactive mine and mill 

wastes from the environment, and improve water quality in 

the Animas River watershed study area. Aspects of our work 

that provide a basis for evaluating the effectiveness of remediation efforts are listed following:

• Loading patterns for metals determined from the various tracer studies (Kimball and others, this volume) 

document loading of metals in surface streams and 

provide important constraints on remediation activities 

that need to be evaluated to improve water quality in 

the watershed.
Howardsville

Gladstone

Eureka

Chattanooga

Animas

Forks

Silverton

107o48' 107o32'

37o

52'

30"

37o

45'

107o40'

0

0

1

2

3

4

5 MILES

6 8 KILOMETERS

M ni eral Creek

Animas River

C

me ne t Creek

An

mi as

River

Mine

EXPLANATION

Mill tailings

Reclamation done with

Private funds

Public and private funds

Public funds

MAY DAY

Mayflower

Mill

SILVER

 WING

Lackawanna

Mill

SUNNYSIDE

Sunnyside

Flotation 

Mill

Animas River 

watershed

study area

Figure 8. Sites where remediation work has been done (through 2003) in the study area (Finger and others, this volume, fig. 1). 

Sites are coded by types of funding used (private; public, that is NPS 319 grants through EPA, severance tax funds through the State 

of Colorado, and funding from the USDA Forest Service or U.S. BLM abandoned mine lands funds; and mixed private/public funding 

sources). Numerous mine adit closings have also been initiated by the State of Colorado using severance tax funds to reduce physical 

hazards. These adit closures do not affect water quality and are not included in this illustration. Data provided by William Simon 

(Animas River Stakeholders Group, written commun., 2004).

14 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Summary and Conclusions 15

• Various remediation scenarios have been evaluated in 

four stream reaches using the tracer data in a chemical 

model (OTIS) that accounts for changes in chemistry 

by in-stream chemical processes (Walton-Day and 

others, this volume) in Cement Creek and the Animas 

River, and in Mineral Creek (Runkel and Kimball, 

2002). The goal was to ascertain whether large anthropogenic sources of contamination could be effectively 

remediated.

• Although large amounts of metals and acidity could 

be removed by remediation of sources in the upper 

Cement Creek basin, changes in metal loads at the 

mouth of Cement Creek would be small because much 

of the metal loading comes from ground-water-fed iron 

bogs and sedge marshes (fig. 5).

• Remediation scenarios in the upper Animas River basin 

are promising, and reduction of dissolved zinc concentrations to below aquatic life standard for sensitive 

aquatic organisms may be possible.

• Prior to the remediation of the Lackawanna Mill site, 

the U.S. Bureau of Land Management in 2000 asked 

the U.S. Geological Survey (Wright, Kimball, and 

Runkel, this volume, Chapter E23) to assess the impact 

of the May Day mine site on water quality in Cement 

Creek. These data provide a geochemical baseline 

needed to evaluate the effect of the repository subsequently built at the May Day mine site for tailings from 

the Lackawanna Mill.

• Phased remediation of the most significant sources of 

metals and acidity in the watershed (as identified in 

Nash and Fey, this volume; Wright, Simon, and others 

this volume) is recommended and will allow real-time 

assessment of remediation efforts (Finger and others, 

this volume).

• Benchmark benthic macroinvertebrate data collected 

in the Animas River watershed provide a quantitative 

evaluation of the benthic community structure and 

health prior to remediation (Anderson, this volume, 

Chapter E20).

• Monitoring of the recovery of sensitive macroinvertebrates along with improvements of water quality 

as remediation proceeds is a proposed method for 

monitoring functional recovery of the aquatic communities (Anderson, this volume; Finger and others, this 

volume).

• Benthic macroinvertebrates are present in tributary 

streams and will reinhabit mainstem reaches upon restoration of water and sediment quality to levels where 

aquatic life can persist (Besser and Brumbaugh, this 

volume; Anderson, this volume).

Figure 9. Silver Wing mine, a privately held, inactive mine site on east side of Animas River. 
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16 Environmental Effects of Historical Mining, Animas River Watershed, Colorado