Thursday, October 2, 2008: 3:20 p.m.
The Saginaw Hill Mining District, located approximately 10 miles southwest of Tucson, Arizona, contains heavily contaminated waste material (47,301 mg/kg of arsenic) due to previous mining activities. Prior to closure, the site was a popular destination for ATV riders and hikers, and a human health risk assessment was performed to assess the risk posed by arsenic to potential receptors.
Recent EPA publications assert that it is best to measure in vivo relative bioavailability (RBA); however, as measurement of arsenic RBA in animals is time-consuming and costly, in vitro procedures are a logical alternative. Dr. John Drexler at the University of Colorado in Boulder has been working cooperatively with EPA Region 8 to develop an in vitro method that can be used to obtain RBA data for lead, arsenic, and other metals in soils. While the in vitro bioaccessibility method was specifically developed for lead, additional research is underway to substantiate the use of the method as a basis for derivation of in vivo RBA data for arsenic. In addition, a 2003 study performed at the University of Florida substantiated the appropriateness of using relative bioavailability data for arsenic in risk assessments as absorption of soil arsenic into the body was found to be less efficient than absorption of arsenic from water.
Recent EPA publications assert that it is best to measure in vivo relative bioavailability (RBA); however, as measurement of arsenic RBA in animals is time-consuming and costly, in vitro procedures are a logical alternative. Dr. John Drexler at the University of Colorado in Boulder has been working cooperatively with EPA Region 8 to develop an in vitro method that can be used to obtain RBA data for lead, arsenic, and other metals in soils. While the in vitro bioaccessibility method was specifically developed for lead, additional research is underway to substantiate the use of the method as a basis for derivation of in vivo RBA data for arsenic. In addition, a 2003 study performed at the University of Florida substantiated the appropriateness of using relative bioavailability data for arsenic in risk assessments as absorption of soil arsenic into the body was found to be less efficient than absorption of arsenic from water.
Four composite surface soil samples from the site were analyzed for bioaccessibility of arsenic. All four samples were collected following the 2004 USGS methodology published by Kathleen Smith that statistically affirms that a 30-increment composite sample can accurately represent 30 individual grab samples. The Drexler in vitro test results indicated a range of bioaccessibility from 3% to 6%. Based on these results, the RMC for arsenic adjusted to reflect site-specific bioaccessibility resulted in a recreational clean-up criterion of 390 mg/kg.