Environmental geochemistry of an epigenetic Pb-Zn-Ag deposit at the abandoned Cecilia mine, Puno region, Peru

Abstract

The abandoned Cecilia Pb-Zn-Ag mine is located at the headwaters of the Lake Titicaca watershed in the Altiplano of Peru. The site is characterized by three months of high precipitation and nine months of limited precipitation. The environmental geochemical characterization of the abandoned mine was done to evaluate environmental risks at the site from mine wastes and mine drainage, and their potential for downstream impacts on water quality in Lake Titicaca. The approach included sampling of mine waste, water, and sediment. Composite mine waste samples were collected from six main piles (four tailings, two waste rock). The surface water was collected from (1) drainage from mine portals; (2) the Cecilia and Crucero rivers upstream of mine influences; (3) the impacted reach of the Cecilia River down to its confluence with the Crucero River; and (4) the Crucero River, which receives drainage from the Cecilia River. Sampling in the dry season did not identify seeps from the waste rock or tailings piles. This study documents the capacity of the site to generate acid mine drainage from the mine waste and underground workings. Mine waste has elevated concentrations of As (up to 883 mg/kg), Cu (up to 20,106 mg/kg), Pb (up to 16,716 mg/kg), and Zn (up to 11,937 mg/kg). Results for dissolved concentrations from leaching experiments on mine waste samples showed high leachability for As (0.001 to 0.95 mg/L), Cu (0.01 to 57.34 mg/L), Cd (0.001 to 1.13 mg/L), Fe (0.42 to 785 mg/L), Zn (0.01 to 91 mg/L), and Mn (0.05 to 279 mg/L) with an acidic pH (2.5 to 6.0). Water chemistry at the site varied on the basis of water type. The Cecilia and Crucero rivers had neutral pH and low concentrations of metals upstream of mine influences. In contrast, samples collected at the mine portal were highly acidic (pH 1.4 to 3.7) with high dissolved concentrations of Fe (up to 4,720 mg/L), Al (up to 400 mg/L), sulfate (up to 19,428 mg/L), As (up to 5.92 mg/L), Cd (up to 9.84 mg/L), Cu (up to 1.56 mg/L), Pb (up to 1.95 mg/L), and Zn (up to 4,065 mg/L), causing an increase in metal concentrations in the river downstream after mixing. Carbonate rocks in the watershed produce alkaline waters that neutralize acid drainage prior to its confluence with the Crucero River. Proposed remediation methods include capping mine waste to limit contact with rainwater and passive treatment of mine portal drainage.