http://www.usgs.gov/blogs/features/slidedeck2/soil-geochemistry-maps/arsenic/

Arsenic in the Soils, USGS

How much of this soil arsenic is available to move

into the biosphere?

Arsenic (two forms in this example)

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

“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.

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)

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)