BACKGROUND LEVELS, GEOCHEMICAL FORMS AND BIOAVAILABILITY OF ARSENIC IN URBAN SOILS OF DETROIT, MICHIGAN

The adverse health effects of arsenic are well established, but almost all previous studies have focused on drinking water. Relatively little is known about health effects of arsenic in urban soils. This is a potential problem in Detroit, Michigan because the background level of soil As often exceeds the state’s residential direwct contact limit of 7.6 mg kg^-1. The higher than normal background level of soil As is attributed partly to the presence of arsenopyrite in the underlying Marshall Sandstone bedrock. We conducted several geochemical/ mineralogical tests on urban soils in Detroit and surrounding western and southern suburbs to determine the geochemical forms, bioaccessibility, and possible anthropogenic sources of As in the surface soil. Seventy surface soil samples were collected. A sequential extraction was done to measure the geochemical forms of As found in each sample. The oxide-occluded fraction of the sequential extraction showed the highest concentrations; with the highest values ranging from 3-7 ppm As. Bioaccessibility was measured using a physiologically-based extraction test. The bioaccessible As fraction in many cases tested above the State of Michigan 7.6 mg kg^-1 residential direct contact limit, some more than double that amount. Additionally, soils were examined using a light microscope for micro-particles of steel-making slag, coal, coal cinders, and ferrous and siliceous microspheres in order to identify possible sources for anthropogenically derived As. Scanning Election Microscope analysis showed the presence of anthropogenic micro-particles that previous studies have shown may contain As. This suggests that the As found in the samples from the Detroit area are derived from both natural and anthropogenic sources.