ARSENIC OCCURRENCE, TRANSPORT, AND EXPOSURE PATHWAYS

As associated with: • Fe, Al, Mn-oxyhydroxides • clay minerals• sulfate minerals• arsenate minerals

As(III)

As associated with: • organoarsenicals• pore waters

oxidized

reduced

organic-rich soils

mineralized soils

As associated with:• Fe, Al, Mn-oxyhydroxides

--may cause oxidation

• sulfide minerals• pore waters

As(V)As associated with: • Fe, Al-oxyhydroxides • organoarsenicals

Mobility mechanisms in soils and sediments

Chemical processes leading to transport in dissolved state

Entrainment of host mineral

Desorption(pH change; competition)

Reduction of As(V) to As(III)

Reductive dissolution

of host mineral

Solid state/physical transport

Coal barge on the Cumberland River, TN

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ARSENATES

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SULFIDESOrpiment, As2S3

Realgar, AsS Arsenopyrite, FeAsS

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ASARCO copper smelter, Tacoma, WASpecial Collections Division, University of Washington Libraries. Photograph by Howard Clifford

FumarolePhotograph by R.L. Christiansen, U.S.G.S.

WEATHERING: OXIDATION AND DISSOLUTION

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Pyrite’s weathering products incorporate arsenic -- sorbed on goethite, substituting for S in sulfates.

Arsenic commonly substitutes for sulfur in pyrite: FeS(2-x)Asx

Pyrite oxidation leads to acid mine drainage; arsenic may be associated with iron oxides

Arsenate adsorptiononto amorphous ironhydroxide

Redox Relationships in Soils

Harvard Open Pit Mine, Jamestown, CaliforniaThe Harvard lode at Jamestown was discovered in 1859 and mined intermittently until 1916. The Sonora Mining Company developed the open pit mine between 1987 and July 1994, producing more than 600,000 ounces of gold.

Arsenic is primarily associated with arsenian pyrite, and with minor cobaltite, gersdorffite, and nickeline.

The pit has been filling with rainwater, runoff, and ground water since mining ceased in 1994. In 2000, at the end of the study period, the pit contained a 90-meter deep alkaline lake with As concentrations of about 700-1000 µg/L.

Arsenic (as arsenate) remains dissolved in the pit lake because it is poorly susceptible to sorption under the pH conditions maintained by equilibrium with wall rock carbonates.

Arsenic is a concern because of the pit's proximity to Woods Creek, a tributary to the Tuolumne River which drains to the San Joaquin Valley.

Smelter-contaminated soils, Puget Sound, WA

map from Tacoma - Pierce County Health Departmentmap from Tacoma - Pierce County Health DepartmentTacoma Smelter Plume ProjectTacoma Smelter Plume Project

Chili peppers roasted over Chili peppers roasted over coal containing up to coal containing up to 35,000 ppm As typically 35,000 ppm As typically adsorbs 500 ppm As, adsorbs 500 ppm As, causing poisoning in the causing poisoning in the Guizhou Province, China.Guizhou Province, China.[1][1]

[1] Finkelman R. B., Belkin H. E., and Zheng B. S. (1999) Health impacts of domestic coal use in China. Proceedings of the National Academy of Sciences of the United States of America ; 96(7), 3427-3431.

[2] Tao S. S. H. and Bolger P. M. (1999) Dietary arsenic intakes in the United States: FDA total diet study, September 1991-December 1996. Food Additives and Contaminants 16(11), 465-472.

[3] Anawar H. M., Akai J., Mostofa K. M. G., Safiullah S., and Tareq S. M. (2002) Arsenic poisoning in groundwater - Health risk and geochemical sources in Bangladesh. Environment International 27(7), 597-604.

[4] Chou C. and De Rosa C. T. (2003) Case studies - Arsenic. International Journal of Hygiene and Environmental Health 206(4-5), 381-386.

Shellfish are the largest dietary source for arsenic. [2]

Nearly 18 million people in Bangladesh are drinking water containing more than 200 µg/L arsenic.[3] The drinking water provisional guideline value suggested by the World Health Organization is 10 µg/L.

Children are at particular risk for arsenic exposure from soil ingestion [4].

Kaye SavageDept. of Earth and Environmental Sciences

Arsenic released from the ASARCO copper smelter in Tacoma, WA, and deposited on forested soils of glacial origin on Puget Sound islands, has migrated vertically in the soil column. As concentrations up to ~800 ppm are present at 10-15 cm depth. We used X-ray absorption spectroscopy to investigate how arsenic, released from the smelter as arsenic trioxide, has interacted with soil constituents, and to explore how arsenic speciation may promote or limit arsenic mobility and bioavailability in this region.

As(III) released from the smelter has been uniformly oxidized to As(V) in near-surface soils; EXAFS data suggests surface complexes on fine soil particulates. Arsenate in a sandy, organic-rich sample is associated with iron, with a small contribution from aluminum. In a sample with a larger clay fraction, aluminum is the main second shell backscattering element. Two potential mobilization pathways are considered: particle-mediated transport, and desorption followed by dissolved transport and resorption during intermittent wetting events.

Arsenic in Ground Water of the United StatesRyker, S.J., Nov. 2001, Mapping arsenic in groundwater: Geotimes v.46 no.11, p.34-36.

Soils may become contaminated with arsenic due to herbicide application, livestock feed operations, runoff from pressure-treated wood, smelting operations, and waste from the semiconductor industry.

Inhalation is a potential risk, particularly for industrial exposures related to smelting or generation of sawdust from pressure-treated wood. Wood and coal smoke are also of concern in domestic settings. [1,4]