USING THE BIOAVAILABILITY OF ARSENIC IN SOIL AS A RISK ASSESSMENT TOOL AT A FORMER MANUFACTURED GAS PLANT SITE

Elevated background levels of arsenic (As) in soil in Michigan present a known challenge when evaluating site-specific risk assessment and remediation goals. The background level of total As within soil often exceeds 7.6 milligrams per kilogram (mg/kg), the Michigan Department of Environmental Quality (MDEQ) Part 201 Nonresidential Generic Cleanup Criteria and Screening Levels Direct Contact (DC) Criteria for total As within nonresidential soils. A graduate research study by the Wayne State University Department of Geology was conducted to demonstrate that As in industrial impacted soils have a low bioavailability by determining geochemical forms, bioavailability and potential sources of arsenic in Metro Detroit. A sequential extraction for As showed that the oxide-occluded fraction of the soil contained the highest percentage of As present, suggesting an anthropogenic source of As in the soils. The bioavailable arsenic (BA As) fraction showed that only 15 to 30 percent of the total As present within the soil was available for digestion by the human body, suggesting that industrial affected soils have a low bioavailability.

Near surface soils at a former manufactured gas plant site were collected at 18 locations with known levels of total As exceeding the (MDEQ) Part 201 direct contact criteria for total arsenic within nonresidential soils. These soil samples were tested for total As and BA As to demonstrate that although the total As exceeds the MDEQ Part 201 generic Non-Residential DC Criteria, the amount of the total As available for digestion and uptake by the human body (the bioavailable concentration of As). The average percentage of BA As measured within these soil samples was 14.2% of the total As in the soil. The average concentration of BA As within these soil samples was 6.1 mg/kg of soil, which demonstrates that at the site the total As available for the average human body to digest is below the As soil Part 201 generic Non-Residential DC Criteria of 37 mg/kg, as well as below the As soil Part 201 generic Residential DC Criteria of 7.6 mg/kg. This study has demonstrated that the total As present at this manufactured gas plant site is present at a level where the amount of Ba As in the soil does not pose an unacceptable risk to human health through direct contact.