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The role of hydrology, geochemistry and microbiology in flow and solute transport through highly heterogeneous, unsaturated waste rock at various test scales Blackmore, Sharon

Abstract

Drainage chemistries from unsaturated waste rock are affected by a number of hydrological, geochemical and microbiological processes. These processes are generally coupled and reliable prediction of solute loads is a difficult task for most mine sites, making it challenging to provide accurate estimates of future water treatment requirements. An assessment of preferential and matrix flow, geochemical controls and microbially-enhanced weathering is beneficial to provide an improved understanding of the dominant drainage controls. A multi-scale waste rock study was implemented at the Antamina mine (Peru) to assess drainage controls using 10-m high pile experiments, 1-m field barrels and 0.8-m laboratory columns. Observed drainage chemistries show a strong seasonal pattern in response to changes in infiltration rates, with increasing concentrations during the dry season and decreasing concentrations during the wet season. These seasonal fluctuations are less pronounced for finer-grained material, likely due to lower proportions of preferential flow, indicating that hydrological processes provide a key control on observed drainage chemistries. Microbiological analyses show iron-oxidizing neutrophiles are ubiquitous to all rock types, whereas proportions of acidophiles are strongly influenced by lithology. Drainage waters are more acidic and contain higher metal loads with increasing proportions of acidophilic microbes. Breakthrough curves of an externally-applied bromide tracer show that infiltration migrates mostly through matrix materials, with a minor proportion following preferential flow paths. Long tails indicate a portion of mass enters extremely slow matrix flow paths and/or immobile domains. Chloride, originating from blasting residues, was used as an internal tracer. Mean residence times from chloride breakthroughs are longer than bromide values for the same spatially-specific region, suggesting a slow release of chloride from low permeability matrix material. Flow and solute transport processes were successfully modeled using a dual-porosity mobile-immobile approach in HYDRUS1D. This research provides an improved understanding of the governing hydrologic and geochemical processes relevant to Antamina waste rock with implications for the large-scale waste rock dumps at site. Other aspects of this research, such as using blasting residues as an internally-applied tracer and using a dual-domain approach to model flow and solute transport, will be of value to other mines with similar conditions.

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