U.S. EPA Contaminated Site Cleanup Information (CLU-IN)

U.S. Environmental Protection Agency
U.S. EPA Technology Innovation and Field Services Division


Chemistry and Behavior

Arsenic exists in two distinct crystalline forms known as yellow arsenic and metallic arsenic. Yellow arsenic has a relatively low density (3.9 g/cc), is metastable, extremely volatile, and readily degrades to its metallic form. The low density implies an As4 structure that is similar to P4. The metallic form is a tin-white mineral that quickly tarnishes to dark gray. Metallic arsenic crystals have a density of 5.73 g/cc, are very brittle, and are good conductors of heat and poor conductors of electricity. Elemental arsenic is not common in nature.

Arsenic's principal ores are sulfides (As2S3, As4S4, and FeAsS), and arsenic sulfides are almost invariably found with other metal sulfides. The hydrogen form of arsenic is arsine, which is a poisonous gas. Arsenic also forms oxide compounds. Arsenic trioxide (As2O3) is a transparent crystal or white powder that is slightly soluble in water and has a specific gravity of 3.74. Arsenic pentoxide (As2O5) is a white amorphous solid that is very soluble in water, forming arsenic acid. It has a specific gravity of 4.32.

The most common forms of arsenic in groundwater are their oxy-anions, arsenite (As+3) and arsenate (As+5). Under moderately reducing conditions, arsenite is the predominant species. In oxygenated water, arsenate is the predominant species. Both anions are capable of adsorbing to various subsurface materials, such as ferric oxides and clay particles. Ferric oxides are particularly important to arsenate fate and transport as ferric oxides are abundant in the subsurface and arsenate strongly adsorbs to these surfaces in acidic to neutral waters. An increase in the pH to an alkaline condition will cause both arsenite and arsenate to desorb. Hence, they can be expected to be very mobile in an alkaline environment. The arsenic oxy-anions are also sensitive to redox conditions, and the speciation differential between them will change with changing redox (Henkel and Polette 1999).

Arsenic also forms organic compounds.


Arsenic in Ground Water of the Willamette Basin, Oregon
S. Henkel and D. Polette.
U.S. Geological Survey Water-Resources Investigations Report 98-4205, 1999

Adobe PDF LogoArsenic Behavior under Sulfate-Reducing Conditions: Beware of the "Danger Zone"
Carol L. Stein, Gannett Fleming, Inc.
EPA Science Forum 2005: Collaborative Science for Environmental Solutions, May 16-18, 2005, Washington, DC.
Contact: Carol L. Stein,

Adobe PDF LogoArsenic Cycling in Hydrocarbon Plumes: Secondary Effects of Natural Attenuation
Cozzarelli, I.M., M.E. Schreiber, M.L. Erickson, and B.A. Ziegler.
Groundwater 54(1):35-45(2015)

NA relies on biodegradation of hydrocarbons coupled with reduction of electron acceptors, including solid-phase ferric iron (Fe(III)). Because As adsorbs to Fe-hydroxides, a potential secondary effect of hydrocarbon NA coupled with Fe(III) reduction is a release of naturally occurring As to groundwater. Anaerobic biodegradation of hydrocarbons coupled to Fe(III) reduction has been well documented at a crude-oil-contaminated aquifer near Bemidji, Minn. Groundwater samples collected at the site annually from 2009 to 2013 showed As concentrations in groundwater in the plume reached 230 µg /L, whereas groundwater outside the plume contained <5 µg /L As. A conceptual model of secondary As release due to hydrocarbon NA can be applied to other sites where an influx of biodegradable organic carbon promotes Fe(III) reduction.

Adobe PDF LogoArsenic in Groundwater: A Summary of Sources and the Biogeochemical and Hydrogeologic Factors Affecting Arsenic Occurrence and Mobility
Barringer , J.L. and P.A. Reilly.
Current Perspectives in Contaminant Hydrology and Water Resources Sustainability; P.M. Bradley (ed). InTech, Rijeka , Croatia . ISBN: 978-953-51-1046-0, Chapter 4: 83-116, 2013

Arsenic Methylation - Lessons from Three Decades of Research
Thomas, D.J. | Toxicology 457:152800(2021)

This report summarizes the increase in knowledge of arsenic biomethylation between 1990 and 2020 and considers the prospects for new discoveries. The current understanding of the role of methylation as a modifier of kinetic and dynamic behaviors of arsenic is the product of research at molecular, cellular, organismic, and population levels. This information provides a basis to refine estimates of risk associated with long-term exposure to inorganic arsenic in environmental media, food, and water. Longer abstract.

ATSDR Toxicological Profile for Arsenic
Agency for Toxic Substances and Disease Registry, Aug 2007

Attenuation of Naturally Occurring Arsenic at Petroleum Impacted Sites
American Petroleum Institute, API Publication 4761, 90 pp, 2011

Naturally occurring arsenic can be mobilized into shallow groundwater by inputs of biodegradable organic carbon, including petroleum hydrocarbons. This manual identifies potential arsenic sources; explains the mobilization, transport, and attenuation mechanisms of naturally occurring arsenic in groundwater at petroleum-contaminated sites; and indicates tools for arsenic assessment and management. The report can be read on line. A summary paperAdobe PDF Logo is also available.

Behavior of Metals in Soils
McLean, J.E. and B.E. Bledsoe.
EPA 540-S-92-018, 25 pp, 1992

This paper covers the fundamental processes that control the mobility of metals (lead, chromium, arsenic, cadmium, nickel, zinc, copper, mercury, silver, and selenium) in soil and describes laboratory methods used to evaluate the behavior of metals in soil.

Environmental Chemistry of Arsenic
Frankenberger, W.T. (ed.)
Marcel Dekker, New York. ISBN: 0-8247-0676-5, 410 pp, 2002

Adobe PDF LogoEnvironmental Fate and Exposure Assessment for Arsenic in Groundwater
J.G. Hering, D. Burris, H.J. Reisinger, and P. O'Day.
SERDP, Project ER-1374, 27 pp, 2008

The fate and transport of arsenic were examined at Tyndall AFB, FL, where arsenic contamination has resulted from application of arsenical herbicides, and at Ft. Devens, MA, where naturally occurring arsenic has been mobilized by application of enhanced reductive dechlorination technology designed to remediate chlorinated solvent contamination. The comparison of to plume evolution, mechanism of arsenic mobilization in the source areas, and potential sequestration mechanisms at the two sites provides insight into the conditions under which MNA could be a feasible remedy for arsenic contamination.

Adobe PDF LogoEnvironmental Fate and Exposure Assessment for Arsenic in Groundwater: Final Report Addendum
O'Day, P. and V. Illera.
SERDP Project ER-1374, 38 pp, 2010

The results of this study indicate that sulfide phases can be a sink for As under sulfate-reduced conditions, particularly at low pH.

Field Study of the Fate of Arsenic, Lead, and Zinc at the Ground-Water/ Surface-Water Interface
U.S. EPA, National Risk Management Research Laboratory, Ada, OK.
EPA 600-R-05-161, 91 pp, 2005

Adobe PDF LogoFinal Report: Arsenic Fate, Transport and Stability Study, Groundwater, Surface Water, Soil and Sediment Investigation, Fort Devens Superfund Site Devens, Massachusetts
Ford, R., K.G. Scheckel, S. Acree, R. Ross, B. Lien, T. Luxton, and P. Clark
USEPA, Office of Research and Development, 193 pp, 2008

This document presents results from the Fiscal Years 2006-2007 field investigation at the Shepley's Hill Landfill Superfund site. The purpose of this study is to provide EPA Region 1 with a technical evaluation of the distribution and flux of arsenic in shallow groundwater adjacent to Red Cove and the fate, transport and stability of arsenic in sediments and surface water following groundwater discharge. Additional information: Ford et al. 2011. Delineating landfill leachate discharge to an arsenic contaminated waterway. Chemosphere 85(9):1525-1537 (Abstract).

Handbook of Elemental Speciation, II: Species in the Environment, Food, Medicine and Occupational Health
R. Cornelis, J. Caruso, H. Crews, and K. Heumann, eds.
John Wiley & Sons, New York. ISBN 0-470-85598-3, 768 pp, 2005.

Covers the speciation of elements from aluminum to zinc, including arsenic, chromium, and mercury.

The Impact of Ground-Water/Surface-Water Interactions on Contaminant Transport with Application to an Arsenic Contaminated Site
Robert Ford, U.S. EPA, National Risk Management Research Laboratory, Ada, OK.
EPA 600-S-05-002, 22 pp, 2005

This document provides a brief overview of the dynamics of chemical processes that govern contaminant transport and speciation during water exchange across the ground-water/surface-water transition zone and presents results from a field study examining the fate of arsenic during ground-water discharge into a shallow lake at a contaminated site.

Adobe PDF LogoImpacts of CCA-Treated Wood on Arsenic Concentrations in Soils and Plants
L. Ma, T. Chirenje, and J. Santos.
Florida Center for Solid and Hazardous Waste Management, Report 06-50892, 28 pp, 2006

The examination of soils and plants that have had 44 or more years of ground contact with chromated copper arsenate (CCA) provided a unique opportunity to study long-term migration of CCA constituents at several sites.

Natural Remediation of Arsenic Contaminated Ground Water Associated with Landfill Leachate
Kenneth G. Stollenwerk and John A. Colman.
U.S. Geological Survey Fact Sheet 2004-3057, 4 pp, 2004.

This fact sheet describes results of studies by the U.S. Geological Survey at the Saco Municipal Landfill, Saco, Maine. The source of arsenic in ground water and effects of landfill leachate on arsenic concentration in ground water are described.

Partition Coefficients for Metals in Surface Water, Soil, and Waste
Allison, Jerry D. (HydroGeoLogic, Inc., Herndon, VA); Terry L. Allison (Allison Geoscience Consultants, Inc., Flowery Branch, GA).
Report No: EPA 600-R-05-074, p , July 2005

This report presents metal partition coefficients for the surface water pathway and for the source model used in the multimedia, multi-pathway, multi-receptor exposure and risk assessment (3MRA) technology under development by U.S. EPA. Literature searches, statistical methods, geochemical speciation modeling, and expert judgment were used to provide reasonable estimates of partition coefficients for antimony, arsenic, barium, beryllium, cadmium, chromium, cobalt, copper, lead, molybdenum, mercury, methylated mercury, nickel, selenium, silver, thallium, tin, vanadium, and zinc.

Relation of Arsenic, Iron, and Manganese in Ground Water to Aquifer Type, Bedrock Lithogeochemistry, and Land Use in the New England Coastal Basins
J.D Ayotte, M.G. Nielsen, G.R. Robinson, Jr., and R.B. Moore.
U.S. Geological Survey Water Resources Investigations Report 99-4162, 70 pp, 1999
Contact: Joseph Ayotte,

Rock-Bound Arsenic Influences Ground Water and Sediment Chemistry Throughout New England
G.R. Robinson Jr. and Joseph D. Ayotte. U.S. Geological Survey, Open-File Report 2007-1119, 18 pp, 2007

Both stream sediment chemistry and the solubility and mobility of arsenic in ground water in bedrock are influenced by host-rock arsenic concentrations. Stream sediment chemistry and the distribution of geologic units are useful parameters to predict the areas of greatest concern for elevated arsenic in ground water and to estimate the likely levels of human exposure to elevated arsenic in drinking water in New England; however, the extreme local variability of arsenic concentrations in ground water from these rock sources indicates that arsenic concentrations in ground water are affected by other factors in addition to arsenic concentrations in rock.