Arsenic in the Soils, USGS http://www.usgs.gov/blogs/features/slidedeck2/soil-geochemistry-maps/arsenic/

How much of this soil arsenic is available to move into the biosphere?

Bioaccessible arsenic is the amount of arsenic that is free to move from the soil into the body of a living organism, typically through ingestion. The extent to which Arsenic is bioaccessible depends on the soil type, pH, presence of other metals, etc. The chemistry of the various forms of arsenic in the environment determines the mobility of each. Contaminated soil at a Superfund site can contain different forms of Arsenic Porewater Soil particle Arsenic (two forms in this example) Two metals that are commonly encountered in soils at hazardous waste sites are arsenic and lead and ingestion of contaminated soils is a human health concern at these sites. While these metals can be harmful to humans if ingested, they are not always present in the soil in a form that can cause harm to humans or wildlife. The extent to which soils contaminated with these metals is cleaned-up depends in part on the extent to which these metals are available to move from the environment into the body of a human or animal. The extent to which a chemical is available to move into the body of a human or animal and be absorbed is referred to as bioaccessibility.

“The current regulatory paradigm for characterizing the level of contamination in soils and sediments generally does not include measures of the actual bioavailability of these contaminants to human or ecological receptors.” SERDP & ESTCP Expert Panel Workshop on Research & Development Needs for Understanding & Assessing the Bioavailability of Contaminants in Soils & Sediments (2008)

If soil is eaten and taken in the stomach, the type and amount of arsenic determines the extent to which arsenic is bioavailable and thus able to be absorbed by the body. In this scenario the “red” form of arsenic is absorbed by the body and the “brown” form is not; it will be eliminated from the body through feces. Various factors (“bioavailabilty processes”) can impact the amount of contaminant able to be absorbed. Bioavailability processes refer to the individual physical, chemical, and biological interactions that determine the exposure to chemicals associated with soils and sediments. NRC, 2003. These processes modify the amount of chemical in soil or sediment that is actually absorbed and available to cause a biological response. NRC, 2003. Bioavailability is a measure of how much of a contaminant is absorbed when people are exposed to that contaminant through inhalation, skin contact or food intake (EPA definition).

Bioavailability Processes The individual physical, chemical, and biological interactions that determine the exposure of plants and animals to chemicals associated with soils and sediments. These processes modify the amount of chemical in soil or sediment that is actually absorbed and available to cause a biological response. National Research Council, 2003.

USEPA Bioavailability Research (Bradham et al) Evaluate arsenic bioaccessibility, bioavailability and speciation Test soils affected by urban and historical land use activities and compare to three standard reference materials Measure total arsenic in soil and biological samples using instrumental neutron activation analysis (INAA) U.S. Environmental Protection Agency Office of Research and Development National Exposure Research Laboratory Human Exposure and Atmospheric Sciences Division Methods Development and Applications Branch Instrumental Neutron Activation Analysis (the gold standard for total arsenic analysis in soil, which is important for us to have an accurate comparison of the bioaccessibility measurements with the mice excrement). 

To assess the bioavailability of Arsenic from soil at a specific Superfund site and more accurately determine the risk to human health, scientists use a mouse model to study and predict the amount of arsenic that is bioavailable to humans at this particular site.

Mice studies enable scientists to calculate the relative bioavailability of soil arsenic at a specific Superfund site which can inform the clean up process and result in a less expensive approach that is still protective of human health.

Barber Apple Orchard NPL Site 25 to 38 percent of the arsenic in soil from the Barber Orchard NPL site was determined to be bioavailable and not the 100% assumed and used in the risk assessment. http://www.epa.gov/region4/superfund/sites/npl/northcarolina/baronc.html

Evaluating Risk “Incorporating bioavailability considerations in the calculation of risk can: optimize the extent of cleanup required to be protective, improve site decision-making, and can be an important factor in balancing the risks caused by remedial action with the risks addressed by remedial action.” -Interstate Technology and Regulatory Council (ITRC) http://www.itrcweb.org/Training?topicID=4 Interstate Technology and Regulatory Council (ITRC)