Bioaccessibility of mercury in soils

The initial risk assessment for the East Fork Poplar Creek (EFPC) floodplai n in Oak Ridge, Tennessee, a superfund site heavily contaminated with mercu ry, was based on a reference dose for mercuric chloride. Mercuric chloride, however, is a soluble mercury compound not expected to be present in the f loodplain, which is frequently saturated with water. Previous investigation s had suggested mercury in the EFPC floodplain was less soluble and therefo re potentially less bioavailable than mercuric chloride, possibly making th e results of the risk assessment unduly conservative. A bioaccessibility st udy, designed to measure the amount of mercury available for absorption in a child's digestive tract (the most critical risk pathway endpoint), was pe rformed on 20 soils from the EFPC floodplain. The average bioaccessible mer cury for the 20 soils was 5.3%, compared with 100% of the mercuric chloride subjected to the same conditions. The alteration of the procedure to more closely mimic conditions in the digestive tract did not significantly chang e the results. Therefore, the use of a reference dose for mercuric chloride at EFPC, and potentially at other mercury-contaminated sites, without inco rporating a corresponding bioavailability adjustment factor may overestimat e the risk posed by the site.