Effects of Long-Term Topsoil Stockpiling on Soil Quality for Use in Copper Mine Site Reclamation in a Semi-Arid Region

Abstract

After mine closure, reclamation of mine-waste can be challenging given the scale of waste deposition. Mine waste on a single copper mine can occupy thousands of acres of land. This large scale mine waste can be mitigated by adding 15-30 cm of soil cover to minimize dust transportation, contain contaminants, and support plant establishment during reclamation. However, sourcing cover material may strip nearby areas of topsoil and in some cases the cover fails to support development of a vegetative cap. Thus, it is important to source cover materials that will efficiently support ecosystem recovery with minimal destruction of adjacent lands. The objective of this research is to assess the effect of long-term topsoil stockpiling on soil quality in a semi-arid region by characterizing soil health metrics associated with successful ecosystem recovery. For this research, a 14-year-old topsoil stockpile on an Arizona copper mine was sampled in 2021. Two soil cores were drilled to 20 and 30 m in depth, respectively, and sampled every 76 cm. In total, 64 soil samples were collected for physicochemical analyses and 192 for soil DNA biomass to understand the soil quality and variability within the two soil cores. Physicochemical analyses included soil texture, general agronomic properties, plant micro- and macro-nutrients, soil mineral composition, selective oxalate and dithionite extractions, and total elemental composition. Depth profile trends revealed that soil DNA biomass decreased below 1000 ng DNA/g dry soil at 5m in depth in both cores. In the North Core, % organic matter (OM), water extractable organic carbon (WEOC), ammonium (NH4), and iron (Fe) increased around 20m in depth which suggests an anaerobic effect with depth on nutrient composition. Similar patterns were found in the South Core for %OM, WEOC, NH4, and Fe around 10m in depth. Soil mineral composition appears to vary minimally across depth and key changes are associated with active mineral phases. Differences in soil chemistry appear to be driven by organic carbon content and anaerobic conditions. This research will contribute to an improved understanding of the value of topsoil stockpiling during active mine operations as a resource for ecosystem regeneration of lands degraded by mine-waste deposition and mining operations.