BIOAVAILABILITY OF ARSENIC AND LEAD IN SOILS FROM THE BUTTE, MONTANA, MINING DISTRICT

Risk assessments for mining sites are typically based on As and Pb bioavailability determined from studies using soluble metal salts. This is an inappropriate analogue of soil ingestion at mining sites due to the lower solubility of As and Pb in naturally occurring mineral assemblages. As and Pb bioavailability in soils from Butte, MT, is further reduced by authigenesis, where galena oxidizes to form a coating of anglesite further armored by sparingly soluble K-jarosite precipitates. Enargite and tennantite were also encapsulated by K-jarosite. When the soil was fed to New Zealand White rabbits, only 11% SIGMA-As and 6% SIGMA-Pb were solubilized in the small intestine. In vitro tests demonstrate that Na2HAsO4.7H2O and Pb(OAc)2 overestimate As and Pb solubility by at least a factor of 5. MINTEQA2 accurately simulated experimental equilibrium As and Pb concentrations, but overestimated their solubility for residence times typical of the gastrointestinal tract.