114-1 Climate-Driven Weathering Controls Lead Bioaccessibility Through Mineralogical Transformation in Legacy Mine Tailings

Session: Environmental Geochemistry and Health (Part I)

Presenting Author:

Robert Root

Authors:

Root, Robert A.¹, Alghzawi, Ma'in Zaid², De Gracia , Xenia Medina³, Chorover, Jon⁴

(1) Department of Environmental Science, The University of Arizona, TUCSON, AZ, USA, (2) Department of Environmental Science, The University of Arizona, TUCSON, AZ, USA, (3) Department of Environmental Science, The University of Arizona, TUCSON, AZ, USA, (4) Department of Environmental Science, The University of Arizona, Tucson, AZ, USA,

Abstract:

Legacy polymetallic mine tailings are major sources of Pb-bearing particulate matter (PM), yet risk assessments still rely largely on total Pb concentrations, overlooking how mineralogy and speciation govern Pb mobility and bioaccessibility. We hypothesize that climate-driven weathering fundamentally alters Pb speciation and, in turn, Pb bioaccessibility and human exposure potential. To test this, we isolated particulate matter from tailings collected at 10-cm depth increments to 2 m in twelve 50- to 100-year-old polymetallic sulfide mine tailings piles spanning a climate gradient from the arid deserts of Arizona to the humid temperate forests of Oregon. Each profile was characterized across the vertical oxidative weathering front using total elemental concentrations, mass-transfer coefficients (τ) to quantify geochemical enrichment and depletion, two-stage in vitro gastric bioaccessibility (IVBA), and Pb and Fe speciation by synchrotron X-ray absorption spectroscopy (XAS) before and after IVBA extraction.

Weathering intensity varied systematically with climate, producing distinct vertical weathering profiles. Surface tailings consistently exhibited the lowest Pb bioaccessibility, whereas the parent material generally exhibited the highest values. Mass-transfer coefficients (τ) quantified climate-dependent redistribution of Pb, with chemically enriched (τ_max) horizons developing within the oxidative weathering profile. Although Pb was enriched within these horizons relative to the parent material, enrichment alone did not predict bioaccessibility. Instead, Pb bioaccessibility within the τ_max horizons varied with climate, remaining below that of the parent material at humid to intermediate sites but increasing to match or exceed parent material values at the driest sites. Synchrotron XAS revealed transformation of primary Fe- and Pb-bearing minerals to secondary weathering products, with increasing incorporation of Pb into jarosite-group minerals corresponding to progressively lower Pb bioaccessibility. Tailings with pH <4 exhibited negligible Pb bioaccessibility, indicating that Pb incorporated into acid-stable secondary weathering phases is highly resistant to gastric dissolution.

Together, these results demonstrate that climate controls Pb bioaccessibility by driving mineralogical evolution during weathering, which in turn alters Pb speciation. Molecular Pb speciation, rather than total Pb concentration, is the primary control on bioaccessibility across weathered mine tailings. These findings provide a mechanistic framework linking climate, weathering, mineral transformation, and Pb bioaccessibility, with direct implications for risk assessment and remediation of legacy mining districts.

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