Understanding Metal Pathways in Mineralized Ecosystems

By Laurie S. Balistrieri, Andrea L. Foster, Larry P. Gough, Floyd Gray, James J. Rytuba, and Lisa L. Stillings

The USGS Pathways of Metal Transfer from Mineralized Sources to Bioreceptors Project studied the physical and biogeochemical processes that influence the distribution, concentration, and bioavailability of potentially toxic metals at historical hard-rock mine sites in the western United States.

U.S. Geological Survey, Reston, Virginia: 2007

Abstract .........................................................................................................................................................1 Introduction.....................................................................................................................................................1 The USGS Pathways Project .......................................................................................................................1 Mercury and fish in California ............................................................................................................3

Lead and endangered waterfowl in Idaho .......................................................................................3

Cadmium and willow in Alaska ...........................................................................................................6

Selenium in wetlands: Case studies in Idaho and Nevada ............................................................6

Metal transport and climate in Arizona ............................................................................................7

Discussion .......................................................................................................................................................7 Acknowledgments .......................................................................................................................................12 Sources of Additional Information ............................................................................................................12 iii

Why was the study done? ..........................................................................................................3 What are the major conclusions? .............................................................................................3 How is the information used? ....................................................................................................3

Why was the study done? ..........................................................................................................3 What are the major conclusions? .............................................................................................3 How is the information used? ....................................................................................................6

Why was the study done? ..........................................................................................................6 What are the major conclusions? .............................................................................................6 How is the information used? ....................................................................................................6

Why was the study done? ..........................................................................................................6 What are the major conclusions? .............................................................................................7 How is the information used? ....................................................................................................7

Why was the study done? ..........................................................................................................7 What are the major conclusions? .............................................................................................7 How is the information used? ....................................................................................................7

Abstract

Successful management of ecosystems containing historical mine wastes requires understanding of processes that are responsible for the distribution, concentration, and bioavailability of potentially toxic elements. U.S. Geological Survey (USGS) scientists recently completed several investigations at historical mine sites in the western United States. These investigations have improved our understanding of how metals are mobilized from mineralized sources, are transported through the environment, and become available to humans and other biota. The new information is being used by Federal, State, and local agencies that manage and remediate abandoned mine lands.

Introduction

The mid 1800s marked the beginning of a long and colorful history of mining in the western United States. That history has left a legacy of approximately 11,000 abandoned hard-rock mine sites. At many of these sites, historical mining activities resulted in adverse impacts to the quality of water and sediment and to the health of humans and other biota. Understanding the processes that influence the distribution, concentration, and bioavailability of potentially toxic metallic elements, such as arsenic (As), cadmium (Cd), lead (Pb), mercury (Hg), selenium (Se), and zinc (Zn), is critical for successful management of chronically affected ecosystems where total remediation of environmental problems is not financially or technically possible. Such understanding of processes can be used to identify and target those pathways that have the greatest immediate and long-term impact on the environment and health of biota. It therefore provides the scientific foundation for making decisions, developing strategy, and assessing mitigation and remediation alternatives by local, State, and Federal agencies charged with minimizing the environmental and health impacts of the toxic elements.

The USGS Pathways Project

The U.S. Geological Survey (USGS) Pathways Project focuses on improving our understanding of the processes that redistribute metals from mineralized sources to human populations and other biological receptors within ecosystems. The major objectives of the Pathways Project are to:

• advance our understanding of the physical, chemical, and biological processes responsible for the cycling of potentially toxic elements in large-scale ecosystems influenced by mineralized deposits and mine wastes,
• identify those pathways that have the greatest chronic or long-term adverse impact to the environment and to the health of biota, and
• develop conceptual and quantitative transport and reaction models that link the concentrations and distributions of elements to specific processes. To meet these objectives, work within the Pathways Project

examines the behavior of various potentially toxic elements in different ecosystems and addresses several interrelated questions:

• How do ecosystem conditions (for example, arid versus wet or vegetated versus non-vegetated) influence the transport and dispersion of elements?
• How does the speciation (specific chemical form) of elements in solution and as particles affect their geochemical mobility and availability to biota?
• How do processes at physical and geochemical boundaries affect element speciation, mobility, and bioavailability?
• What are the relations among different types of microbial communities, the processes that transform elements between various chemical forms, and the availability of elements to biota?
• Can conceptual and quantitative models be developed that link element concentrations and distributions in mineralized ecosystems to specific physical and biogeochemical processes and that help us to understand the possible impacts of natural and anthropogenic changes on element distributions in these systems? Selected major accomplishments of the USGS Pathways

studies that addressed these issues are highlighted in the following sections.

Dissolved Metals Mining and Natural Processing Weathering

mixing •

hydrologic transport •

reduction/oxidation •

dissolution/precipitation • Particulate

adsorption/desorption •

organic/inorganic Metals complexation •

biological productivity and respiration •

Why was the study done?

Historical placer gold dredging in the floodplains of many rivers in the western United States has modified river geomorphology and impaired the spawning and rearing habitats of salmonid species. In addition, the dredge tailings contain mercury (Hg), an element used in the mining process to concentrate gold. Large-scale river-restoration projects in mining-affected watersheds may mobilize mercury from the tailings and downstream sediment derived from them and consequently lead to increased Hg levels in fish. Knowledge of the processes that control Hg cycling in the environment is needed to plan and manage river-restoration programs in order to minimize Hg release from sediments, chemical transformations due to methylation, and accumulation in fish.

What are the major conclusions?

(silt and clay sizes) of contaminated sediments, with Hg concentrations as much as 300 times higher than in the bulk material.

gold recovery process has been converted to organicbound phases and mercury sulfide, with only 15-40 percent remaining as elemental Hg.

is readily leached from contaminated silts and sands by water containing organic acids formed from the degradation of plant material.

tailings, including dredge ponds and constructed wetlands ponds in tailings, typically show moderate to high rates of Hg methylation. The methylation process results in methyl Hg, which is a more bioavailable form of Hg.

contaminated to be used in river-restoration projects, but coarser tailings can be moved and used to improve habitat, if the fine sediment is first removed.

How is the information used?

Study results have been used by the U.S. Bureau of Reclamation in the Trinity River Restoration Project and by the U.S. Bureau of Land Management and U.S. Bureau Hg is concentrated in finer grain-size fractions

Elemental Hg that was introduced during the

Colloidal (submicrometer size) mercury sulfide

Contaminated sediments associated with dredge

Sands and silts in dredge tailings may be too

The USGS Pathways Project 3

of Reclamation in the restoration of lower Clear Creek, California. In the Trinity River area, tailings and contaminated sediments that were removed from open floodplain channels were placed above the 100-year flood level and not vegetated in order to minimize the potential release of Hg. Wetlands, originally planned for areas where contaminated sluice sands and silts were found, were sited elsewhere. At Clear Creek, tailings that are moved or used for gravel injection in floodplain restoration are now routinely evaluated for potential release of Hg and, if necessary, treated to minimize Hg release.

Why was the study done?

A century of historical mining, milling, and smelting of world-class silver (Ag), lead (Pb), and zinc (Zn) deposits, disposal of mill waste into rivers, and periodic natural flooding have resulted in the dispersion of metal-enriched water and sediment throughout the Coeur d’Alene River Basin, northern Idaho. In 1983, the U.S. Environmental Protection Agency (EPA) listed an area of 21 square miles in the heart of the basin as a Superfund site. In 1998 the EPA began applying Superfund requirements to many other areas throughout the basin. Many studies indicate that the contaminated materials pose health risks to humans and other biota in the basin. Understanding how metals cycle through this ecosystem is essential for minimizing those health risks.

Marshes in the lower Coeur d’Alene River valley are prime resting and feeding areas for migratory birds. These marshes, however, are enriched in toxic metals, and waterfowl deaths attributed to Pb poisoning during feeding have been reported for this area. During certain seasons of the year, reddish-brown material forms along the edge of the river in the lower valley. This material is easily re-suspended and transported downstream; it adheres to plant surfaces after floodwaters recede and may be a potential source of metal uptake during feeding by waterfowl. Investigations therefore examined this material and the processes responsible for its composition and formation.

What are the major conclusions?

Relative to water in the Coeur d’Alene River,

water that is in the pores of levee-bank sediment along the river is acidic and contains high concentrations of sulfate (SO4), calcium (Ca), magnesium (Mg), and metals, including arsenic (As), cadmium (Cd), cobalt (Co), copper (Cu), chromium (Cr), iron (Fe), manganese (Mn), lead (Pb), and zinc (Zn).

Toxic Lead – A Hazard to Waterfowl in the Lower Coeur D’Alene River Valley

Sediment Layers and Associated Lead (Pb) Contents

to oxidation of metal sulfide minerals, dissolution of carbonate minerals, and biologically mediated decomposition of organic matter in bank sediment.

basin or after floodwaters recede, porewater drains from bank sediment and mixes with the Coeur d’Alene River water. Chemical reactions during the mixing process result in the formation of the metal-enriched, reddish-brown material.

indicate that the reddish-brown material is an iron oxide, which adsorbs other toxic metals.

How is the information used?

The results of this study add to the overall understanding of how metals cycle through the Coeur d’Alene River Basin and how they may impact the health of humans and other biological receptors. That knowledge was used by EPA to help develop long-term goals for cleanup and recovery from the effects of historical mining in the basin.

Why was the study done?

Previous studies in ecosystems containing mineralized deposits have indicated a direct link between high cadmium (Cd) concentrations in willow leaves and twigs and the occurrence of disease in ptarmigan (grouse) that feed on the willow. Because willow is a preferred source of food for moose in Alaska, investigations of Cd cycling were conducted in both mineralized and nonmineralized areas throughout Alaska to determine if willow and other plants accumulate Cd and, if so, to define the geochemical conditions that result in bioaccumulation of Cd. Understanding of Cd cycling within these ecosystems is important for minimizing adverse impacts to the health of browsing animals, such as moose, and the health of people who hunt those animals.

What are the major conclusions?

mineralized bedrock accumulates Cd in concentrations 4 to 10 times higher than willow growing in nonmineralized soil. The enrichment of elements in porewater is due

During annual lowering of water levels in the

Chemical analyses and geochemical modeling

Willow growing in soils developed from Cd levels in willow are as much as 10 to 100

times greater than those found in other plants, such as green alder, feather moss, and soil lichen, collected from the same area.

Mineralized areas in Alaska are likely sources

for natural occurrences of potentially harmful levels of Cd in willow, which pose a health threat to browsing animals.

How is the information used?

Based on our Cd study and to examine links between Cd levels in willow and moose, a database of element concentrations in moose tissue was developed by USGS with assistance from the Alaska Department of Fish and Game and the University of Alaska. Samples of moose kidney from harvested as well as road-kill animals are analyzed and compared with similar moose data from Sweden and Finland and with Cd data from willow.

Why was the study done?

Selenium (Se) toxicity, resulting from bioaccumulation in their forage, is a serious threat to livestock, particularly cattle and sheep, in the western United States. High concentrations of Se, released into surface waters during the mining of phosphate deposits in southeastern Idaho and subsequently taken up by plants, are implicated in the deaths of sheep grazing in that area. Wetland soils and plants are known to concentrate Se and thereby promote its entry into the food chain. Understanding how Se cycles through wetland systems will aid in minimizing bioaccumulation by plants and consequent health risks to foraging livestock.

Studies were conducted in two wetlands to assess transport and retention of Se within these systems. One of the study areas is a wetland downstream from the phosphate deposits in Idaho. The other is a wetland in a nature park affected by runoff from Las Vegas, Nevada. These wetlands were compared by evaluating their Se mass balances. A mass balance determines the amount of an element that remains or is retained in the wetland by comparing the amount of the element that enters the wetland with the amount that leaves the wetland. Elements can enter or leave the wetlands by surface streams, by ground water, or through the atmosphere.

What are the major conclusions?

The two wetlands differ in both flow volume and

Se concentrations in their inlet and outlet waters. Water entering or leaving the Idaho wetland had lower flows, but higher Se concentrations, than water entering or leaving the Nevada wetland.

The Idaho wetland retained 611 grams of Se (88

percent of the Se in the inflow) during a five-month period, whereas the Nevada wetland retained 5,800 grams of Se (24 percent of the Se in the inflow) during a four-month period. The amount of Se retained in the wetland likely represents accumulation of Se by plants and wetland sediments, although gains or losses to the wetlands from the ground water and atmosphere were not measured.

How is the information used?

Land-management agencies are changing their management practices in order to decrease the amount of Se delivered to the studied wetlands. The U.S. Forest Service manages the Idaho wetland; they are now using soil amendments and seed mixtures to immobilize Se within the waste-rock source, hoping to decrease both the concentration of Se in forage plants in the wetland as well as the concentration of Se in drainage from the waste to the wetland. The Nevada wetland is managed by the Clark County Department of Parks and Recreation; they now use another source of inlet water with lower Se concentrations in order to decrease the amount of Se entering their wetland.

Why was the study done?

The Patagonia Mountains and southern Santa Rita Mountains in southeastern Arizona were mined for silver (Ag), lead (Pb), copper (Cu), and zinc (Zn) intermittently from the 1600’s to the mid-1960’s. Our studies examined the transport and chemical behavior of metals in several arid to semiarid watersheds (Harshaw Creek and Alum Gulch drainages) containing these abandoned mine lands. The goal was to evaluate how climate and other ecosystem conditions influence metal cycling in mining-affected ecosystems. Such knowledge is vital for developing robust models that describe potential environmental impacts of mining activity in different climate settings.

What are the major conclusions?

• Seasonal precipitation and acid mine drainage, which contains metals, such as cadmium (Cd), cobalt (Co), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn), are the dominant sources of water to the study areas.
• During dry periods, acid rock drainage evaporates and readily soluble metal sulfate salts are deposited on rocks and stream sediment. During storm events, these salts dissolve and release acid and metals back to solution. Downstream decreases in acid and metal

concentrations after a storm event depend on the ability of the ecosystems to neutralize or buffer the acid produced during salt dissolution and on the volume of water from the storm event. If the buffering capacity of the ecosystem is small, acid and metals are transported far downstream in the dissolved phase. If the buffering capacity of the ecosystem is large, neutralization results in precipitation of aluminum-iron-oxyhydroxide minerals, adsorption of metals, and release of carbon dioxide gas.

How is the information used?

A major outcome of the research has been an improved understanding of the links among weather conditions, landscape, mineral precipitation and dissolution, and metal transport. The U.S. Forest Service and State of Arizona Environmental Quality Agency are using this information to develop remediation plans for this mining-affected area.

Discussion

These case studies conducted under the U.S. Geological Survey (USGS) Pathways Project have yielded important new information about the ways in which metals are mobilized from rocks and mining waste, transported and chemically modified, and accumulated in various biota. Many of the metals, including arsenic (As), cadmium (Cd), lead (Pb), mercury (Hg), selenium (Se), and zinc (Zn), are potentially toxic to animals and humans. The results of these studies provide a better understanding of the physical and biogeochemical processes that influence the distribution, concentration, and bioavailability of the metals in mineralized ecosystems, not only in historical mining areas, but also in planning and developing new mines.

Climate Influences Metal Dispersion from Arizona Mining Areas

This improved understanding of the metal pathways allows the land-management agencies responsible for areas affected by mining or mineralization—including the U.S. Bureau of Reclamation, U.S. Bureau of Land Management, U.S. Environmental Protection Agency, U.S. Department of Agriculture, U.S. Forest Service, and State agencies such as the Alaska Department of Fish and Game and the Arizona Environmental Quality Agency—to better plan for mitigating the impacts of chemical toxicity in sediment, water, and biota. The research results of the USGS Pathways Project have also highlighted remaining questions and pointed the way to promising avenues of further research.

Acknowledgments

The work in the USGS Pathways Project was done with the support of Kathleen Johnson, Coordinator of the USGS Mineral Resources Program, numerous colleagues in the USGS Mineral Resources Program, and our various Federal, State, and local collaborators. The manuscript benefited from the reviews of Steve Box, Tom Frost, Sue Kropschot, and Bronwen Wang, from discussions with Dave Frank, and from the technical edit by Peter Stauffer, and design and layout by Jeanne DiLeo.

Sources of Additional Information

Ashley, R.P., Rytuba, J.J., Rogers, R., Kotlyar, B.B., and Lawler, D., 2002, Preliminary report on mercury geochemistry of placer gold dredge tailings, sediments, bedrock, and waters in the Clear Creek Restoration Area, Shasta County, California: U.S. Geological Survey Open-File Report 2002–401, 47 p. Ashton, K.L., and Duarte, O.A., 2003, Soil characterization

Balistrieri, L.S., Box, S.E., and Tonkin, J.W., 2003, Modeling precipitation and sorption of elements during mixing of river water and porewater in the Coeur d’Alene River basin: Environmental Science & Technology, v. 39, p. 1547-1554. Box, S.E., Bookstrom, A.A., and Ikramuddin, M., 2005, Stream sediment geochemistry in mining-impacted streams; sediment mobilized by floods in the Coeur d’Alene-Spokane River drainage, Idaho and Washington: U.S. Geological Survey Scientific Investigations Report 2005-5011, 57 p. Gough, L.P., Sanzolone, R.F., Lamothe, P.J., Ager, C.M.,

Slowey, A.J., Rytuba, J. J., and Brown, G.E. Jr., 2005,

Willard, P., and Stillings, L.L., 2005, A mass balance for

Additional information about the recently completed “Pathways of Metal Transfer from Mineralized Sources to Bioreceptors” project is at the USGS Western Mineral Resources Web site [http://minerals.usgs.gov/west/projects/path.htm].at the World’s Fair Mine site, Patagonia Mountains, Santa Cruz County, SE Arizona; implications for base metal transport during storm runoff events [abs.]: 31st Annual GeoDaze Geoscience Symposium, GeoDaze 2003, p. 55.

Foster, A.L., and Crock, J.G., 2003, Bioaccumulation and mobility of cadmium in willow and soils, Alaska—implications for the health of browsing animals [abs.]: U.S. Geological Survey Open-File Report 2003-097, p. 22.

Speciation of mercury and mode of transport from placer gold mine tailings: Environmental Science & Technology, v. 39, p. 1547-1554.

selenium in the upper pond of the Clark County Wetlands Park, Nature Preserve, Las Vegas, NV [abs.]: Geological Society of America Abstracts with Programs, v. 37, no. 7, p. 178.

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