Processes Affecting the Geochemical 

Composition of Wetland Sediment

By Mark R. Stanton, David L. Fey, Stanley E. Church, and Charles W. Holmes

Chapter E25 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

Abstract .....................................................................................................................................................1033

Introduction...............................................................................................................................................1034

Field and Analytical Methods ................................................................................................................1034

Core Locations.................................................................................................................................1034

Core and Auger Samples ...............................................................................................................1034

Forest Queen Detailed Study ...............................................................................................1034

Aqueous Samples and Analyses ..................................................................................................1036

Acid-Volatile Sulfide (AVS) and Acid-Extractable (6N HCl) Element Analyses ...........1036

Alteration Types and Geology .......................................................................................................1036

Mining History .................................................................................................................................1037

Results and Discussion ...........................................................................................................................1037

South Fork Mineral Creek ..............................................................................................................1037

Core 97ABS321 .......................................................................................................................1037

Core 97ABS322 .......................................................................................................................1040

Aqueous Chemistry ...............................................................................................................1041

Mineral Creek ..................................................................................................................................1041

Core 999292 .............................................................................................................................1041

Aqueous Chemistry ...............................................................................................................1043

Cement Creek ..................................................................................................................................1043

Core 97ABS324 .......................................................................................................................1043

Premining Sediment .....................................................................................................1045

Mining Sediment ...........................................................................................................1046

Postmining/Remediation Sediment ............................................................................1046

Iron and Aluminum Chemistry at the Site ..........................................................................1048

Animas River ....................................................................................................................................1048

Sulfate Reduction as a Metal Sequestration Mechanism ..............................................1048

Forest Queen Wetland Site ..................................................................................................1048

Core Descriptions, HCl-Extractable Metal Concentrations, and AVS Recovery ...........1051

Core FQ3 .........................................................................................................................1051

Acid-Extractable Element Concentrations and AVS ......................................1051

Core FQ4 .........................................................................................................................1054

Total Element Concentrations ............................................................................1054

Acid-Extractable Metal Concentrations and AVS ..........................................1054

Core FQ5 .........................................................................................................................1054

Total Element Concentrations ............................................................................1054

Acid-Extractable Element Concentrations and AVS ......................................1056

Core FQ7 .........................................................................................................................1056

Total Element Concentrations ............................................................................1056

Acid-Extractable Element Concentrations and AVS ......................................1056

Core FQ9 .........................................................................................................................1059

Acid-Extractable Element Concentrations and AVS ......................................1059

Summary of Acid-Extractable Element Concentrations and Acid-Volatile Sulfide

for Animas River Cores .............................................................................................1059
Conclusions...............................................................................................................................................1062

Importance of Metal-Rich Inflow as a Control on Metal Content of Wetlands ....................1062

Relative Importance of Different Sorbents or Contributors within Individual Wetlands .........1062

Spatial Relationship of Metal Enrichment within Forest Queen Wetland .............................1063

Stability of Wetlands and Wetland Functions Through Time and Implications 

for Wetland Management and Water Quality ..............................................................1063

References Cited......................................................................................................................................1063

Figures

 1. Map of Animas River watershed study area, showing wetland core locations, 

drainages, and generalized distribution of major rock units and alteration 

types ..........................................................................................................................................1035

 2. Downhole plot of total Al, Ca, Fe, Cu, Pb, Zn, and As in sediment from 

core 97ABS321, South Fork Mineral Creek ........................................................................1039

 3. Downhole plot of total Al, Ca, Fe, Cu, Pb, Zn, and As in sediment from 

core 97ABS322, South Fork Mineral Creek ........................................................................1040

 4. Plot of the sum of trace elements (Cu, Pb, Zn, and As) versus pH in water 

samples from different depositional environments, Animas River watershed 

study area ................................................................................................................................1042

 5. Downhole plot of Al, Ca, Fe, Cu, Pb, Zn, and As in sediment from core 999292, 

Mineral Creek ..........................................................................................................................1044

 6. Downhole plot of total Al, Ca, Fe, Cu, Pb, Zn, and As in sediment from 

core 97ABS324, Cement Creek .............................................................................................1045

 7. Sketches showing variations in deposition of clastic (detrital) and organic 

sediment and iron oxyhydroxide during premining, mining, and postmining 

periods, site of core 97ABS324, Cement Creek .................................................................1047

 8. Plots of iron versus aluminum in core 97ABS324 (a mixed organic/iron wetland) 

and core 97ABS327 (an iron bog) ..........................................................................................1049

 9. Sketch map of Forest Queen wetland and adjacent area ...............................................1050

 10. Plot of acid-extractable major- and trace-element concentrations with depth, 

Forest Queen core FQ3 ..........................................................................................................1053

 11–13. Plot of total element concentrations, organic matter, AVS yield, and percentage 

of acid-extractable elements with depth:

 11. Forest Queen core FQ4 ...................................................................................................1055

 12. Forest Queen core FQ5 ...................................................................................................1057

 13. Forest Queen core FQ7 ...................................................................................................1058

 14. Scatter diagrams of iron, lead, copper, and zinc versus S2–

(AVS), showing 

relationships of trace-metal content to total recoverable sulfide in core FQ7 ............1060

 15. Downhole plot of acid-extractable major- and trace-element concentrations, 

Forest Queen core FQ9 ..........................................................................................................1061

 16. Contour maps showing maximum concentrations of total iron at three different 

depths in Forest Queen wetland............................................................................................1062
Tables

 1. Summary statistics for total concentrations of major (Al, Fe, Ca (weight percent)) 

and trace (Cu, Pb, Zn, As (ppm)) elements in wetland sediment, South Fork 

Mineral Creek and Mineral Creek .........................................................................................1038

 2. Individual and summary statistics of aqueous major- and trace-element 

concentrations (μg/L), calcium and sulfate concentrations (mg/L), and water 

chemistry parameters from wetlands, South Fork Mineral Creek and Mineral 

Creek .........................................................................................................................................1043

 3. Water chemistry parameters from five Forest Queen monitoring wells .......................1051

 4. Summary statistics for acid-extractable concentrations of major and trace 

elements from Forest Queen wetland cores ......................................................................1052

 5. Summary statistics for total concentrations of major and trace elements from 

Forest Queen wetland cores .................................................................................................1052
Abstract

Geochemical analyses of sediment core and water samples 

from five wetlands along four major drainages of the Animas 

River watershed study area were undertaken to characterize 

wetland chemistry in an acid drainage regime. The objectives 

of this study were to determine the capability of the wetlands to 

retain or release metals produced by acid weathering processes 

that occur in the area, to examine the past and present effect of 

these weathering processes on the geochemistry of the wetland 

systems, and to investigate how these processes affect the composition of wetland sediment. Geochemical profiles in wetland 

cores and dating of cores by 14C, 210Pb, and 137Cs chronology 

provided records of historical changes in the metal flux to the 

various wetlands.

Analysis of nine sediment cores for major- and traceelement abundances (total and HCl-extractable concentrations), organic and mineral matter content, acid-volatile 

sulfide, and secondary iron oxyhydroxide revealed that, 

compared to iron bogs (iron-rich wetlands) in the same region, 

wetland sediment is characterized by higher organic content 

(10–90 weight percent) and generally lower concentrations of 

iron and aluminum. Mean concentrations from all nine cores 

range from 1.5 to 34.5 weight percent iron and from 3.9 to 

6.9 weight percent aluminum. In individual samples, however, 

maximum concentrations of total iron and aluminum are as 

high as 50.1 and 12.0 weight percent, respectively.

Trace-element content of wetland sediment varies widely. 

Maximum total concentrations of trace elements in wetland 

sediment are as follows: Cement Creek, 770 ppm copper, 

1,600 ppm lead, 7,000 ppm zinc, and 88 ppm arsenic; Mineral 

Creek, 330 ppm copper, 700 ppm lead, 2,600 ppm zinc, and 

57 ppm arsenic; South Fork Mineral Creek, 760 ppm copper, 190 ppm lead, 3,000 ppm zinc, and 37 ppm arsenic. The 

ranges of acid-extractable trace-element concentrations are 

copper, 17–200 ppm; lead, 17–640 ppm; zinc, 70–1,720 ppm; 

and arsenic, from below detection to 48 ppm. The percentage 

of an individual element that is acid extractable ranges from 

0 to about 15 percent. Based on these iron, aluminum, and 

trace-element values, the wetlands appear to concentrate some 

elements within solid phases.

Four water samples from South Fork Mineral Creek 

showed low concentrations of dissolved iron and aluminum 

(<1 ppm), zinc (<50 ppb), and arsenic (below detection; 

<10 ppb). The sample from the westernmost locality was 

highly acidic (pH 3.2) with relatively higher concentrations 

of dissolved major and trace elements, but sediment from the 

associated wetland had among the lowest total abundances of 

solid-phase metals. This results from decreased metal adsorption at lower pH and low availability of metals from surrounding rock units that are not highly mineralized or hydrothermally altered. The easternmost wetland sediment had slightly 

higher total metal abundances, and copper, in particular, was 

enriched in the sediment. These two samples illustrate the 

increasing importance of adsorptive processes with higher pH, 

and the increased availability of metals from altered or mineralized rocks (or windborne metal-bearing detritus) farther east 

along South Fork Mineral Creek.

A water sample from mainstem Mineral Creek had 

2–18 times higher concentrations of elements than South Fork 

Mineral Creek samples, a result of Mineral Creek draining an 

acid-sulfate alteration zone. A nearby wetland sediment was 

also enriched in most elements, probably through deposition 

of windborne metal-rich detritus.

One site, the Forest Queen wetland along the upper 

Animas River, was cored in detail (five cores) to determine 

the role of sulfate reduction and resulting sulfide mineral 

formation in the sequestration of metals from influent acidic, 

metal-rich water. Acid-leaching (6N HCl) of sediment with 

recovery of acid-volatile sulfide (AVS) was used to examine associations of metals with authigenic sulfide minerals. 

High concentrations of extractable metals generally were 

not associated with the highest AVS yields, indicating that 

although metals may be sequestered in iron monosulfide 

phases, the process is unevenly distributed throughout the 

wetland. The maximum total concentrations of trace elements 

in Forest Queen wetland sediment are copper, 215 ppm; lead, 

1,600 ppm; zinc, 3,250 ppm; and arsenic, 73 ppm. Proximity 

to the source water and metal flux to core sites are important 

factors in metal enrichment. Element enrichment in the upper 

60 cm and deeper than 80 cm indicate two possible flow paths 

and (or) sources of metal in the wetland.

Chapter E25

Processes Affecting the Geochemical Composition 

of Wetland Sediment

By Mark R. Stanton, David L. Fey, Stanley E. Church, and Charles W. Holmes
Water samples showed mixtures of dissolved ferric and 

ferrous iron in the presence of varying levels of dissolved sulfide, suggesting that several redox processes were occurring in 

close spatial proximity in ground water. Inflow surface water 

had a pH of 4.7, ground water within the wetland had pH near 

7, and the pH of outflow water was 5.9. These pH values suggest that processes that buffer or neutralize incoming acidic 

waters are occurring in the Forest Queen wetland but may not 

be active in all areas.

The studied wetlands act to trap metals through 

adsorption onto organic matter or detrital mineral surfaces, or 

through precipitation as authigenic metal sulfides or iron oxyhydroxide. Iron oxyhydroxide in near-surface layers of some 

wetland sediment sequesters substantial amounts of metals. 

This is primarily a seasonal phenomenon associated with 

lowered water tables and precipitation of metal-bearing iron 

solids on the surface. In some areas, iron oxyhydroxide forms 

at depth as a postdepositional coating and performs a similar 

role in trace-element adsorption.

Introduction

Elevated concentrations of dissolved major and trace 

elements and extreme acidity characterize the chemistry of 

surface water and inactive mine drainage in the Animas River 

watershed study area (Church and others, 1997). In this report, 

we use the concentrations of iron, aluminum, calcium, copper, 

lead, zinc, and arsenic in wetland sediment to indicate historical trends in metal deposition during premining and postmining periods. The geochemical behavior of these elements in an 

acid system may help provide a record of variations in supply 

of sediment, solubility of minerals, and mobility of elements 

(Fortin and others, 1993; Jenne, 1968).

The ability of the wetland environments to trap and 

retain toxic elements is of interest as a possible control on 

local water quality. The two major processes operating in 

wetlands to scavenge and sequester metals are coprecipitation 

(formation of authigenic sulfide or iron oxyhydroxide minerals) and adsorption onto mineral surfaces or organic matter 

(Schwertmann and Cornell, 1991; Konhauser and others, 

1993; Sholkovitz and Copland, 1981; Szalay, 1964). However, 

mechanistic details of these processes such as surface complex 

formation or ion exchange are not considered in this overview.

Field and Analytical Methods

Core Locations

Five wetlands along four major tributaries in the Animas River watershed study area—South Fork Mineral Creek, 

Mineral Creek, Cement Creek, and Animas River—were 

sampled (fig. 1). Two cores are from wetlands along South 

Fork Mineral Creek (cores 97ABS321, 97ABS322), and one 

core is from a wetland near Mineral Creek downstream from 

its confluence with the South Fork (core 999292). The core 

near Cement Creek (97ABS324) is from a hybrid wetland that 

contains both organic matter and iron oxyhydroxide. The other 

wetland is the Forest Queen along the upper Animas River; we 

took five individual cores (FQ3, FQ4, FQ5, FQ7, FQ9) at this 

site.

Core and Auger Samples

To take the cores, we used a 1.5 to 3 m length of 5-cm 

inside-diameter PVC pipe, driving it into the organic+clastic 

sediment layers until resistance barred further penetration. 

Individual drives could not be made deeper than about 3 m 

by this method, and often, two or three insertions in the same 

location were made to ensure that the maximum thickness 

of sediment was sampled. A rubber seal placed in the top 

opening of the pipe after insertion provided vacuum that held 

the sediment column in place during extraction. After being 

pulled, the core length in the pipe was measured, excess PVC 

pipe was removed, and cores were sealed with PVC endcaps 

secured by cloth tape.

Because of sediment compression, the length of a 

recorded core was from 5 to 15 percent less than the total 

depth of coring. Cores from South Fork Mineral Creek, 

Mineral Creek, and Cement Creek were not corrected for compression. Subsamples from these four cores were taken at 1 cm 

intervals to provide adequate analytical coverage. Selected 

intervals of peat and organic-rich peaty muck were sent to a 

commercial laboratory for 14C age dating; other intervals were 

chosen for use in 210Pb and 137Cs chronology.

Forest Queen Detailed Study

At the Forest Queen wetland, we used PVC pipe to 

take large-diameter cores by the process just described. We collected smaller cores (1.25 cm diameter), using a PVC-sleeved 

soil sampler, at the edges of the wetland and at locations where 

large-diameter coring proved unworkable. The small-diameter 

cores were no deeper than approximately 0.5 m. Cores were 

taken approximately 2 m adjacent to piezometers that were 

installed and sampled 2 weeks earlier as part of a water-quality 

study directed by the Colorado School of Mines (Pavlik and 

others, 1999). Additional shallow (≤0.5 m) grab samples were 

taken with an auger, placed in polyethylene bags, and flushed 

of air with N2(g). Core and auger samples were stored in the 

dark at 4°C until they were treated in the laboratory, approximately 2 weeks after collection.

Upon opening a core, we immediately described 

the stratigraphy. Rather than subsampling cores in discrete 1-cm intervals, we selected the sample intervals from 

Forest Queen cores based on visually identifiable similarities 

in color, organic matter, clastic content, and grain size. The 

1034 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Affecting Composition of Wetland Sediment 1035

selected intervals ranged from one to tens of centimeters in 

length. Sample depths were corrected for compression by 

proportioning the total amount of compression to the percentage of core length that a recovered interval occupied. The 

samples were composited and processed for analysis under an 

N2(g) stream in a glove bag to inhibit oxidation, but were not 

dried. On some surfaces, variable amounts of red to brown 

staining indicative of iron oxyhydroxide were seen. This likely 

did not result from oxidation during short-term core storage 

because during the coring operations, we observed iron oxyhydroxide as deep as 1 m.

Samples were air-dried and sieved (60 mesh) to remove 

grains of fine sand or larger material. Organic matter (loss on 

ignition or LOI, weight percent) and ash content (100 minus 

LOI, weight percent) were determined in cores 999292, FQ4, 

FQ5, and FQ7 by ashing, while in other cores, these parameters 

were estimated by the proportion of peat plus wood fragments 

to clastic sediment. Loss on ignition is used as a proxy for 

organic matter content of the samples. Clastic-dominated samples with no visible organic matter were not ashed. The organicrich samples were shredded in a Wiley mill and ashed at 550°C 

for 24 hours to obtain the loss on ignition and ash content.

Figure 1. Animas River watershed study area, showing wetland core locations, drainages, and generalized distribution 

of major rock units and alteration types. Wetland cores are 97ABS321, 97ABS322, 999292, 97ABS324, and five cores from 

the Forest Queen site.

PROP

PROP

Mi ddle Fork

South

Fork

Min eral Creek

Cem

e

n

t

Creek

A n ima s

River

107°45' 107°37'30"

37°52'30"

37°48'45"

37°56'15"

3929

4206

4075

3958 4075 4107

4111

Howardsville

Silverton

Eureka

Ross

Basin Red Mtn Pass

Red Mtn

No. 3

Sultan Mtn

PROP

PROP

PROP

PROP

PROP

QSP

WSP

QSP

PROP

PROP

QSP

QSP

Silverton

Volcanics

San Juan Formation

volcanics

Sultan Mtn

stock

Pennsylvanian-Jurassic

sedimentary rocks Precambrian

rocks

Burns Member of

Silverton Volcanics

Approximate boundary of

Silverton caldera

AS

AS

COLORADO

Durango

Grand

Junction

DENVER

Animas River watershed

study area

WSP

3792

4206 Mountain peak

 and elevation

Sample site Alteration type

 and number

Regional propylitic

Quartz-sericite-pyrite

Weak sericite-pyrite

Acid-sulfate

Geologic contact

PROP

WSP

QSP

AS

0123 MILES

01234 KILOMETERS

97ABS324

97ABS321

97ABS324

FOREST

QUEEN

97ABS322

999292

EXPLANATION

Prospect

Gulch

 (meters)
All samples were powdered to <230 mesh (57 μm 

(micrometers)) in a ceramic shatterbox. A split of the powdered sample was digested in a mixture of acids and analyzed 

by inductively coupled plasma–atomic emission spectrometric 

(ICP-AES) analysis for 40 major and trace elements (Briggs, 

2002). Element concentrations in ashed samples were reported 

on an as-received basis.

Aqueous Samples and Analyses

Water samples were collected along South Fork Mineral 

Creek (one from each cored wetland and two from surface 

water between the sites) and Mineral Creek (one surface water). 

At all sites, dissolved oxygen (DO, ppm), pH (standard units), 

temperature (°C), and specific conductivity (microsiemens/

centimeter, μS/cm) were measured in place; the meters were 

calibrated with standards 5–10 minutes before sampling. At 

selected sites where iron staining or an iron precipitate was 

visible, we measured ferrous iron (Fe2+) and total iron (Fe(tot)) on 

site within 15 minutes of collection and filtration, using a portable spectrophotometer (Hach Company, 1997, User’s Manual). 

Ferric iron (Fe3+) was determined by difference. At the Forest 

Queen wetland, five water samples associated with the sediment 

cores were analyzed on site for total aqueous sulfide.

Samples were drawn into a 60-mL (milliliter) polyethylene syringe, then passed through a 0.45 μm polycarbonate 

syringe filter into an HCl-washed polyethylene sample bottle. 

For cations, one 60-mL aliquot was acidified to pH <2 with 

concentrated nitric acid; for anions, a filtered, unacidified 

aliquot (30 mL) was collected. Cation concentrations were 

determined by inductively coupled plasma–mass spectrometry 

(Lamothe and others, 2002). The major anions fluoride, chloride, nitrate, and sulfate were determined by ion chromatography (Theodorakos, 2002).

Acid-Volatile Sulfide (AVS) and Acid-Extractable 

(6N HCl) Element Analyses

The term “acid-volatile sulfide” (AVS) is usually 

restricted to iron sulfides that are soluble in 6N HCl (Berner, 

1970): greigite (Fe3

S4), amorphous FeS (FeS(am)), and mackinawite (crystalline FeS(c)). Pyrite (FeS2

) is insoluble in 6N HCl. 

The AVS definition is expanded here to include other HClsoluble metal sulfides such as PbS (galena), ZnS (sphalerite), and Cu2

S (chalcocite) that may have been present in the 

sediment.

Thirty-six samples from Forest Queen cores FQ4, FQ5, 

and FQ7 were analyzed for total and HCl-extractable metals, 

and AVS. (HCl-extractable metals are also referred to as 

acid-extractable or acid-soluble metals.) Samples from cores 

FQ3 and FQ9 were analyzed only for HCl-extractable metals 

and AVS. The samples were not dried prior to AVS treatment, 

so an undetermined concentration of metal in each sample 

was associated with the aqueous phase (residual water). Aqueous analyses (Thomas Wildeman, Colorado School of Mines, 

written commun., 1998) showed lead and zinc present in 

water from 103

 to 105 times less than in most solids. Thus, the 

total amount of metal associated with residual water in the core 

samples was small relative to that in the solid phases.

The sample (10–50 g, wet weight) was combined with 

6N HCl (25 mL/10 g) and a 10 percent SnCl2

 suspension 

(10–20 mL per sample), the latter to prevent Fe3+ from oxidizing sulfide to higher oxidation states, such as S0

. The mixture 

was heated to 90°C for 1/2 hour under an N2 atmosphere to 

release AVS (Rice and others, 1993). H2

S(g) released during 

this acid/heating step was swept by N2 gas into a series of 

three glass traps. The first trap contained phosphate buffer 

(pH 4.0); the second and third traps contained 0.1 M AgNO3

that precipitated the sulfide as Ag2

S. The total amount of sulfide was determined gravimetrically after the recovered Ag2

S 

was weighed, and was reported as parts per million (ppm) 

total sulfide (S2–

(AVS)). After AVS recovery, the sample residue 

was rinsed with deionized water (DI), air-dried (25°C), and 

weighed. The filtered HCl solution was analyzed for 40 majorand trace-element concentrations by ICP-AES.

Alteration Types and Geology

Several types of hydrothermal alteration associated with 

mineral deposits are present in the Animas River watershed 

study area (Bove and others, this volume, Chapter E3). A 

generalized depiction of alteration types and geology in the 

areas where the cores were taken is in figure 1. Each alteration 

type is characterized by suites of minerals that represent differences in intensity of the hydrothermal alteration. Regional 

propylitization (PROP) is the weakest grade; minerals are 

primarily altered feldspars with little or no sulfides except 

pyrite. Weak sericite-pyrite (WSP) is a low-grade alteration/

mineralization where the pyrite is not high in ore metals. 

Quartz-sericite-pyrite (QSP) is medium to high-grade alteration with associated higher (than WSP) metal abundances in 

pyrite. (Vein-related quartz-sericite-pyrite alteration (V-QSP; 

Bove and others, this volume) is limited to much smaller areas 

compared to the other alteration assemblages, and is therefore 

not considered separately in this chapter.) The highest grade 

alteration is acid-sulfate (AS), noted by a variety of sulfide 

minerals including tetrahedrite, galena, sphalerite, and chalcopyrite (Bove and others, this volume).

South Fork Mineral Creek drains an area of diverse geology, including WSP/QSP-altered rocks of the subeconomic 

porphyry copper-molybdenum deposit of peak 3,792 m 

(fig. 1). The deposit is hosted by altered Oligocene San Juan 

Formation volcanics, the dominant rock unit immediately 

north of the drainage. Unaltered Pennsylvanian-Permian 

through Jurassic sedimentary rocks crop out to the south and 

west, and variably altered San Juan Formation volcanics are 

present farther west in the drainage (Yager and Bove, this 

volume, Chapter E1, pl. 1). Southeast of peak 3,792 m across 

South Fork Mineral Creek is the porphyritic Sultan Mountain 

stock (fig. 1), an intrusive of quartz monzonite-diorite composition (Yager and Bove, this volume). Mineral Creek also 

1036 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Affecting Composition of Wetland Sediment 1037

drains an area of diverse geology and multiple alteration types, 

from unaltered PROP and WSP/QSP types along its western 

border to dominantly AS- and QSP-altered rocks to the east 

and north (fig. 1). Cement Creek is dominated on the west by 

acid-sulfate and QSP-altered rocks of the Silverton Volcanics 

that make up the Oligocene Silverton caldera. The caldera 

margins are approximated by the large circular outline in 

figure 1; Mineral Creek generally follows the western boundary of the caldera. Rocks east of Cement Creek are primarily 

propylitically altered with scattered zones of WSP and QSP 

alteration. Rocks along the Animas River near the Forest 

Queen mine (Abandoned Mined Lands Inventory (AMLI) 

site # 195) belong to the Burns Member of the Silverton 

Volcanics; these are dominantly propylitically altered with 

small areas of WSP east of the river and QSP west of the river 

(fig. 1). The Silverton Volcanics are host rocks for most of the 

mineral deposits in the Animas River watershed study area. 

See Yager and Bove (this volume and pl. 1) and Bove and others (this volume) for a detailed compilation and overview of 

the geology and alteration types in the area.

Mining History

Mining began in the study area in the 1870s (Jones, this 

volume, Chapter C); several larger mines along South Fork 

Mineral Creek began operation in the 1900s. Peak years of 

production for most mines along South Fork Mineral Creek 

and Mineral Creek were between 1890 and 1950; a few mines 

along Cement Creek continued operation up until 1991 (Bird, 

1999). Numerous mines and the large Sunnyside Eureka Mill 

(AMLI site # 164) operated in the Animas River valley north 

of the Forest Queen site.

Results and Discussion

South Fork Mineral Creek

Core 97ABS321

This wetland is at the toe of a large alluvial fan (100 m 

wide) and appears to be sustained by ground water flowing through the fan. Abundant green algae are found on the 

water surface, and photosynthetic sulfur bacteria are present on the sediment surface (5–10 cm deep water). Mosses, 

sedges, grasses, and woody riparian zone plants are abundant 

throughout the wetland. The steep topography and attitude of 

the country rock above the site direct surface drainage away 

from the area. The perennial stream feeding the fan turns west 

200 m upstream from the site and meets South Fork Mineral 

Creek 500 m west, so the streamflow volume to the fan and 

resulting discharge to the wetland are probably low yearround. The downslope distance where the stream turns west 

to the bottom of the fan is approximately 400 m.

The site is located about 2 km west of the structural 

margin of the Silverton caldera near the southwest edge of peak 

3,792 m (fig. 1). A subeconomic porphyry copper-molybdenum 

deposit is hosted by the WSP-QSP altered rocks of this peak. 

A few kilometers west and southwest of the site are outcrops of 

unmineralized Pennsylvanian-Permian through Jurassic sedimentary rocks that are overlain by weakly altered (PROP) volcanic rocks of the San Juan Formation. The San Juan Formation 

volcanics host ore deposits and mines farther west up South Fork 

Mineral Creek. Surface drainage from both sedimentary and 

volcanic rocks is not highly acidic (pH 5–6), and pH is likely 

controlled by reaction with limestone present in the Permian 

Cutler, Triassic Dolores, and Jurassic Morrison and Wanakah 

Formations (Yager and Bove, this volume, pl. 1).

The core sediment is dominated by organic matter and peat 

with orange-brown iron staining present in every interval. A 14C 

date from a depth of 30.5 cm gave an age of 710±40 years B.P. 

Assuming a constant depositional rate, an estimated (average) 

sediment accumulation rate is 30 cm/710 yrs = 0.04 cm/yr. 

Any discernible mining effect from the past 130 years should 

be confined to approximately the upper 6 cm. The sediment deposition rate is 40 percent slower than in the Forest 

Queen wetland, where an estimated average accumulation 

rate of about 0.1 cm/yr was observed (Vincent and Elliott, this 

volume, Chapter E22).

Summary statistics from the four cores along all drainages except the Animas River are listed in table 1. Selected 

major and trace elements from core 97ABS321 are plotted 

versus depth in figure 2. Prominent maxima are near the bottom of the core for copper (51–53 cm), aluminum (58–60 cm), 

and iron and zinc (60–63 cm). Several peaks of high concentrations of lead and zinc are below 6 cm. Thus, abundance 

maxima for most elements are well below the 6 cm depth and 

therefore predate historical mining. Most element concentrations in the upper 11 cm are low compared to intervals below, 

and in general, most metals show slight increases with increasing depth (fig. 2). (Molybdenum, which is not shown in fig. 2, 

was below the detection limit of 4 ppm in 43 samples and no 

greater than 5 ppm in the remaining 20 samples. This suggests that molybdenum from the peak 3,792 m deposit is either 

immobile or is not reaching the depositional site.)

The steadily increasing concentrations with depth suggest 

that dissolved metals may have been transported to the site 

through time and sorbed onto organic material. Aluminum 

and lead likely originate from windblown, or in the case of the 

spikes at 14 cm and 55–60 cm, perhaps fluvial clastic detritus.

Total lead and aluminum concentrations and ash content in premining sediment exhibited similar patterns (fig. 2), 

suggesting that their variations are the result of increases or 

decreases in detrital deposition. Copper in premining sediment 

exhibited a pattern of variation similar to that of organic content and showed the highest correlation (r = 0.57) with organic 

matter, suggesting that copper was transported to the site as a 

dissolved species and sorbed by organic material. Detectable 

arsenic in samples below 11 cm showed only a weak correlation with iron (r = 0.26).
The variations in total copper, lead, and zinc above 6 cm 

(fig. 2) may be the result of mining upstream. The initial 

increase in these elements after mining began is followed by a 

decrease in concentration in recent time, suggesting that mining produced only a short-term increase in the concentration 

of these elements in sediment. One abundant source of lead 

and zinc is windborne fine-grained mine and mill waste from 

upstream areas. A second potential source of lead and zinc is 

the stream that flows down the alluvial fan. The stream could 

carry dissolved lead and zinc into the fan, and suspended lead 

and zinc might reach the wetland as a result of heavy precipitation or surface runoff events.

In the upper 2 cm, copper and organic matter showed 

contrasting behavior, indicating that copper is not associated 

with organic matter but was from (mining-related?) detritus. 

Zinc behaved as aluminum did in the upper 2 cm, suggesting that zinc originated from a detrital phase. Otherwise, the 

low and relatively invariant concentrations of most elements 

in the 0–6 cm interval suggest that mining activities have had 

little or no observable effect on the wetland.

The lack of noticeable enrichment in most elements 

since mining began may be influenced by the location of the 

wetland on a bench 0.5–1 m above the present base level 

of South Fork Mineral Creek. The wetland is isolated from 

this major stream and receives little clastic deposition except 

during high-flow events (spring runoff or floods), so miningrelated fluvial detritus from upstream areas is not likely to be 

deposited in the wetland.

Normalization of element data to aluminum (used as 

an indicator of detrital solids) indicates that lead is primarily 

associated with detritus (windborne or fluvial), whereas iron, 

copper, zinc, and arsenic are associated with either iron solids 

or organics. Normalization of the element data to iron produces geochemical profiles of copper and zinc that are similar 

to the organic content profile, whereas arsenic and iron are 

associated in secondary iron oxyhydroxide. The presence of 

an iron oxyhydroxide coating on almost all detrital and organic 

sediment in the core indicates that it formed as a postdepositional solid. Aluminum normalized to iron has a geochemical 

profile that shows behavior opposite that of iron, indicating 

that the two metals were not co-deposited. Iron, copper, and 

zinc were likely transported in ground water to the wetland 

where copper and zinc were adsorbed by organic material and 

iron was precipitated (probably as a result of an increase in 

local pH). Thus, element enrichment in the core was accomplished by both detrital (aluminum and lead) and aqueous 

(iron, copper, and zinc) processes.

The lack of significant concentrations of most metals in 

the upper sediment (0–6 cm) is the result of the dominant rock 

types (unmineralized sedimentary and weakly altered volcanic 

Table 1. Summary statistics for total concentrations of major (Al, Fe, Ca (weight percent)) and trace (Cu, Pb, Zn, As (ppm)) elements 

in wetland sediment, South Fork Mineral Creek and Mineral Creek.

Al Fe Ca Cu Pb Zn As

Core 97ABS321, South Fork Mineral Creek

 (Arsenic based on 37 values that were above detection limit of 10 ppm)

Number 63 63 63 63 63 63 37

Mean 5.7 2.8 1.0 49 44 400 21

Minimum 2.2 0.91 0.4 21 9 100 10

Maximum 8.4 7.1 1.6 110 110 1,400 37

Median 5.9 2.9 1.0 43 47 310 23

Core 97ABS322, South Fork Mineral Creek

 (Arsenic based on 24 samples that were above detection limit of 10 ppm)

Number 78 78 78 77 73 78 24

Mean 4.7 1.5 0.6 130 36 630 14

Minimum 1.2 0.1 0.2 5 6 65 10

Maximum 12 5.2 1.4 760 190 3,000 38

Median 4.0 0.6 0.5 90 18 520 10

Core 999292, Mineral Creek

Number 28 28 28 28 28 28 26

Mean 3.9 5.1 1.2 100 205 970 28

Minimum 1.4 1.9 0.6 25 11 95 10

Maximum 6.0 9.8 1.7 330 700 2,600 57

Median 4.0 4.6 1.2 70 175 655 25

Core 97ABS324, Cement Creek

Number 74 74 74 74 74 74 67

Mean 6.9 8.7 0.42 200 640 1,720 48

Mean, 21–0 cm 6.8 13.2 0.3 220 1,080 2,330 73

Mean, 74-22 cm 7.0 6.8 0.5 190 460 1,490 40

Minimum 1.3 3.8 0.11 1 120 260 19

Maximum 8.8 44 0.63 770 1,600 7,000 88

Median 7.1 6.6 0.41 180 470 1,100 43

1038 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Affecting Composition of Wetland Sediment 1039

rocks) drained by the wetland, the inability of these rocks to 

generate or sustain acid drainage, and as a consequence, the 

lack of acidity to mobilize substantial concentrations of metals 

from the host rock. In addition, the younger upper sediment has 

not been present for a sufficient length of time for ground-water 

transport of dissolved metals and adsorption onto organic matter 

to produce the degree of enrichment found in deeper sediment.

The fact that the maximum values for most elements 

predate mining indicates that historical “background” values 

in these organic-rich sediments have exceeded present-day 

values. In addition, only minor variations in element 

concentrations are seen above about 6 cm when mining 

would have begun. The relative lack of fluvial clastic 

deposition, the long flow path from the upslope water 

source to and through the alluvial fan, and higher element 

concentrations in deeper premining sediment, all suggest 

that episodic transport of metal-bearing ground water 

through the alluvial fan from the upslope altered rocks of 

peak 3,792 m has led to metal enrichment of organic matter 

at the site.

Figure 2. Downhole plot of total Al, Ca, Fe (weight percent), and Cu, Pb, Zn, and As (ppm) in sediment from core 97ABS321, South 

Fork Mineral Creek.

65

55

45

35

25

15

5

0.1 1 10 100 1,000

Wt pct org

As ppm

Zn ppm

Pb ppm

Cu ppm

Ca pct

Al pct

Fe pct

CONCENTRATION, IN WEIGHT PERCENT OR PARTS PER MILLION

Wt pct ash

DEPTH, IN CENTIMETERS

Start

of mining

C-14 date

710 yr B.P.

50 80

(Arsenic values

below detection

are plotted as

10 ppm)

EXPLANATION
Core 97ABS322

This core site is 0.5 km downstream (east) of core 

site 97ABS321 on South Fork Mineral Creek, about 1.5 km 

west of the Silverton caldera margin. Host rocks are San Juan 

Formation volcanics that are weakly altered (WSP) and 

mineralized in this area (fig. 1). Directly north of this site is 

the WSP-QSP-altered, subeconomic, porphyry copper-molybdenum deposit of peak 3,792 m. This wetland is potentially 

subject to a greater influence of acid drainage because ground 

water draining south from peak 3,792 m could intersect it. 

Surface water from peak 3,792 m drains into South Fork 

Mineral Creek downstream from this site and should not 

affect metal deposition in the wetland.

The 14C age date at approximately 66 cm core depth was 

1,060±130 years B.P. An estimated average accumulation rate 

is 0.06 cm/yr, and in the 130 years since mining began, about 

8 cm of material would have been deposited in the wetland.

Samples from this core had among the lowest abundances of total iron from all sites; iron exceeded 5 weight 

percent in only two samples (fig. 3). An interval of relatively 

high iron concentration is in premining sediments in the 

80

70

60

50

40

30

20

10

0

0.1 1 10 100 1,000 10,000

As ppm

Zn ppm

Pb ppm

Cu ppm

Ca pct

Al pct

Fe pct

Start of

mining

C-14 date

1,060 yr B.P.

(Arsenic values

below detection

are plotted as

10 ppm)

CONCENTRATION, IN WEIGHT PERCENT OR PARTS PER MILLION

DEPTH, IN CENTIMETERS

EXPLANATION

Figure 3. Downhole plot of total Al, Ca, Fe (weight percent), and Cu, Pb, Zn, and As (ppm) in sediment 

from core 97ABS322, South Fork Mineral Creek.

1040 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Affecting Composition of Wetland Sediment 1041

30–50 cm interval. Similarly, aluminum ranges from about 

6 to 10 weight percent in this 30–50 cm interval, among the 

highest values for aluminum in the core. The highest aluminum value in this core, 12 weight percent at 75 cm, is the 

highest aluminum concentration of all nine cores that were 

analyzed. Gray clay and silt prominent in the deepest intervals 

(70–78 cm) are the likely sources of the elevated aluminum.

Iron and aluminum generally decline in concentration 

above 8 cm, that is, in the postmining period. In the uppermost 

two samples, aluminum and calcium show minor decreases, 

a possible indication of the cessation of mining activities 

approximately 30–40 years ago. Iron shows a slight increase 

in the top 1 cm, so a source of iron remains available to the 

wetland.

Copper (760 ppm) and zinc (3,000 ppm) maxima are 

found in premining sediments at the 50 and 44 cm depths, 

respectively. The copper concentration remains essentially 

constant at about 130 ppm above 8 cm. The rather low and 

constant concentration of copper above 8 cm suggests that 

mining did not greatly enhance the flux of copper to this 

wetland. However, copper is, on average, 2.6 times higher in 

this core than in the upstream core (97ABS321). The higher 

copper values could indicate greater proximity to the porphyry 

copper-molybdenum deposit of peak 3,792 m. Arsenic was 

below the detection limit of 10 ppm in most sampled intervals, 

and the maximum value was 37 ppm (38 cm). The low arsenic 

concentration reflects its low abundance in the nearby rocks 

and in ground water and surface water that drain into the wetland (Bove and others, this volume).

Similar to core 97ABS321, only lead and zinc show 

analytically significant variation in concentration in modern 

times. Lead shows a distinct spike in concentration at 6 cm, 

or about 100 years ago. This value (130 ppm) is anomalous 

compared to other lead values (15–20 ppm) in this upper 8 cm 

of section. Likewise, normalization of trace-element data to 

aluminum or iron showed similar behavior and associations as 

in core 97ABS321. Aluminum and lead appear to be of detrital 

origin, whereas copper and zinc are more closely associated 

with organic matter or secondary iron oxyhydroxide, and iron 

and arsenic are associated with iron oxyhydroxide.

A regression plot of lead versus aluminum (not shown) 

from the core samples indicates a strong positive correlation (r = 0.92) with aluminum, suggestive of lead hosted by 

detrital clay or feldspar, but the 6 cm sample lies farthest off 

the regression line. This 6 cm sample displays a weaker correlation of lead with iron (r = 0.64). The lack of correlation 

of the 6 cm, high-lead sample with both aluminum and iron 

suggests that lead is present neither in detrital clay or feldspar, 

where it is often a trace constituent, nor in ferric oxhydroxide 

(Zanker and others, 2002). This was the only sample where 

lead exhibited such a distinct spike of high concentration in 

the postmining era. The opening of numerous mines upstream 

and prospects on the slopes of peak 3,792 m may have led to 

an episodic increase in local movement of mine, mill, or ore 

waste by wind or water.

Similar to core 97ABS321, the wetland of 

core 97ABS322 has been only slightly affected by mining 

(primarily by lead and zinc) in modern times, and historically, major- and trace-element abundances have been equal 

or higher. The WSP- and QSP-altered rocks north of the 

wetland are the most likely sources of metals to this site on 

the South Fork Mineral Creek drainage. Copper is significantly elevated in core 97ABS322 compared to the upstream 

core (which is not as directly affected by drainage from peak 

3,792 m). High concentrations of all metals except copper 

in the bottom of the core (approximately 72–78 cm) suggest 

addition of these elements from a source less directly influenced by peak 3,792 m.

Aqueous Chemistry

Water chemistry in the wetlands along South Fork Mineral 

Creek contrasts strongly with that of iron bogs (Stanton, Yager, 

and others, this volume, Chapter E14) and that of other major 

drainages and associated wetlands in Cement Creek and 

Mineral Creek. South Fork Mineral Creek sites contain the 

lowest levels of dissolved major and trace constituents of any 

group of waters sampled for this study (also see Bove and others, this volume). Figure 4 illustrates the low concentrations 

of (the sum of) the four trace elements copper, lead, zinc, and 

arsenic versus pH in organic-rich wetlands compared to water 

samples from other environments in the watershed study area. 

The four samples associated with the wetlands on South Fork 

Mineral Creek, even acidic waters (pH=3.2), have low concentrations of dissolved trace elements (solid squares; fig. 4) 

compared to other drainages. In these four samples, sulfate 

did not exceed 130 ppm, iron and aluminum did not exceed 

1 ppm, and the means for trace-element concentrations were all 

below 10 μg/L except zinc at 50 μg/L (table 2). The mean pH is 

relatively high (4.5) with a maximum of pH 5.7 in one sample. 

The water chemistry in the South Fork Mineral Creek wetlands 

reflects (1) the favorable composition of the rock units for 

not generating metal-rich drainage (even though the pH may 

be acidic), (2) the lack of continual deposition of significant 

amounts of mineralized detrital material, and (3) the ability of 

the wetland sediment to remove some of the dissolved constituents from influent water.

Mineral Creek

Core 999292

This core was obtained from the largest (50×50 m) beaver 

pond in the southern portion of the Mineral Creek drainage. The 

site is about 1 km southeast of the confluence of South Fork 

Mineral Creek and mainstem Mineral Creek. The upper 20 cm 

of the core is high in organic content (as much as 70 weight 

percent). Any inorganic (clastic) sediment was probably winddeposited, because the site is 100 m west of the main Mineral 

Creek channel and 1 m above the high-stage level, and likely 
not subject to fluvial deposition. This site is at the foot of Sultan 

Mountain (fig. 1), a peak composed of unaltered and unmineralized intrusive rocks of the Sultan Mountain stock.

No 14C date was obtained from this core, but 210Pb age 

dating indicates that at about 20 cm, a minimum in 210Pb activity is found. A best-fit model indicates that sediment from 

the year 1900 should be found at 25 cm, giving an average 

depositional rate of approximately 0.25 cm/yr. Although the 

depositional rate is unlikely to have been constant, material 

above about 25 cm was probably deposited since the beginning of the mining era.

A 137Cs maximum is present in the bottom of the 50-cm 

core. Cesium-137 is a byproduct of nuclear atmospheric testing, which ended in the United States in 1965. Cesium-137 is 

removed from the atmosphere through precipitation events; 

normally, it is rapidly and strongly adsorbed and fixed within 

the upper layers of modern (40–50 year old) sediment. The 

peak of 137Cs distribution can be used as a dating tool to determine the calendar year 1963±2 years in sediment (Robbins 

and Edginton, 1975). The mobility of 137Cs should be limited 

by sorption, but its presence in deeper sediment in the core (as 

old as 200 years) suggests that 137Cs was added to the lower 

portion of the core by ground water.

Unlike the South Fork Mineral Creek cores, organic content and abundance of most reported major and trace elements 

in the Mineral Creek core 999292 generally exhibit a progressive increase from about 30 cm (about the time mining started; 

fig. 5). Modern (0–20 cm) iron, copper, zinc, and arsenic are 

consistently higher than premining values found at depths 

of 20–50 cm. One possible explanation for the increases is 

a greater contribution of windblown material derived from 

nearby areas of mining activities. This suggests that mining 

has had a noticeable impact on deposition in the wetland and 

that organic matter is a possible host of metals.

Figure 4. Plot of the sum of trace elements (Cu, Pb, Zn, and As; μg/L) versus pH in water samples from different depositional 

environments, Animas River watershed study area. Four samples from South Fork Mineral Creek wetlands are shown by solid 

squares; one water sample from a Mineral Creek wetland is shown by the solid triangle.

0

500

1,000

1,500

2,000

2,500

Cu+Pb+Zn+As, IN MICROGRAMS PER LITER

3 3.5 4 4.5 5 5.5 6

pH, IN STANDARD UNITS

Wetland, bar

Wetland

Stream

Spring

Seep

Iron bog

Adit

EXPLANATION

1042 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Affecting Composition of Wetland Sediment 1043

A strong correlation of arsenic with iron was observed 

in this core. Although total iron and arsenic concentrations are 

much lower in this core compared to iron bog samples from 

acid-sulfate altered terranes, the correlation coefficient was 

high for arsenic versus iron (r = 0.83). This correlation reflects 

the known strong affinity of arsenic for solid iron oxyhydroxide (Smedley and Kinniburgh, 2002). Additionally, iron and 

arsenic generally mimic the LOI curve (in core intervals where 

these three analyses are available), suggesting that the iron 

solids with coprecipitated(?) arsenic are retained on organic 

matter. The fact that arsenic in the upper 30 cm is about 

two to five times higher than in the lower intervals (below 

30 cm) indicates that material with a higher arsenic content 

has been deposited and retained in the wetland since the start 

of mining.

In contrast to the reach studied along South Fork 

Mineral Creek, main Mineral Creek was the site of numerous major mines along its course (Church, Mast, and others, this volume, Chapter E5), and numerous mine-waste 

piles are present in close proximity to the site (within 

1–2 km). The wetland is sustained by drainage from Sultan 

Mountain to the southwest, but because the intrusive stock 

is unaltered and unmineralized, the likelihood that high 

concentrations of trace elements were dissolved and transported from that source area is not great. Thus, windblown 

transport of metal-rich mining waste from sites along main 

Mineral Creek is the preferred explanation for recent metal 

enrichment in this wetland.

Aqueous Chemistry

One water sample from a small (<5 m diameter) wetland 2.5 km below the South Fork–Mineral Creek confluence was analyzed (solid triangle in fig. 4). The sample site 

is 300 m southeast and across Mineral Creek from the site of 

core 999292 but has no connection to the drainage of that site. 

The wetland exists on the surface of a 1-m thick bar of alluvium adjacent to the streambank that was probably deposited 

before mining began. Concentrations of dissolved iron and 

aluminum were about 10 times higher and zinc 6 times higher 

than water in South Fork Mineral Creek and were probably the 

result of Mineral Creek’s being a drainage for the acid-sulfate 

altered rocks of the Silverton caldera. Mean values of copper, 

lead, and arsenic are comparable to those of the South Fork 

Mineral Creek water samples. Compared to water from other 

environments in the area, all five wetland water samples clearly 

contain much lower concentrations of (total) dissolved trace 

elements (fig. 4).

Cement Creek

Core 97ABS324

This wetland, informally known as the east upper bog, 

is about 30 m east of the acidic, iron-rich upper bog of 

Cement Creek (Stanton, Yager, and others, this volume), and 

10 m east of the creek itself (Church, Fey, and Unruh, this 

Table 2. Individual and summary statistics of aqueous major- and trace-element concentrations (μg/L), calcium and sulfate 

concentrations (mg/L), and water chemistry parameters from wetlands, South Fork Mineral Creek and Mineral Creek.

[ND, not detected]

Al Fe Ca SO4 Cu Pb Zn As pH SpC1DO2

South Fork Mineral Creek wetlands

Surface water at core 

site 97ABS321....... 950 470 39 100 8.0 ND 50 5 3.25 890 6.5

Ground water between 

sites of 97ABS321 

and 97ABS322....... 100 230 43 130 2.6 1.3 7.3 ND 5.66 270 0.5

Surface water between 

sites of 97ABS321 

and 97ABS322....... 750 640 42 120 2.1 1.8 32 ND 4.56 290 7.0

Ground water at core 

site 97ABS322....... 740 410 45 120 3.7 1.7 36 ND 5.10 280 7.0

Mean 635 440 42 120 4.1 1.6 31 5 4.5 430 5.2

Number 4 4 4 4 4 3 4 1 4 4 4

Minimum 104 170 39 100 2.1 1.3 7.3 5 3.2 270 0.5

Maximum 950 640 45 130 8.0 1.8 50 5 5.7 890 7.0

Median 740 440 42 120 3.2 1.7 34 5 4.8 280 6.8

Mineral Creek wetland

4,950 7,390 12.7 80 2.1 0.8 190 2 4.30 210 3.5

1

SpC, specific conductance in microsiemens/centimeter (μS/cm).

2

DO, dissolved oxygen in parts per million (ppm).
volume, Chapter E12, fig. 21). In common with most wetlands, it has a variety of grasses, aquatic plants, and algae 

growing in or around a small shallow pond (10 m diameter, 

1 m deep). The drainage basin of Cement Creek includes the 

acid-sulfate zone farther upstream and on slopes to the west, 

and propylitically altered rocks to the east (fig. 1). Ground 

water that sustains the wetland originates from the propylitically altered but unmineralized rocks on the east side of 

Cement Creek. This wetland is an uncommon hybrid type 

in which iron and organic matter coexist in relatively high 

abundance in the upper 22 cm. Organic matter is higher 

in the upper 22 cm than in most iron bogs but lower than 

in most wetlands, ranging from about 1 to 10 weight percent. Iron concentrations are remarkably uniform throughout 

this core, generally 5–10 weight percent (fig. 6). The presence of iron oxyhydroxide throughout the sediment column 

suggests that the iron staining is primarily postdepositional. 

The five intervals in which iron concentrations approach 

25 weight percent provide clues to the depositional and geochemical history of the site.

Figure 5. Downhole plot of Al, Ca, Fe (weight percent), and Cu, Pb, Zn, and As (ppm) in sediment from 

core 999292, Mineral Creek.

50

45

40

35

30

25

20

15

10

5

0

0.1 1 10 100 1,000 10,000

Wt pct org

As ppm

Zn ppm

Pb ppm

Cu ppm

Ca pct

Al pct

Fe pct

1900

CONCENTRATION, IN WEIGHT PERCENT OR PARTS PER MILLION

DEPTH, IN CENTIMETERS

EXPLANATION

Core 999292

A.D.

about

1044 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Affecting Composition of Wetland Sediment 1045

Premining Sediment

Sediment from 74 cm to approximately 22 cm records 

continuous deposition of Cement Creek-derived clastic 

material (pebbles, gravel, sand) and shows good correspondence with the geochemical and sedimentological characteristics of premining sediment at other sites in Cement Creek 

(Church, Fey, and Unruh, this volume, fig. 21, site B13). Thus, 

the sediment likely represents premining conditions in the wetland. With the exceptions of copper and zinc, element abundances are fairly uniform throughout the 74–22 cm interval. 

The greater variability and covariation of copper and zinc in 

deeper horizons (32–72 cm) may be controlled by temporal 

variability in ground-water chemistry or delivery of metal-rich 

detritus (fig. 6).

Figure 6. Downhole plot of total Al, Ca, Fe (weight percent), and Cu, Pb, Zn, and As (ppm) in sediment from core 97ABS324, 

Cement Creek.

As ppm

Zn ppm

Pb ppm

Cu ppm

Ca pct

Al pct

Fe pct

Railroad

construction

Silty clay,

fine-grained

sediment

Fine-grained

sediment

Silty clay

with thin

fibrous

organic

layers and

small to

large 

pebbles

Fine sand

Coarse to

fine sand

Fine sand

Fine sand

with pebbles,

silt

Coarse sand,

silty/clayey

Organics,

peat

Core 97ABS324

0.1 1 10 100 1,000

(Arsenic values

below detection

are plotted as

10 ppm)

70

60

50

40

30

20

10

0

Description

CONCENTRATION, IN WEIGHT PERCENT OR PARTS PER MILLION

DEPTH, IN CENTIMETERS

EXPLANATION
Mean iron, lead, zinc, and arsenic are 2–3 times higher 

in the upper 21 cm portion of the core compared to mean 

values below 22 cm (table 1). Total aluminum and copper 

concentrations in mining-related sediment are about the same 

as in premining sediment (table 1).

Mining Sediment

The upper 21 cm of the core are affected by anthropogenic activities that altered the depositional environment. A 

railroad bed built on the western margin of the depositional 

site in late 1899 (Jones, this volume) dammed an eastern channel of Cement Creek and created a small artificial closed basin 

(fig. 7). The creation of a closed basin is clearly recorded in 

the lithology (increased organic matter and decreased detrital 

content) and trace-metal chemistry of this core.

The most striking feature in the core is the sharp 

decrease in the concentrations of all reported elements except 

iron in the 19–21 cm interval (fig. 6). Iron shows a complementary steep increase in concentration to more than 40 weight 

percent. The decrease of all elements except iron is the result 

of a temporary pause in deposition of metal-bearing clastic 

material because of barriers put in place during railroad grade 

construction.

However, organic matter from nearby conifers, grasses, 

and aquatic plants continued to be deposited over this 3 cm 

interval, and ground water from the east side of Cement Creek, 

low in most elements except iron because of the lack of mineralization, also continued to reach the area. Aluminum and the 

four trace elements are depleted in the 19–21 cm interval compared to samples above and below. Detectable trace-element 

concentrations (copper, lead, zinc) in the 19–21 cm layers show 

only weak relationships to solid-phase iron. The complete lack 

of arsenic indicates that none was available in the aqueous 

phase, indicating that the ground water feeding the site came 

from the east side of Cement Creek (Bove and others, this volume). These observations suggest that neither coprecipitation 

of aluminum and trace metals with iron oxyhydroxide nor sorption of trace metals to organic matter occurred. As a result, an 

iron-rich, aluminum- and trace-element-poor organic sediment 

formed in the 19–21 cm interval.

Even though aluminum and arsenic are known for their 

ability to coprecipitate with iron oxyhydroxide (Langmuir and 

Whittemore, 1971; Smedley and Kinniburgh, 2002), the lack 

of solid-phase aluminum suggests that the pH of resident water 

was 5.0 or less (Bigham and Nordstrom, 2000; Carlson and 

Kumpulainen, 2000). Another reason for the lack of aluminum 

is that it sorbs poorly to organic matter as the pH increases or 

decreases from pH 5.0, where maximum aluminum adsorption 

to relict algal material has been noted (Sneddon and Pappas, 

1991). Additionally, adsorption of divalent metal cations to 

iron oxyhydroxide surfaces increases as the pH increases 

(Forbes and others, 1976). The greatly reduced concentrations of lead, copper, and zinc in the 19–21 cm interval appear 

to confirm that a low-pH, low-metal water occupied the site 

while the 3 cm of iron-organic sediment formed. A lack of 

available dissolved trace elements for adsorption to iron 

oxyhydroxide (Jenne, 1968) or organic matter (Sneddon and 

Pappas, 1991) is probably the most important factor affecting 

sediment composition in the 19–21 cm interval.

The brief nonclastic depositional episode (about 20 years, 

based on the depositional rate described in the last paragraph 

of this section) was followed by a return to comparable majorand trace-element concentrations in the solids as a result of the 

mining era and resumed clastic deposition. This is represented 

by sediment from about 4 cm to 18 cm. During this period, 

solid-phase element concentrations initially rose steeply, then 

returned to levels 2–3 times higher than premining sediment 

(fig. 6). Iron was an exception and decreased to premining 

levels such as those seen below 22 cm. This is a dilution effect 

in that other elements and clastic material were once again 

being deposited along with iron in the sediment. The return 

of measurable arsenic to the 4–18 cm interval indicates that 

the detrital or aqueous source material was derived, at least in 

part, from the acid-sulfate zone upstream and to the west.

Two major reasons exist for renewed clastic deposition 

during the mining period. First, the railroad fell into disrepair 

around World War I (Jones, this volume) and the creek ultimately breached the railroad bed near the north end of the 

former channel to allow flow back into the basin. Second, a 

large volume of mine and mill waste was placed into upstream 

areas during the peak years of mining. The uniformity of all 

element concentrations in the 4–18 cm interval suggests that 

most material did have similar compositions as a result of 

ore-related separation and preparation processes. Milling processes increased the solid load carried by Cement Creek, and 

some of this detritus was deposited in the wetland.

Detritus accumulated at a relatively rapid rate in the 

wetland: 18 cm of material was deposited in approximately 

80 years (1900–1980), a rate of about 0.22 cm/yr. The similarity of detritus from 4 to 18 cm (silty clay with small to large 

angular pebbles and organic fibers) suggests that all material in 

this interval was deposited under the same conditions. During 

World War II, the railroad rails were removed, and the bed and 

wooden ties left in place have become cemented by 10–15 cm 

high mounds of fine-grained iron oxyhydroxide to form a permanent barrier to deposition from Cement Creek (fig. 7).

Postmining/Remediation Sediment

Above the 4 cm level, nearly all elements show a decrease 

in abundance of 1 to 5 times less than in the levels below. Iron 

is again an exception, increasing from about 5 to 35 weight percent in the top 3 cm of the core. At present, the basin no longer 

receives clastics, as evidenced by the lack of coarse-grained 

material (sand, pebbles) in the upper 3 cm. Organic matter is 

still being deposited, as evidenced by pine needles and fibrous 

organic matter identified in the upper 2 cm.

One possible explanation for the decline in element concentrations (except for iron) is the result of changes in transportation and processing of ore beginning in 1963 (Jones, this 

volume). Metal-rich detritus no longer reached the depositional 

1046 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Affecting Composition of Wetland Sediment 1047

Figure 7. Variations in deposition of clastic (detrital) and organic sediment and iron oxyhydroxide during 

premining, mining, and postmining periods, site of core 97ABS324, Cement Creek.

Cement Creek

Railroad bed

East upper bog

depositional basin

Upper iron bog

Ground water

from acid-sulfate

altered rocks

Ground water continues to

deposit iron oxyhydroxide

around bog site

Thick mound of

accumulated

iron oxyhydroxide

Ground water

from acid-sulfate

altered rocks

Ground water from 

propylitically altered rocks;

low concentrations of trace

elements Stream channels of

Cement Creek

Present-day stream channel

 of Cement Creek

Upper iron bog Cement Creek Site of east upper bog

Not to scale

Fe

Fe

Premining

Upstream tributaries transport metal-bearing

water and clastics to eastern depositional site

Al, Cu, Pb, Zn, As

Al, Cu, Pb, Zn, As

Cement Creek

Railroad bed

East upper bog

depositional basin

Upper iron bog

Ground water

from acid-sulfate

altered rocks

Ground water from 

propylitically altered

rocks continues to supply

dissolved iron to form 

iron oxyhydroxide

Thin mound of

accumulating

iron oxyhydroxide

Core 97ABS324

Present-day stream channel

 of Cement Creek Premining sediment deposited

by Cement Creek

Premining sediment deposited

by Cement Creek

Fe

Mining

Brief interval (≈20 yrs) of nonclastic deposition (railroad barrier),

followed by resumption of deposition of clastics and

mining-related detrital material

Al, Cu, Pb, Zn, As

Organic matter and

iron oxyhydroxide

accumulate during

nonclastic episode.

Detritus reintroduced

during mining era

Organic matter and

iron oxyhydroxide

accumulate

Treatment/holding ponds

upstream from site

WEST EAST

Postmining and remediation

Alluvial bar

Mining cessation (1978)

Upstream water treatment (1994) results in:

Removal of trace elements by iron flocs upstream of site

Limited fluvial clastic deposition in east bog site

Flow
site and reduced the supply of mining-related detritus to the 

wetland. Upstream remediation begun in 1994 and continuing 

to the present included construction of treatment holding ponds 

to reduce the sediment content of water allowed to go downstream. Iron flocs forming in upstream ponds could reduce the 

trace-element content of the treated water via coprecipitation, 

but the water could still contain dissolved iron. Similar to the 

formation of iron oxyhydroxide at 21–19 cm, perhaps iron-rich 

but trace-element-poor water released to Cement Creek eventually reached the depositional site to produce solid iron phases.

However, a more likely situation is that ground water 

from the east side of Cement Creek has again become the major 

contributor of dissolved iron to the site. Interestingly, five of 

the seven samples in which arsenic was not detected came from 

the 19–21 and upper 2 cm intervals, representing points in time 

when arsenic-poor ground water from the east side of Cement 

Creek was the dominant source to the site. Solid samples in 

which arsenic was detected are most likely from detritus and 

water from the upstream (north) reaches of Cement Creek or 

ground water from slopes to the west that drain the arsenicbearing acid-sulfate alteration zone (figs. 1 and 7).

Iron and Aluminum Chemistry at the Site

The measured pH of 4.5 allows a metal-deficient iron 

oxyhydroxide to precipitate. Aluminum is largely controlled 

by gibbsite solubility and nearly conservative up to pH 5.0 

(Bigham and Nordstrom, 2000), and thus, should remain in 

solution and not precipitate. The solid-phase data for highiron sediment are consistent with aluminum chemistry in 

acid waters, that is, a decrease in solid-phase aluminum is 

observed where iron oxyhydroxide dominates the sediment 

(fig. 8). Thus, an iron-rich but metal-depleted iron oxyhydroxide mixed with organic matter and fine-grained (windblown?) 

detritus has been deposited at the site since mining ended.

The five iron-rich samples (1–2 cm and 19–21 cm) are 

characteristic of iron oxyhydroxide that forms in iron bogs 

(compare figs. 8A and 8B). Three of these are samples from 

19–21 cm, the interval that contains the highest levels of iron 

in this core. The other two are from the top 2 cm of the core. 

These five iron-rich samples all have a similar genesis that is 

related to (1) a lack of detrital deposition and (2) the dominance of ground water from the east side of Cement Creek.

Animas River

Sulfate Reduction as a Metal Sequestration 

Mechanism

The Forest Queen wetland was studied in detail to 

determine if it is capable of attenuating influent metal and 

SO4

2– concentrations through formation of metal sulfides, 

and thus improving effluent water quality. The nearly unique 

location of the wetland—adjacent to an acidic, metal-bearing 

outflow from the Forest Queen mine adit—makes it well 

suited for examination of geochemical processes that may 

affect sediment composition and water quality in an acid 

drainage regime. Abundant carbonaceous matter and dissolved sulfate in the wetland make it a potential site of sulfate 

reduction and removal and storage of dissolved metals in 

authigenic sulfides. Iron, copper, lead, zinc, and arsenic 

form sulfide minerals upon reaction with aqueous H2

S (total 

sulfide = H2

S(aq) + HS–

(aq)). In the absence of acid water 

(pH<5) or dissolved oxygen, these sulfides are generally 

insoluble.

Sulfide in organic sediments is produced via reduction 

of sulfate ion. At surface temperatures and pressures, this is 

accomplished through the action of sulfate-reducing bacteria, primarily of the genus Desulfovibrio (Berner, 1970). 

These microbes couple sulfate reduction with organic carbon 

oxidation to produce sulfide (S2–) and ultimately H2

S. Sulfate 

reduction is an anaerobic process that occurs in the absence 

or at very low (sub-ppm) levels of molecular oxygen (O2). 

Sulfate reduction with organic matter oxidation is summarized by the reaction SO4

2– + 2CH2

O → H2S + 2HCO3

–

, where 

CH2O = organic matter. Active sulfate reduction in the Forest 

Queen wetland was evidenced by an H2S odor in many core 

and water samples. Sulfate reduction leads to an increase in 

pH via HCO3

–

 production. Most dissolved metals become less 

soluble as pH increases towards neutral and alkaline values; 

pH values as high as 7 (neutral) were observed in the wetland. 

Metal complexation by certain organic functional groups and 

sorption onto mineral surfaces is also enhanced at higher pH 

(Forbes and others, 1976). Thus, processes that contribute 

to a rise in pH may help mitigate elevated concentrations of 

soluble metals.

Forest Queen Wetland Site

The Forest Queen wetland is 10 km northeast of Silverton 

along the Animas River at an altitude of approximately 2,990 m 

(fig. 1). The surface area is approximately 1 hectare, and the 

organic+sediment layer ranges from less than 0.1 m to at least 

3 m in thickness in areas that were cored. Core locations are 

shown in figure 9.

During the fall sampling, the wetland surface was 

generally dry except for a 1–2 m wide arc of wet vegetation 

around the west edge of the iron oxyhydroxide pool (fig. 9). 

In contrast, during a visit the following spring (no sample 

collection), 3–4 cm of water covered about 70 percent of 

the wetland surface, and an odor of H2S was noted in every 

monitoring well. Drying and lowered water levels allow air 

diffusion and iron and organic matter oxidation to take place at 

greater depths in wetland sediment. Thus, processes of sulfate 

reduction, FeS formation, and FeS oxidation are all subject to 

variability as a result of seasonal changes.

Figure 8 (facing page). Plots of iron versus aluminum in A, 

core 97ABS324 (a mixed organic/iron wetland), and B, core 

97ABS327 (an iron bog).

1048 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Affecting Composition of Wetland Sediment 1049

1

2

3

4

5

6

7

8

9

0 5 10 15 20 25 30 35 40 45

IRON, IN WEIGHT PERCENT

ALUMINUM, IN WEIGHT PERCENT

r

2

 = 0.84

97ABS324

A

0

1

2

3

4

5

6

7

ALUMINUM, IN WEIGHT PERCENT

5 10 15 20 25 30 35 40 45 50

IRON, IN WEIGHT PERCENT

r

2

 = 0.71

97ABS327

B
As a result of geologic and anthropogenic processes, 

surficial material in and around the wetland is composed of 

differing amounts of country rock, road and railroad aggregate, mine-waste rock and tailings, and river-transported 

tailings and sediment from upstream locations (fig. 9). Rocks 

surrounding the site are mainly propylitically altered volcanics of the Burns Member of the Silverton Volcanics, but a 

few isolated areas of WSP (east) and QSP (west) alteration 

are present (fig. 1). Landslides of unknown age from slopes 

above the wetland have deposited country rock rubble into the 

wetland area. Most country rock in the area contains visible 

pyrite, but the extent of mineralization of landslide-deposited 

rock is unknown. Areas east (upslope) and north (valley of the 

Animas River) of the wetland were mined from the late 1890s 

Adit

FQ5

FQ9

FQ7

FQ4

FQ3

Access road

Iron

oxyhydroxide

pool

Inflow

Inflow

Inflow

Outflow

Old railroad

grade

Pond

Pond

Pond

MINEWASTE

PILE

Auto

road

Outflow

FOREST

QUEEN

WETLAND

Culvert

Culvert

Animas

River

Conifers

Conifers

0 10 METERS

Willows and 

grasses

Willows

 and 

grasses

N

Figure 9. Forest Queen wetland and adjacent area. Individual coring locations (numbers prefixed by “FQ”) are shown 

by solid squares.

1050 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Affecting Composition of Wetland Sediment 1051

until the early 1950s (Jones, this volume; Bird, 1999). Railroads hauled the ore, and abandoned railroad beds are adjacent 

to the west and south edges of the wetland (fig. 9). A 4 m-high 

mine-waste rock mound is at the northeast margin of the 

wetland. Water enters the wetland from the collapsed Forest 

Queen mine adit, which now resembles a flowing spring. At the 

time of the October 1997 sampling, the adit inflow had a pH of 

4.7 and contained 0.66 ppm Zn, 0.014 ppm Pb, 400 ppm SO4

2–, 

24 ppm Fe, and 1.8 ppm Al (Pavlik and others, 1999). Water 

flows from the adit down slope for about 10 m, passes through 

a narrow aluminum culvert, and enters a 5 m diameter by 1 m 

deep pool located 1⁄2 meter above the wetland surface. The pool 

(fig. 9) is encrusted with several millimeters of iron oxyhydroxide. Primary surface water flow is directly west out of the 

pool. Water exiting the wetland ultimately discharges into the 

Animas River. The effluent water contains dissolved zinc that 

ranges from 0.12 to 0.17 ppm and dissolved lead from 0.001 

to 0.008 ppm; pH of outflow water at the south edge of the 

wetland was 5.9 (Pavlik and others, 1999). Thus, at the time of 

sampling, the concentrations of lead and zinc in outflow water 

were only about 25 percent of those in the adit inflow. Aluminum was also lower (0.33–0.81 ppm versus 1.8 ppm), but iron 

was higher in some outflow water (1.9–32 ppm versus 24 ppm), 

indicating that iron is mobile in the wetland environment.

Table 3 summarizes water chemistry with respect to 

dissolved iron species and sulfide concentrations in the Forest 

Queen wetland. Because the samples were composited through 

the entire depth of the well, they represent multiple levels in 

the wetland rather than discrete zones. However, these results 

suggest that several redox processes are occurring in close 

proximity within the sediment column. Water chemistry of the 

Forest Queen was monitored for a year both pre- and postremediation; the data are discussed in Church, Mast, and others 

(this volume, figs. 7, 8, 12, and table 7).

Core Descriptions, HCl-Extractable Metal 

Concentrations, and AVS Recovery

Core stratigraphy, interval descriptions, total and acidextractable major-element (Al, Fe, Ca) and trace-element 

(Cu, Pb, Zn, and As) abundances, loss on ignition (LOI), 

ash content, and total acid-volatile sulfide (AVS) recovery 

(S2–

(AVS) ; ppm) are illustrated in a series of downhole plots. 

Acid-extractable and total element concentrations are summarized in tables 4 and 5. The loss on ignition (LOI, weight 

percent) and the ash content (100–LOI) provide estimates of 

organic matter and mineral matter, respectively. The midpoint 

depth of each extracted core interval is used to identify samples in tables or on downhole plots. For example, in core FQ7, 

the midpoint of the 107–125 cm interval is 116 cm; this value 

is used to place the data point on the downhole plot.

Core FQ3

This core was taken 10 m southwest of the iron oxyhydroxide pool; the core site is not in the path of the main 

outflow. Analyzed samples covered a depth interval from 41 

to 163 cm. Total (unleached) metal concentrations were not 

determined for this core, so only acid-extractable values are 

presented. The samples contained the lowest concentrations 

of acid-extractable metals of the five Forest Queen cores, and 

neither copper nor arsenic was detected in any acid-leach 

solution (table 4).

Acid-Extractable Element Concentrations and AVS

From the surface to the bottom of the core, extractable 

element concentrations vary only slightly as illustrated by 

the nearly vertical lines in figure 10. Extractable iron, lead, 

and zinc have their maximum abundances in the 41–71 cm 

interval. Iron decreased from its maximum extractable 

concentration (240 ppm) to the bottom of the core, whereas 

extractable aluminum values clustered in a narrow range from 

27 to 34 ppm in all intervals. Besides iron and aluminum, zinc 

was the only metal present in detectable concentrations in the 

6N HCl solution from all four intervals (fig. 10). Other trace 

metals (for example, lead) had detectable concentrations in the 

41–71 cm interval but were undetectable in the two or three 

deeper intervals. If metal sorption was actively occurring at the 

site of core FQ3, it appears to have happened in the uppermost 

71 cm, which consisted almost wholly of organic matter.

The 41–71 cm sample was black and had similar organic 

content (50–75 percent, primarily peat) to the deeper levels 

but no H2

S odor. The three deeper intervals were also black 

but had equal or higher organic matter content (approximately 

Table 3. Water chemistry parameters from five Forest Queen monitoring wells.

[Total iron, ferrous iron, and total sulfide concentrations are in mg/L. ND, not detected; --, not determined]

Sample No. pH Temp1 SpC2 DO3 Fe(tot) Fe2+ Fe3+ Total sulfide

FQ3 6.9 7.8 1,970 ND 0.64 0.06 0.58 0.07

FQ4 7.0 8.6 1,350 ND 0.11 ND -- 0.91

FQ5 6.9 9.7 1,340 ND 1.29 ND -- 0.29

FQ7 5.8 8.7 1,130 ND 1.45 0.98 0.47 0.66

FQ9 6.3 10.5 1,100 ND 0.32 ND -- 0.89 1

Temp, temperature in degrees Celsius (°C).

2

SpC, specific conductance in microsiemens/centimeter (μS/cm).

3

DO, dissolved oxygen in parts per million (ppm).
Table 4. Summary statistics for acid-extractable concentrations of major and trace elements from Forest Queen wetland cores.

[All values in parts per million (ppm). <LLD, insufficient number of samples above detection to calculate parameter]

Al Fe Ca Cu Pb Zn As

Core FQ3

Mean 30.5 97.8 47.5 <LLD 0.2 3.2 <LLD

Number 4 4 4 <LLD 2 4 <LLD

Minimum 27.0 28.0 30.0 <LLD 0.1 1.6 <LLD

Maximum 34 240 58 <LLD 0.2 5.9 <LLD

Median 30.5 61.5 51.0 <LLD 0.2 2.6 <LLD

Core FQ4

Mean 68.5 477 33.3 0.2 0.8 6.0 <LLD

Number 6 6 6 4 6 6 <LLD

Minimum 17.0 240 14.0 .05 0.6 1.1 <LLD

Maximum 210 750 84 0.3 4.0 13 <LLD

Median 34.5 460 22 0.2 0.2 6.0 <LLD

Core FQ5

Mean 117 197 51.4 0.6 2.6 5.7 <LLD

Number 9 9 9 7 9 9 <LLD

Minimum 6.5 31.0 22 0.1 0.2 170 <LLD

Maximum 210 350 80 1.1 5.4 12 <LLD

Median 120 240 50 0.5 2.7 4.9 <LLD

Core FQ7

Mean 300 255 65.7 1.5 7.7 12.3 <LLD

Number 10 10 10 10 10 10 <LLD

Minimum 88 44 17.0 0.2 0.6 1.5 <LLD

Maximum 600 630 100 3.4 27 20 <LLD

Median 290 230 65.5 1.4 6.7 12.0 <LLD

Core FQ9

Mean 59.0 133 34.0 0.2 0.5 4.1 <LLD

Number 3 3 3 1 3 3 <LLD

Minimum 23 78 13.0 0.2 0.5 2.3 <LLD

Maximum 92 200 55 0.2 0.6 7.1 <LLD

Median 62 120 34 0.2 0.5 2.9 <LLD

Table 5. Summary statistics for total concentrations of major and trace elements from Forest Queen wetland cores.

[Al, Fe, and Ca in weight percent; Cu, Pb, Zn, and As in parts per million (ppm)]

Al Fe Ca Cu Pb Zn As

Core FQ4

Mean 3.9 34.5 1.4 140 142 2,030 34

Number 6 6 6 6 6 6 6

Minimum 2.2 6.1 1.0 44 95 170 9.0

Maximum 8.1 48.7 2.3 215 240 3,250 48

Median 2.3 44.5 1.2 145 130 2,600 42

Core FQ5

Mean 5.8 18.8 1.8 115 350 1,500 38

Number 9 9 9 9 9 9 9

Minimum 2.2 3.4 0.5 80 90 535 6

Maximum 8.4 50.1 5.4 150 525 2,810 58

Median 7.0 9.4 1.1 110 425 1,210 39

Core FQ7

Mean 8.5 3.7 0.8 120 635 1,120 40

Number 10 10 10 10 10 10 8

Minimum 7.6 3.0 0.6 52 230 625 8.0

Maximum 10.9 5.4 1.0 180 1,600 2,250 73

Median 8.2 3.4 0.8 115 480 860 34

1052 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Affecting Composition of Wetland Sediment 1053

75 percent, primarily wood fragments) and a strong H2

S odor. 

The black color in all four intervals and H2

S odor in the lower 

three intervals suggested the presence of FeS, yet trace-metal 

abundances in the AVS-leach solution of these sample intervals were low compared to cores FQ4 and FQ5 (table 4).

The low concentrations of extractable metals in core FQ3 

were unexpected because the moderate to high yields of 

S2–

(AVS) below 71 cm indicated abundant FeS and thus potential immobilization of metals in AVS solids. The relative lack 

of extractable metals in core FQ3 suggests that the area was 

not well connected hydrologically with inflow (surface and 

possibly subsurface) water at depths greater than about 71 cm. 

Alternatively, metals may be most strongly associated with 

iron oxyhydroxide above 41 cm. When the core was extracted, 

the wetland surface was damp but not flooded, and upper core 

material was coated by large amounts of iron oxyhydroxide, 

suggesting little recent flow of water into the area and oxidation 

of aqueous iron.

Black,

 75 percent

organic matter,

wood fragments;

minor clastic

sediment;

strong H2S odor

Black,

 75 percent

organic matter,

wood fragments;

minor clastic

sediment;

strong H2S odor

Dark brown to 

black, 50-75

percent

organic matter,

wood fragments;

minor clastic

sediment; 

strong H2S odor

Black,

50-75 percent

organic matter;

no wood

fragments

or H2S odor;

minor clastic

sediment

 

132

102

163

71

41

0.01 0.1 1 10 100 1,000

AVS ppm

 Zn ppm

 Pb ppm

 Ca ppm

 Fe ppm

 Al ppm

DEPTH, IN CENTIMETERS

Core FQ3

CONCENTRATION, IN PARTS PER MILLION

(Values below

detection limits

are not plotted)

EXPLANATION

wood

fragments

Description

Figure 10. Plot of acid-extractable major- and trace-element concentrations with depth, Forest Queen core FQ3.
Core FQ4

This core was taken 10 m directly west of the iron oxyhydroxide pool and directly in the path of the major surface outflow from the pool (fig. 9). The length of analyzed core covers 

the interval from 20 to 152 cm. Sediment above approximately 

81 cm was primarily organic rich, clast-free material; sediment 

below about 81 cm contained clay and sand mixed with organic 

matter. In addition to composite samples, a “discrete” 1 cm 

sample at 41 cm was taken to examine the effect that compositing had on the metal values. The 20–41 cm interval and the 

41 cm discrete sample were macroscopically identical, consisting of homogeneous, iron-stained organic matter. Total metal 

concentrations between the composite and discrete samples differed by ±1.8 percent for aluminum and ±5.3 percent for iron.

Total Element Concentrations

Total iron, aluminum, and calcium concentrations in the 

upper four organic-rich intervals (0–81 cm) varied only slightly 

from their mean values over these intervals. Iron varied by 

±7.5 percent, aluminum varied by ±3.5 percent, and calcium 

varied by ±14 percent. Copper, lead, zinc, and arsenic showed 

similar uniformity in abundances in the upper organic intervals. 

Thus, the relatively homogeneous organic horizon (down to 

about 81 cm) showed fairly uniform metal concentrations, suggesting a similar source of metals and mode of sequestration in 

these intervals. Periodic flushing of the pool (spring runoff?) 

probably results in transport of additional amounts of solid iron 

oxyhydroxide to surficial sediment in this part of the wetland.

In the 99–117 cm interval, most metal concentrations 

decreased but then showed slight enrichment in the bottom 

interval at 147 cm. For example, total iron at 147 cm increased 

from 6.1 to 15.9 weight percent. The sharp drop in metal 

concentrations, followed by an increase, suggests that another 

source of metal (other than percolating surface water) is available to the sediment.

Acid-Extractable Metal Concentrations and AVS

The percentages of extractable metals ranged from 

around 0.5 to 4 percent within the organic layers up to the 

81 cm depth (fig. 11). Below the 61–81 cm interval, the percentages of extractable iron, lead, and zinc increased down to 

the 98–117 cm black, organic-rich interval. The percentage of 

extractable iron then showed a sharp decrease in the bottom 

interval whereas the percentages of extractable lead and zinc 

increased.

In contrast to the variation in extractable iron, extractable aluminum generally increased with depth and reached its 

maximum extractable abundance (1.5 percent; 210 ppm) in 

the deepest interval. Zinc and lead tended to follow aluminum 

in this respect. The downhole increases in extractable metals 

such as aluminum are likely the result of a larger proportion of 

silicate sediment in the deeper levels of the core.

HCl-extractable lead and zinc were found in relatively 

higher concentration in two different intervals in core FQ4. 

High extractable zinc and lead concentrations were at 41 cm 

and at the bottom of the core (fig. 11). This result is similar 

to core FQ3 in that a peak in extractable zinc was seen in the 

uppermost (organic) 60 cm of core. Perhaps zinc is being 

adsorbed by organic matter as metal-bearing water flows 

over and (or) percolates into the surficial organic layers. 

Alternatively, the dominantly dark gray color of sediment in 

the upper 60 cm was indicative of FeS, and moderate AVS 

yields were obtained from these intervals, suggesting that 

perhaps metals were also being sequestered into AVS-related 

phases. The coincidence of high zinc with high AVS at 41 cm 

strongly suggests that the extractable zinc is within an AVS 

solid. Although both organic matter adsorption and authigenic 

sulfide mineral formation processes may be operative in the 

upper 60 cm, the relative magnitude of each is not quantifiable 

with the data at hand. Extractable copper was present only in 

the four upper organic samples, but not in silicate sediment 

intervals below approximately 81 cm.

AVS (acid-volatile sulfide) data from core FQ4 indicated 

that sulfate reduction ceases below approximately 102 cm in 

this area. No sulfide was recovered from the 6N HCl leach of 

two of the deepest intervals in this core (fig. 11).

In the interval 41–61 cm, extractable iron, copper, lead, 

and zinc contents are probably more closely associated with 

organic matter or iron oxyhydroxide because S2–

(AVS) recovered 

from the 6N HCl leach was low (16.8 ppm). Below about 

81 cm, extractable zinc and lead increased whereas AVS 

recovery dropped to zero. The black organic layer centered 

at 108 cm showed lower total zinc and lead concentrations 

compared to lighter colored organic layers. Black lenses at 

142–152 cm initially suggested the presence of FeS, but the 

zero yield of AVS indicated only organic sources of the black 

coloration. Thus, in the deepest intervals (below about 81 cm), 

zinc and lead were associated with organic matter but not 

iron oxyhydroxide or FeS, because the latter two phases were 

essentially absent.

Core FQ5

Core FQ5 was taken about 10 m northwest of the iron 

oxyhydroxide pool; it represents an interval from 0 to 183 cm. 

The area receives substantial inflow from the iron oxyhydroxide pool as a result of a breach in the pool wall. Similar 

to core FQ4, the upper 61 cm is organic rich with increasing 

clastic content in deeper intervals.

Total Element Concentrations

Total metal concentrations were uniform for almost 

the entire depth of this core, with increases of 2 to 5 times 

as much iron, calcium, and zinc near the base (165–173 cm) 

compared to the interval immediately above. Total metal 

concentrations in approximately the upper 60–80 cm 

were similar to those in core FQ4: iron ranged from 37 to 

Figure 11 (facing page). Plot of total element concentrations, 

organic matter, AVS yield, and percentage of acid-extractable 

elements with depth, Forest Queen core FQ4.

1054 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Affecting Composition of Wetland Sediment 1055

Light gray brown;

organics and peat

dominant (50-75

pct), mainly roots

w/ some wood chips;

visible Fe-oxide

staining at bottom

of interval

Dark gray; >50 

percent peat and

organic matter (roots);

oxidized Fe stains on

50 percent of surface

Dark-gray and red

(Fe-oxide)-stained

organic matter and

peat ( 75 percent) 

Organic rich

( 60 percent)

w/ red Fe staining;

minor clay and sand;

H2S odor in interior

Clay w/ high organics;

a few black lenses

(FeS?)

Clay + organics,

small black lenses

(FeS?)

Black, organic rich

w/ clay

Black, organic rich

Gray, coarse sand

Gray clay w/

30 percent organics

Mixed gray clay

and sand;

minor black lenses

(FeS?)

0

20

41

61

81

98

117

127

142

152

Description Core FQ4

CONCENTRATION, IN WEIGHT PERCENT OR PARTS PER MILLION PERCENT LEACHED

Values below detection limits

are not plotted

*All arsenic leachate values

were below the detection limit

of 0.2 ppm

As above

 

No sample

analyses in

0-20 cm

interval

No sample

analyses in

81-98 cm

interval

No sample

analyses in

117-142 cm

interval

Total As ppm

Total Zn ppm

Total Pb ppm

Total Cu ppm

Total Ca pct

Total Al pct

Total Fe pct

0.1 10 1,000 2 4 6 8

Pct As leached*

Pct Zn leached

Pct Pb leached

Pct Cu leached

Pct Ca leached

Pct Al leached

Pct Fe leached

EXPLANATION

DEPTH, IN CENTIMETERS

AVS ppm

Wt pct org

Sand

Clay

Silty/clayey

 sand

FeS(?)

 lenses 

Organic

 matter

 50 percent

 organic matter

Sediment Type

0 60 20 40 80
50 weight percent, and aluminum was relatively uniform at 

about 2–2.5 weight percent. The similar iron and aluminum 

values in cores FQ4 and FQ5 are likely the result of their 

close proximity to surface flow out of the iron oxyhydroxide 

pool. And as in core FQ4, an enrichment of iron occurred 

in a deeper interval (165–173 cm), to 9.4 weight percent.

Acid-Extractable Element Concentrations and AVS

The percentage of extractable metals generally increased 

with increasing depth (fig. 12), with lower extractable concentrations in the uppermost intervals (above 81–102 cm). Recoverable sulfide was variable in the intervals above 102 cm but 

showed some sharp increases with depth. Extractable copper 

showed strong correlations with organic matter (r = 0.93) and 

with acid-volatile sulfide (AVS) (r = 0.94); extractable zinc 

showed a strong correlation with organic matter (r = 0.92). 

Correlation coefficients of extractable copper, lead, and zinc 

with total iron were less than 0.6.

The organic-rich intervals (from 81 to 145 cm) were 

separated by a gray layer of silt-clay sediment at 102–122 cm 

(fig. 12). This silt-clay layer showed evidence of iron oxidation, primarily rootlets whose outer surfaces were colored 

red (assumed to be iron oxyhydroxide). This clastic layer has 

extractable metal contents similar to those in the relatively 

organic rich layers that encompass it, indicating that both 

clastic and organic layers can contain similar amounts of 

HCl-extractable metals.

Two black intervals with a strong H2S odor (122–145 

and 145–165 cm) produced the highest AVS yields in the core, 

and showed high extractable copper, lead, and zinc concentrations, strongly suggesting that metals are associated with AVS. 

In contrast, the interval immediately beneath (165–173 cm) 

showed a sharp decrease in all acid-extractable element concentrations even though AVS remained moderately high. This interval contained 20 percent clay, along with some woody material; 

it had a slight H2

S odor. The bottom interval (173–183 cm) 

exhibited an increase in extractable lead and zinc concentrations, but the AVS yield decreased to only 15 ppm. These 

variations in extractable metal concentrations and AVS indicate 

that a significant AVS yield can, but does not always, indicate 

enrichment of extractable metals in AVS.

The similarity of total metal abundances in clastic and 

organic horizons in core FQ5 suggests that both types of sediment are equally capable of sorbing metals in the wetland. 

The residence of metals within specific mineral phases in the 

clastic sediment or specific compounds in the organic fraction 

is not known. The extractable metals are likely loosely bound 

on mineral surfaces or within highly acid soluble phases like 

(noncrystalline) iron oxyhydroxide.

Core FQ7

Core FQ7, about 45 m west of the iron oxyhydroxide 

pool on the west edge of the wetland, encompasses a 0–246 cm 

interval; the site is surrounded on three sides by ponded water 

(fig. 9). Core FQ7 is the deepest of the cores, and it illustrates 

the complexity of the wetland stratigraphy, as organic-rich 

upper layers are underlain by increasingly clastic (clay, silt, 

and sand) sediment with depth. Iron staining was present in the 

upper 0–16 cm and in the 59–68 cm interval. Except for deeper 

clastics, the sediment was dark gray brown to black with several intervals exhibiting an H2

S odor.

Total Element Concentrations

Total major-element concentrations in core FQ7 were 

much lower than in core FQ4 or FQ5 and showed little variation with depth (fig. 13). For example, iron ranged from 3 to 

3.5 weight percent in intervals to approximately 190 cm. In 

contrast, total iron in cores FQ4 and FQ5 approached 50 weight 

percent in the upper 60–70 cm. In common with cores FQ4 and 

FQ5, however, a slight increase (to 5 weight percent) in the total 

iron concentration was seen near the bottom of the core (about 

200 cm). The low total iron concentrations probably reflect 

the lack of transport of solid or aqueous iron to this area of the 

wetland.

Total aluminum was higher in most intervals in core FQ7 

than in comparable intervals in core FQ4 or FQ5. The increasing clastic content in core FQ7 is likely the result of an 

increasing contribution to sediment deposition by the Animas 

River in lower intervals and windborne deposition in higher 

intervals. A slight enrichment of zinc and lead was present in 

the 41–71 cm interval, and this coincides with a peak in AVS 

recovery and a black zone of organic matter, suggesting that 

lead and zinc may be hosted in an AVS phase.

Acid-Extractable Element Concentrations and AVS

Figure 13 shows variations in extractable aluminum and 

iron in deeper intervals (approximately 183–246 cm) that suggest significant changes in local chemistry or sediment composition. The average organic matter content was lower in this 

core (40.7 weight percent) compared to core FQ4 (58.9 weight 

percent) and core FQ5 (43.3 weight percent), which suggests 

that a higher proportion of clastic material has been deposited 

along with the organic material. Because the sulfide content 

was fairly uniform over the 150–246 cm interval, and iron 

oxyhydroxide staining was minor, increasing clastic content 

is likely responsible for the varying acid-extractable element 

concentrations.

Scatter diagrams of iron, copper, zinc, and lead versus 

S2–

(AVS) (fig. 14) indicate two sample populations in core FQ7. 

This is interpreted as two different associations of each 

metal—an amount associated with monosulfide or organic 

matter, and an amount associated with sparingly soluble sulfide minerals. Detrital sulfide-bearing minerals within clasticrich horizons are derived from sources such as mineralized 

country rock, or deposit-related material spilled from railroad 

cars or used for road ballast. (The proximity of this core to the 

Figure 12 (facing page). Plot of total element concentrations, 

organic matter, AVS yield, and percentage of acid-extractable 

elements with depth, Forest Queen core FQ5.

1056 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Affecting Composition of Wetland Sediment 1057

Black,

some red staining,

strong H2S odor

Black,

organic rich,

strong H2S odor

Brown, peat and

organic rich,

minor clastic 

sediment,

no H2S odor

20 percent clay,

woody fragments,

minor H2S odor

Dark gray,

some black

specks/lenses

(FeS?). Strong 

H2S odor

Slightly sandy

Gray sand, silt,

and clay.

Red stringers of

organic rootlets

(Fe-oxidation?)

Core FQ5

CONCENTRATION, IN WEIGHT PERCENT OR PARTS PER MILLION

Med. gray,

sandy and clayey,

organic rich and

visible Fe-oxide

Black, 50 percent

organics

Dk. brown, >50

percent organics

Dark gray to

black, 30 percent

organics

Description

122

145

165

173

183

102

81

76

69

61

0

Total As ppm

Total Zn ppm

Total Pb ppm

Total Cu ppm

Total Ca pct

Total Al pct

Total Fe pct

2 4 6 8

Pct As leached*

Pct Zn leached

Pct Pb leached

Pct Cu leached

Pct Ca leached

Pct Al leached

Pct Fe leached

PERCENT LEACHED

EXPLANATION

0.1 1,000 10

DEPTH, IN CENTIMETERS

AVS ppm

Wt pct org

Values below detection limits

are not plotted

*All arsenic leachate values

were below the detection limit

of 0.2 ppm

Wood

 fragments

Sand

Clay

Silty/clayey

 sand

FeS(?)

 lenses

Organic

 matter

Sediment Type

15 75 45 105

(144)

> 50 percent

 organic matter

 50 percent

 organic matter
Core FQ7

Dark gray clay/silt, wood frags.

Sandy w/ yellow grains

mixed w/ gray, finer grained

matrix, some organic matter

(rootlets, sm. wood fragments)

Red-brown peat and

organic material,

minor clastic sediment;

small lens of gray clays

Dark-gray clay, silty; FeS2(?);

 10 percent organic matter

Brown to dark-gray clay,

minor sand and silt;

 20 percent organic matter 

(wood fragments),

very slight H2S odor

Sandy;

minor Fe-oxide

staining (yellow

and orange),

FeS2(?), minor

organic matter

Dark gray,

approx. 50 percent

gray clay and sand;

50 percent organic

matter (rootlets,

wood fragments)

Dark brown to red,

50 percent organic

matter (peat, root mat);

minor clastic

sediment

Clayey,

organic rich (peat);

dark-gray to

black zone

and gray

sandy/clayey

lenses

Dark-gray to

gray sands,

minor

organics

Description

221

226

246

206

183

173

150

102

71

41

0

0.1 10 1,000 20 40 60 4 8 12

CONCENTRATION, IN WEIGHT PERCENT OR PARTS PER MILLION PERCENT LEACHED

DEPTH, IN CENTIMETERS

Values below detection limits

are not plotted

EXPLANATION

Total As ppm

Total Zn ppm

Total Pb ppm

Total Cu ppm

Total Ca pct

Total Al pct

Total Fe pct

Pct As leached

Pct Zn leached

Pct Pb leached

Pct Cu leached

Pct Ca leached

Pct Al leached

Pct Fe leached

AVS ppm

Wt pct org

Wood

 fragments

Sand

Clay

Silty/clayey

 sand

Organic

 matter

Sediment Type

 50 percent

 organic matter

1058 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Affecting Composition of Wetland Sediment 1059

old railroad bed and present auto road suggests that contamination from detrital material is a possibility in this area of the 

wetland.) The clastic material, particularly if it consisted of 

larger grains or FeS2

, ZnS, or PbS, would not easily dissolve 

in 6N HCl. However, much of the clastic material is at too 

great a depth to have been deposited by anthropogenic means 

(mining, railroad and road building). Thus, it appears that the 

original detritus deposited at the base of the Forest Queen 

wetland was enriched in extractable lead and zinc.

The gently sloping lines on the right sides of figure 14, 

views A–D, indicate slightly increasing metal content with 

large increases in monosulfide. Steeply sloping lines on the 

left sides of views A–D, where metal content increases sharply 

with only slight increases in monosulfide, indicate the depositrelated sulfide (detrital) component.

A traverse of the wetland from east to west (from 

core FQ3 towards core FQ7) indicates a change in the sediment from a high organic content to an increasingly clastic 

content, particularly below about 60 cm. Ages for the deeper 

clastic sediments were not determined, but based on ages of 

similar depth horizons in other dated wetlands from Mineral 

and Cement Creeks, they were probably deposited by the 

Animas River prior to building of the railroad and automobile 

road.

Vincent and Elliott (this volume) obtained 14C dates for 

several layers in the Forest Queen wetland. An accumulation 

rate of approximately 1 m peat (organic matter) per 1,000 years 

(0.1 cm/yr) was postulated for the “growth” of the wetland. 

Thus, the metals found in sediment below about 100 cm probably were deposited prior to mining. However, more exact 

methods (for example, 210Pb geochronology or isotopic work) 

would be needed to determine the timing of metal fixation during wetland accumulation.

Core FQ9

This core was taken approximately 25 m southwest of 

the iron oxyhydroxide pool. Core FQ9 was a short interval 

(0–76 cm) and consisted almost wholly of organic material. 

Because of the limited length and consistent composition, 

only three samples were analyzed from this core. (Only 

HCl-extractable concentrations and AVS recovery are given 

because the samples were not analyzed for total metal concentrations.) Organic content was 60–80 weight percent in the 

upper 61 cm; iron oxyhydroxide staining was noted primarily 

in the upper 0–20 cm. In the lowest 61–76 cm, organic matter 

made up about 40 percent of the sample and iron oxyhydroxide staining was not observed. Unidentified mineral(s) in the 

61–76 cm interval resembled pyrite, but the presence of FeS2

in the sample was not confirmed.

Acid-Extractable Element Concentrations and AVS

The middle interval (20–61 cm) produced the largest 

sulfide yield of all samples (430 ppm). The moderate to high 

yields of sulfide from all three core samples indicated high 

sulfate reduction activity in this part of the wetland. However, 

the abundant iron oxyhydroxide in the upper 20 cm indicated 

that a strongly reducing environment was not present in shallow horizons at the time of sampling.

As figure 15 illustrates, acid-extractable element abundances varied little and showed similar trends with depth in this 

core. Extractable iron, aluminum, and calcium showed systematic but slight increases in concentrations from the 0–20 cm 

interval to the 61–76 cm interval.

Despite the moderate to high sulfide yields from all 

intervals, extractable trace metals were relatively low in these 

core samples (fig. 15). Acid-extractable zinc ranged from 

2.3 to 7.1 ppm, and similar to most cores, was in greatest 

abundance in the uppermost sample (0–20) cm rather than in 

deeper samples. Extractable lead was present in low concentration, ranging from 0.49 to 0.61 ppm. Extractable aluminum 

and iron concentrations were among the lowest of all cores (23 

and 78 ppm, respectively, in the uppermost interval); in other 

cores, extractable aluminum and iron in the top interval generally exceeded 30 ppm and 130 ppm, respectively.

High AVS yields in core FQ9 were associated with only 

relatively small concentrations of extractable metals, suggesting that perhaps metal trapping by monosulfide is inefficient. 

Another explanation for the low extractable metals is that the 

flux of dissolved metals to the site is low. Considering that 

the site of core FQ9 is 25 m downgradient from the source 

surface water, and that surface water likely does not reach this 

site except during high flow in the spring, low metal flux is 

the preferred explanation for the relative lack of enrichment 

in this core. The low extractable iron and aluminum in the top 

interval also point to a low metal flux.

Summary of Acid-Extractable Element 

Concentrations and Acid-Volatile Sulfide 

for Animas River Cores

Yields of acid-volatile sulfide (ppm S2–

(AVS)) varied 

widely from sample to sample in each core. The highest yields 

(≥200 ppm) came from dark (black, brown), organic layers. 

The lowest yields (0–10 ppm) came from lighter colored or red 

(iron oxyhydroxide stained) organic or silicate sediment layers. 

Intermediate AVS yields (≥10–200 ppm) were from dark-gray 

to brown organic layers and from silicate sediment with small 

black lenses that appeared to be fresh FeS forming in place. 

Inspection of figures 10 through 13 and 15 illustrates that AVS 

yield is not a reliable indicator of extractable metals concentrations. The variable trace-metal concentrations associated with 

the AVS indicate that, in the absence of a significant AVS fraction, extractable metals are associated with other phases, such 

as organic matter, clastic (silicate or sulfide) sediment, and iron 

oxyhydroxide.

Figure 13 (facing page). Plot of total element concentrations, 

organic matter, AVS yield, and percentage of acid-extractable 

elements with depth, Forest Queen core FQ7.
Figure 16 shows three contour maps for total iron concentration at depths of approximately 50 cm, 110 cm, and 

180 cm from cores FQ4, FQ5, and FQ7, respectively. (The 

iron concentration in core FQ4 at 180 cm was estimated using 

the mean iron value calculated with the six overlying intervals 

that had an iron analysis.) As these maps show, total iron in 

each core is relatively higher in the upper intervals, then drops 

to low levels in intermediate intervals, then increases once 

again towards the bottom intervals. Similar contour plots were 

obtained for most other reported elements. The increase in 

iron concentration near the bottom of most cores suggests that 

an additional, deeper source of metals is available for adsorption or that the abundance of HCl-extractable detrital minerals 

is greater at depth.

Acid-soluble iron and aluminum exceeded several hundred parts per million in many samples; this is not unexpected, 

because of the significant contribution of dissolved aluminum 

and iron into the wetland from the nearby adit. Aluminum and 

iron concentrations in AVS leachates were relatively uniform 

downhole (100–500 ppm) in most cores. Increases in extractable iron were usually seen when a red or brown zone (indicative of oxidized iron) was encountered; extractable aluminum 

concentrations generally increased where a clastic sediment 

layer was present.

Examination of the percent leached plots in figures 11 

and 12 (cores FQ4 and FQ5) shows that extractable lead and 

zinc concentrations generally increased with depth and from 

south to north. The increase in core FQ5 extractable metal 

0

1

2

3

0 5 10 15 20

COPPER, IN PARTS PER MILLION

TOTAL AVS, IN PARTS PER MILLION

LEAD, IN PARTS PER MILLION

TOTAL AVS, IN PARTS PER MILLION

0

2

4

6

8

10

0 5 10 15 20 0

4

8

12

16

20

0 5 10 15 20

ZINC, IN PARTS PER MILLION

TOTAL AVS, IN PARTS PER MILLION

100

200

300

400

500

600

700

0 5 10 15 20

IRON, IN PARTS PER MILLION

TOTAL AVS, IN PARTS PER MILLION

0

A

B

C

D

Figure 14. Scatter diagrams of A, iron; B, lead; C, copper; and D, zinc versus S2–

(AVS), showing relationships of tracemetal content to total recoverable sulfide in core FQ7. (An outlying sample (40–59 cm) is excluded. When included, it 

“forces” a strong correlation with lead, but the other three metals show little change in correlation with sulfide.)

1060 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Affecting Composition of Wetland Sediment 1061

concentrations resulted from its receiving direct surface flow 

out of the iron-coated pool and its location at the base of the 

mine-waste pile, which could have provided an additional supply of mobile metals.

The percent leached plots in figures 11 and 13 (cores FQ4 

and FQ7) show that extractable lead and zinc concentrations 

tend to decrease from east to west. Again, the decrease in core 

FQ7 results from its having been taken farther away from the 

source water than core FQ4.

These results suggest that lead and zinc fixation happens 

closest to the adit (source) water and that fixation decreases 

as water percolates from the upper organic layers downward 

to the middle intervals. Some high percentages of extractable 

lead and zinc are found in the deepest parts of cores FQ4, 

FQ5, and FQ7 (figs. 11–13). Similar to the results for total 

iron, these higher values of extractable lead and zinc suggest 

other source(s) of metals for sorptive sites or higher concentrations of HCl-soluble minerals.

Figure 15. Downhole plot of acid-extractable major- and trace-element concentrations, Forest Queen core FQ9.

Core FQ9

Brown to

reddish-brown

root mat.

Nearly 80-90

percent peat

plus organics;

minor sand

and clay

Dark brown,

50-75 percent

peat and organics,

mainly roots w/ some

wood fragments,

Fe-oxide staining

at bottom of sample

Gray clay w/

30-40 percent

peat and organics

(black).

Weathered

FeS2(?) in

clay zone

Description

61

76

20

0

CONCENTRATION, IN PARTS PER MILLION

0.1 1 10 100 1,000

AVS ppm

 Zn ppm

 Pb ppm

 Cu ppm

 Ca ppm

 Al ppm

 Fe ppm

DEPTH, IN CENTIMETERS

(Values below

detection limits

are not plotted)

EXPLANATION
Conclusions

Importance of Metal-Rich Inflow as a Control 

on Metal Content of Wetlands

Wetland sediments sampled in this reconnaissance of 

the Animas River watershed study area vary in their ability 

to act as sinks for dissolved metals. Wetlands from South 

Fork Mineral Creek that receive relatively metal poor drainage showed the lowest levels of metal enrichment. The site of 

core FQ3 in the Forest Queen wetland did not receive sufficient 

inflow of metal-bearing surface water, and as a result, core FQ3 

had among the lowest extractable metal concentrations in that 

wetland. Furthermore, core FQ7 showed total iron concentrations about 10 times lower than cores FQ4 and FQ5, whose 

sites were located closest to the source inflow.

In contrast, wetlands along Mineral and Cement Creeks 

receive both non-mining affected and mining-related acid 

drainage as well as eolian deposition of mining (waste) 

material originating from acid-sulfate altered rocks. The 

abundance of metals in cores from these wetlands was highly 

dependent on variations in supply, but consistently recorded an 

overall greater flux of dissolved metals to these sites. Sediment 

directly in the path of surface flow in the Forest Queen wetland 

(cores FQ4 and 5) showed evidence of enhanced metal flux and 

as a result, higher total and extractable metal concentrations. 

Nearly every core studied showed higher total metal concentrations in deeper sediment, even those along South Fork Mineral 

Creek, which strongly suggests some interaction with metalbearing ground water.

Relative Importance of Different Sorbents 

or Contributors within Individual Wetlands

Adsorption of metals by organic matter is clearly operative but appears to play a more prominent role in accumulation of elements in some wetland sediment (for example, core 

999292). In most wetland sediment, clastic detritus appears 

to correlate with lower abundances of total and extractable 

metals.

Iron monosulfide provides a host for metal removal and 

storage but is highly variable in abundance in individual wetlands; it is therefore unlikely to be a major contributor to metal 

fixation in the studied wetlands.

In the Forest Queen wetland, total iron approached 

50 weight percent in some near-surface samples. Total and 

extractable trace-element concentrations were correspondingly 

FQ3

FQ4

FQ5

FQ7 FQ9

FQ10

20

32

40

8

8 20 32 40 48

0 10 20 30 40 50 60

10

20

15

25

0

-5

5

Contour interval = 4.0 (total iron, weight percent)

A

DISTANCE, IN METERS

DISTANCE, IN METERS

FQ3

FQ4

FQ5

FQ7 FQ9

3

1

1

1 3 5

DISTANCE, IN METERS

DISTANCE, IN METERS

0 10 20 30 40 50 60

10

20

15

25

0

-5

5

Contour interval = 0.4 (total iron, weight percent)

B

DISTANCE, IN METERS

DISTANCE, IN METERS

Contour interval = 2.0 (total iron, weight percent)

10

20

15

25

0

-5

5

0 60 10 20 30 40 50

FQ3

FQ4

FQ5

FQ7 FQ9

FQ10

9

5

1

5 9 13

1

C

FQ10

Figure 16. Contour maps showing maximum concentrations 

of total iron at three different depths in Forest Queen wetland. 

A, depth 50 cm; B, 110 cm; C, 180 cm. Core locations shown 

are approximate and based on using the point labeled FQ10 

as the origin.

1062 Environmental Effects of Historical Mining, Animas River Watershed, Colorado
Processes Affecting Composition of Wetland Sediment 1063

higher in these horizons, indicating that solid iron oxyhydroxide is important in removing dissolved species from surface 

water. Compared to the adit and upstream (surface) waters, 

lead, zinc, and aluminum are lower in the wetland outflow, 

but iron is leaving the wetland in concentrations greater than 

in the influent waters. At a pH of 5.9 in outflow water, most 

iron should precipitate as a solid phase. Thus, iron detected 

in outflow water was probably a colloidal form.

The composition of the Forest Queen sediment in 

cores FQ4 and 5 approached that of the Cement Creek samples 

with respect to some trace elements (compare tables 1 and 

4), illustrating that coprecipitation with iron oxyhydroxide 

and adsorption by organic matter can produce high degrees of 

enrichment. Secondary iron oxyhydroxide found at depth as a 

postdepositional coating in wetlands along South Fork Mineral 

Creek is a strong adsorbent for arsenic.

In some wetlands, the lack of high abundances of elements in the solids may be the result of a lack of detrital 

deposition (97ABS321), a lack of precipitation/coprecipitation 

from the aqueous phase, or poor adsorptive capacity in sediment layers.

Spatial Relationship of Metal Enrichment 

within Forest Queen Wetland

Proximity to input source.—Higher total and extractable 

iron and aluminum were in near-surface horizons closest to 

the adit source water (cores FQ4 and 5), whereas lower total 

and extractable concentrations of these metals were present in 

near-surface horizons in cores located farthest away from the 

source water (cores FQ7 and 9). The lowest leachable metal 

concentrations were generally in clastic or organic sediment 

located farthest away from or outside the path of the inflow 

source.

Shallow versus deep enrichment.—The presence of both 

shallow (above about 60 cm) and deep (below 80 cm) intervals 

of elevated extractable metals implies that dissolved metals may be introduced along two different flow paths in the 

wetland sediment. The most obvious path is surface flow and 

downward percolation of metal-bearing surface (adit) water. 

Another potential path is shallow subsurface flow of (metalrich? acidic?) ground water that upwells beneath the wetland 

(Pavlik and others, 1999). If either flow path includes sulfatereducing zones, then metals could become initially entrapped 

by sorptive reaction or coprecipitation with authigenic iron 

monosulfide phases.

Role of AVS.—Results from the Forest Queen wetland 

study indicate that sulfate reduction was active, but variable 

with depth and location during the fall sampling period. When 

sampling took place, the wetland appeared to be drying out, 

as evidenced by low water level. The area around cores FQ3, 

4, 5, and 9 showed the highest yields of AVS that indicate 

FeS formation. However, the highest concentrations of HClextractable metals were not always associated with black, 

AVS-bearing samples. High extractable metal concentrations 

also were present in organic (but generally AVS/H2

S-poor) 

intervals and in intervals with prominent red iron oxyhydroxide solids.

Although no single explanation for the variability in 

extractable metal concentrations in sediment is evident, we 

speculate that observed seasonal variations in the hydrologic 

flow path in the wetland produce fluctuations in the delivery 

of dissolved metals to sites of sulfate reduction or iron oxidation within the sediment column. Alternatively, other potential 

sorbents along any flow path include organic matter or secondary iron oxides. Other undefined hydrologic, geochemical, or 

biological factors must be influencing the mobility of metals 

that enter (via the adit and streams) or exit (via outflows) the 

wetland, such that some of the wetland metal load reaches 

the Animas River. This results, in part, from sulfate reduction 

that is discontinuous with depth and distance throughout the 

wetland in the fall.

Stability of Wetlands and Wetland Functions 

Through Time and Implications for Wetland 

Management and Water Quality

Wetlands in the Animas River watershed study area 

will likely continue to sequester metals within organic, acidvolatile sulfide, and iron oxyhydroxide solid phases. However, 

redistribution of metals among these phases is likely to occur 

in response to seasonal fluctuations in water table depths. 

Such changes will affect the redox conditions in near-surface 

layers and the stability of iron phases and organic matter. In 

the absence of sulfate reduction to buffer pH, metals could be 

redissolved by acidic inflow water that would also promote 

iron oxyhydroxide dissolution and desorption of metals from 

organics. In addition, low water levels in the fall season would 

allow AVS and organic matter oxidation and release of sorbed 

or coprecipitated metals. Thus, the combination of seasonal 

variations in water flow and variable flow paths produces 

temporal and spatial variability in the removal, storage, and 

release of metals in sediment of the different wetlands.

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1064 Environmental Effects of Historical Mining, Animas River Watershed, Colorado