Superfund Research Program
Role of Mineral Genesis, Dissolution, and Sorption on Arsenic Fate in Contained Waste Sites
- Project Summary
Final Progress Reports
The Role of Mineral Genesis, Dissolution, and Sorption on Arsenic Fate project's work on the Iron King Mine Humboldt Smelter Superfund (IKMHSS) site (Dewey-Humbolt, AZ) has revealed that elevated lead and arsenic exposure of potential health concern can occur off-site due to a chain of geochemical and physical processes occurring with the mine tailings. The tailings contain significant mineralogically bound concentrations of lead and arsenic. The project work shows that the waste mineral forms containing lead and arsenic undergo geochemical oxidative weathering near the surface of the tailings heap. The tailings near-surface material including efflorescent salt deposits, as well as the dust generated from it, are being characterized using a diversified suite of physical and chemical methods. In addition the bioaccessibility of the lead and arsenic in the materials is being measured using a set of in vitro bioassays in an overall effort to determine the impact of mine tailings dust on human health. Using a fine fraction dust generator specifically built for project purposes, the dust fractions of most concern for human inhalation, PM10 (aerodynamic diameter <10µm) and PM2.5 (<2.5µm), can be isolated and the arsenic and lead concentrations in the fractions measured. Additionally, efflorescent salt, surface crust, and bulk composited samples from the tailings heap were collected, isolated by grain size, and leach tested with an in vitro bioassay using simulated alveolar and gastric fluids. In all cases gastric fluid extracted much more arsenic and lead (>10X) than lung fluid, attributed to the low pH of the synthetic gastric fluid. Simulated gastric fluid extractions yielded arsenic concentrations of 300-650 ppm As and 150-420 ppm Pb, while simulated alveolar fluid extractions concentrations were 10-50 ppm As and ~4 ppm Pb. As much as 35% of the total arsenic and 10% of the total lead in the bulk tailings samples was extractable and 85% of the total As and 100% of the lead in the efflorescent salts was released by the simplified simulated body fluids. The release of arsenic and lead from mine tailings into simulated body fluids in the tens to hundreds of ppm range suggests that dust inhaled or ingested from these tailings may create a significant health hazard. The dust analysis showed that metal(loid) contaminants are more concentrated in the PM10 fraction than in the larger size fractions. Thus, the particle size fraction which is both more readily transported in the atmosphere and are more subject to human inhalation and ingestion has the highest concentration of these toxins. The PM10 particles from the fractionator were shown to carry order of magnitude higher arsenic and lead concentrations than measured in the average tailings bulk mass. In addition, the transport of contaminated dust from the tailings was traced to surrounding soils by identifying Pb and Sr isotopic signatures. The present results emphasize the importance of size-dependent characterization methods for contaminated dust sources. The finer particles are most likely to be transported off-site, most readily enter the respiratory system, and have potentially higher bioavailability due to their high specific surface area.