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Selected Major Accomplishments of the Pathways of Metal Transfer from Mineralized Sources to Bio-receptors-Project

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In chronically impacted systems where total remediation of environmental problems is not financially or technically feasible, process-based understanding of metal mobilization and transport is vital in identifying those pathways that have the greatest immediate and long-term impacts on the health of biota.  Those critical pathways then can be targeted during the cleanup efforts.  USGS investigations at historical mine sites in the western US focus on improving our understanding of processes that influence the physical and biogeochemical mobility of potentially toxic elements, such as arsenic (As), cadmium (Cd), mercury (Hg), lead (Pb), selenium (Se), and zinc (Zn).  Such pathways link release of elements from mineralized sources to their uptake by biological organisms.

Also see Understanding metal pathways in mineralized ecosystems: by Balistrieri, L.S., Foster, A.L., Gough, L.P., Gray, Floyd, Rytuba, J.J., and Stillings, L.L., 2007, U.S. Geological Survey Circular 1317, 12 p.

Cadmium Mobility and Bioaccumulation by Willow in Alaska Mercury Mobilization and Bioaccumulation in Fish in California Zinc and Other Metal Concentrations in the Coeur d’Alene River Basin, northern Idaho Selenium Flux and Retention in Wetlands in Idaho and Nevada Metal Transport and Loading in Arid to Semi-arid Watersheds in Arizona
Flow chart showing pathway processes
Flow chart showing fate of mercury in the environment

Cadmium Mobility and Bioaccumulation by Willow in Alaska

Mt. Eielson mining district, Alaska

Why was the study done?
Investigations of the uptake and transport of cadmium (Cd) were conducted in mineralized and non-mineralized areas throughout Alaska in order to understand whether and under what conditions willow (Salix) bioaccumulates the metal.  The increased concentration of Cd in willow (leaves and twigs), as compared to other plant species, is unusual and could be detrimental to the health of browsing animals as well as people who hunt them.

What are the major conclusions?

  • Willow growing in soils developed from mineralized bedrock accumulated Cd in concentrations 4 to 10 times higher than willow growing in non-mineralized soil.
  • Cd levels in willow were as much as 10 to 100 times greater than those found in other plants collected from the same area. 
  • Mineralized areas, therefore, may be important loci for the natural occurrence of potentially harmful levels of Cd in willow, which is the preferred browse of moose.
  • Understanding the relationships among cadmium in bedrock, soil, willow, and moose has implications for the health of both humans and animals.

How is the information used?

Based on our Cd study, a database of moose tissue analyses was initiated 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 compared with similar data from Sweden and Finland.  The Alaska willow and soil data are also the basis for an ongoing joint study with the Norwegians.
The Mt. Eielson mining district, Denali National Park and Preserve, with Grant Creek and typical mineralized bedrock in the foreground, a historic cabin dating to the days of mineral prospecting in the district in the near background, and the Thorofare River and the park road in the far background. Willow, high in Cd, was found throughout the area.

Mercury Mobilization and Bioaccumulation in Fish in California

Trinity River Restoration Project
Trinity River Restoration Project. Placer dredge tailings that are on flood plains and in riverbank deposits contain a large reservoir of organic and sulfide bound Hg even though the tailings typically have relatively low concentrations of total Hg (< 10 ppm).

Why was the study done?
Historic placer gold dredging in the flood plains of many rivers in the western US has modified river geomorphology and impaired salmonid spawning and rearing habitats.  Large-scale river restoration projects in mining-impacted watersheds may mobilize mercury (Hg) from dredge tailings and increase Hg levels in fish.  Knowledge of the processes that control Hg mobilization, methylation, and biotic uptake is needed to plan and manage river restoration programs so that Hg accumulation in fish is minimized.

What are the major conclusions?

How is the information used?
Study results have been used by the US Bureau of Reclamation in the Trinity River Restoration Project, and by the US Bureau of Land Management and US Bureau of Reclamation in the restoration of lower Clear Creek.  In the Trinity River area, tailings and contaminated sediments removed from open flood plain 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 moved or used for gravel injection in flood plain restoration are now routinely evaluated for potential release of Hg and selected or treated to minimize Hg release.

Zinc and Other Metal Concentrations in the Coeur d’Alene River Basin, northern Idaho


Map showing core sample locations, Coeur d' Alene River, ID

Metal-enriched floc forms along the river’s edge as a result of physical and biogeochemical processes – annual water level lowering, porewater draining from metal-rich bank sediment, mixing of different source waters, mineral precipitation, & adsorption.

Image showing porewater sampling technique

Porewater was collected from levee bank sediment along the lower Coeur d’Alene River using a sipper array.  Sippers, which are tubes of variable length with fine netting at the bottom, are inserted into the sediment and porewater is pulled up the tube using a syringe.  Reddish-brown floc is visible in the river bottom to the upper right of the sipper array.

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 flooding have resulted in metal-enriched water and sediment throughout the Coeur d’Alene River Basin.

During certain seasons of the year, reddish-brown floc forms along the edge of the lower Coeur d’Alene River.  The floc 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 examined processes responsible for the composition and formation of the floc.

 What are the major conclusions?

How is the information used?
The results add to the overall understanding of how metals cycle through the Coeur d’Alene River Basin and impact the health of humans and other biological receptors.  That knowledge, in part, was used by the US Environmental Protection Agency to develop long-term goals for cleanup and recovery from historic mining effects in the basin.

Selenium Flux and Retention in Wetlands in Idaho and Nevada

  Idaho Wetland Nevada Wetland

Study period


8 May--1 Oct 2002


12 Aug-15 Dec 2004


Se (µg/L) 34-5,000 8-19

Flow (L/s)






Se (µg/L) <6-66 10-22
Flow (L/s) 0.18-2.3 77-118
Pie chart Se distribution between the Nevada and Idaho wetlands.  The area of each circle is proportional to the total Se delivered to each wetland, 24,100 g over 4 months (NV), and 700 g over 5 months (ID).

Why was study done?
USGS research was conducted to understand the flow of selenium (Se) through two wetlands, and its pathway to bioaccumulation in the food chain.  Wetlands are known to concentrate Se and promote its entry into the food chain.

What are the major conclusions?

How is the information used?
Land management agencies have begun to change their management practices in order to decrease the mass of Se delivered to these wetlands.  The US Forest Service manages the Idaho wetland; they are 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 as well as the concentration of Se in drainage from the waste to the wetland.  The Las Vegas wetland is managed by the Clark County Department of Parks and Recreation; they now use another source of inlet water with lower Se concentrations (<5 µg/L), to decrease mass influx to their wetland.


Metal Transport and Loading in Arid to Semi-arid Watersheds in Arizona

Image of stream at low volume

Bubbles on Harshaw Creek

Low-volume, but perennial discharge in stream; white efflorescent salt deposits develop as ring and in-stream crusts near flowing stream. White precipitate and foam (gas bubbles) from the reaction of acid and alkaline streams in Harshaw Creek.  Bubbles form when CO2-gas is released during neutralization reactions.

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.  USGS studies examined the movement and chemical behavior of metals in Harshaw Creek and Alum Gulch, several arid to semi-arid watersheds containing these abandoned mine lands. 

What are the major conclusions?

How is the information used?

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


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