Extensive gold mining has left a legacy of metal-contaminated mine wastes across the state of California, where large volumes of mine tailings and waste rock often contain elevated concentrations of naturally-occurring arsenic. Characterizing trends in arsenic speciation and solubility as a function of particle size can provide insight into the processes that control arsenic transport from mine-impacted sites, including mobilization during rainfall and flooding events that may threaten groundwater supplies.

Mine waste and adjacent background soil samples from the Randsburg mining district in Southern California were collected and weighed prior to particle size separation to generate 11 size range-specific fractions (from >2830 um down to <20 um). Each fraction was weighed and analyzed using ICP-MS for concentrations of 48 separate elements including arsenic, iron, and other potentially toxic metals. Microspectroscopic methods were then applied to selected size fractions of mine wastes, utilizing X-ray fluorescence and X-ray absorption spectroscopy to assess the speciation, distribution, and correlation of metals of interest. Finally, the availability of arsenic from mine wastes through exposure to water or passive ingestion or inhalation was examined by leach extractions using water, a simulated gastric fluid (SGF), and a simulated lung fluid (SLF) medium.

Results demonstrate that arsenic is typically more concentrated in the finer grain size fractions, often by over an order of magnitude, potentially increasing its mobility, reactivity, and bioavailability. Changes in arsenic speciation with particle size corresponding to the removal of more soluble phases and formation of secondary minerals or sorbed arsenic species (i.e. to iron oxyhydroxides) may partially offset the potential toxicity of arsenic in fine-grained fractions as demonstrated through the water leach extractions. However, SGF studies show that in acidic conditions arsenic is still preferentially released in the finer fractions, even when corrected for surface area effects. Arsenic solubility in the samples studied was found to increase as follows: water < SLF < SGF, with approximately an order of magnitude solubility difference between each medium.