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Unsaturated hydrology, evaporation, and geochemistry of neutral and acid rock drainage in highly heterogeneous mine waste rock at the Antamina Mine, Peru

University of British Columbia

Physical and geochemical heterogeneities in mine waste rock complicate the prediction and assessment of waste rock effluent water quantity and quality. The objective of this research is to provide a holistic conceptual understanding of the hydrological and geochemical processes that control effluent water quantity and quality, and the complex interactions among processes at the field scale. To this end, a prodigious dataset from three experimental waste-rock piles at the Antamina Cu-Zn-Mo skarn-deposit mine was compiled and analyzed. Analyses included solid-phase mineralogy and physical characteristics; effluent and pore-water hydrology and geochemistry; and an aqueous tracer study. The instrumented piles (36 m x 36 m x 10 m) are each composed of a single rock type and are exposed to almost identical atmospheric conditions, isolating the effect of rock type on hydrological regimes. Physical waste rock heterogeneities result in highly variable hydrology that is strongly dependent on material particle size distributions and especially the presence of large boulders. The hydrological regimes include wide ranges of velocities for matrix flow (<2-12 cm/day), preferential flow (40-2000 cm/day), and pressure-induced wetting fronts (7-105 cm/day), all of which are strongly influenced by antecedent water content and precipitation patterns resulting from a two-season (wet/dry) climate. Evaporation is also highly variable among waste rock types on annual (24%-75% of precipitation) and multi-year timescales (28%-59% of precipitation). Mineralogical heterogeneities result in material-specific, temporally and spatially variable circum-neutral to acidic geochemical conditions (pH 4.6-8.5). Other geochemical controls on solute concentrations include precipitation and dilution of secondary minerals, sorption, and pH effects from CO₂ degassing. Furthermore, hydrology strongly influences effluent aqueous geochemistry, leading to solute concentrations and loadings that can fluctuate by several orders of magnitude between wet and dry seasons (e.g., Pile 2 Cu loadings: August-2010, 1.8x10-7 mg/(kg·wk); February-2011, 1.6x10-1 mg/(kg·wk)). Hydrological controls on aqueous geochemistry include seasonal solute accumulation; solute flushing through matrix flow paths of variable velocities; mixing and dilution at the pile base; and seasonal changes in moisture content that control internal CO₂ concentrations. The results highlight the need to account for unsaturated hydrology during the prediction and assessment of aqueous geochemistry from waste rock.

Text

2014-04-17

Attribution-NonCommercial-NoDerivs 2.5 Canada

http://hdl.handle.net/2429/46492

Geological Sciences

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/2.5/ca/