Can soil and plant health, and ecosystem services, be improved via reclamation efforts in heavy metal-contaminated alluvial mine tailings deposits?

Abstract

The Leadville Mining District in central Colorado, USA began in 1860, extracting over $2 billion USD (current value) in precious metals over its 140 year history.  Unfortunately, mining operations and pyritic deposits contributed to metals contamination in surface/ground waters, ultimately decimating parts of the food chain.  Some sediments contained > 30,000 ppm available Zn; subsequently, sites were devoid of vegetation leading to enhanced erosional losses.  The location was deemed a US EPA Superfund site in 1983.  In 1998, a study was initiated to prove that alluvial mine tailings’ acid-generating potential can be reduced via lime application (224 Mg ha-1), and the addition of biosolids (224 Mg ha-1) in conjunction with native seed mixes could help reclaim these locations. In 2019, we revisited Leadville to access reclamation success on soil health, plant metal accumulation, and the potential positive or negative long-term environmental effects.  Four transects were located on-site, corresponding to past locations that contained varying heavy metal concentrations yet with ~ the same soil pH.  Five soil samples (to 15 cm) were collected along each transect; all plants were collected within a 1-m quadrant next to each soil sampling location.  Five additional soil samples were collected on-site in areas represented by seeps, where no plants were growing and soil surface metal salt precipitates were present.  We utilized the Soil Management Assessment Framework (SMAF) to ascertain changes in soil health between on-site locations, and overlaid this data with plant metal concentration data.  We found that soil organic C, aggregate stability, bulk density, microbial biomass C, pH, EC, and extractable P and K were indicative of soil reclamation success.  Unfortunately, SMAF does not ascertain differences in bioavailable heavy metals, which were still elevated at locations across the site, leading to excessive plant heavy metal accumulation even though plants did not show toxicity symptoms. However, the presence of plants has helped stabilize these alluvial mine tailings, protecting them from loss via water erosion, leading to improved ecosystem function and services. We anticipate this work leading towards a framework to ascertain soil-plant health on heavy metal contamination mine lands globally.