Legacy mine tailings in semiarid environments present significant health risks to nearby communities from off-site wind and water transport of toxic fugitive dusts. Without remediation, tailings can remain barren for decades to centuries, providing a point source of toxic metal(loids). Successful mitigation of toxins (As, Pb) from tailings is often limited to containment. Capping tailings with soil or gravel is an accepted, although expensive, strategy to reduce erosion. Revegetation via assisted direct planting (aka phytostabilization) has the potential to be a cost-effective and self-sustaining alternative “green-technology” to expensive capping. The impact of phytostabilization, and requisite added carbon and irrigation, on mechanisms of microbial composition dynamics and metal(loid) speciation remain poorly known. Contaminant mobility is being investigated with a highly-instrumented greenhouse mesocosms (1m dia x 0.5 m deep) study using advanced microbiological tools and synchrotron x-ray based molecular probes. Composted treatments initially neutralized the near surface acid tailings (~2 to ~6.5), but after 9 mo, mesocosms experiments showed a decrease back to pH 2. The exception was the root zone of <i>Atriplex lentiformis</i>, which buffered the pH for 12 months. Rhizosphere microbiota experienced a 5-log increase in the compost-amended compared to control greenhouse mesocosms, a metric of improved soil conditions. Weathering of the primary sulfidic minerals, indicated by Fe and S speciation was shown to control the mobility, speciation and bioavailability of both As and Pb via sequestration in (meta)stable neo-formed phases as plumbo-jarosite and As(V) substituted in hydronium jarosite, with important implications for health risk management. We show that the disequilibrium imposed by phytostabilization results in an increase of heterotrophic biomass that is concurrent with a time series of geochemical transformations, which controls the species, fate, and bioavailability of toxic metal(loid)s.

Stabilization of metalliferous mine tailings during mesocosm-scale phytostabilization

253rd National Meeting (2017)

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