Isotope Discrimination of Source Waters, Flowpaths, and Travel Times at an Acid-Generating, Lead–Zinc–Silver Mine, Silver Valley, Idaho, USA

Abstract

Precipitation infiltrates into the lead–zinc–silver Bunker Hill Mine, oxidizes pyrite, and produces acidic waters that discharge from the mine portal. The metasedimentary geology and alteration from 100+ yr of mining provide a heterogeneous environment for source water infiltration and flow within the mine. A university–industry partnership was developed to trace the mine water sources, flowpaths, and travel times to identify potential areas for infiltration reduction. Snowpack, creek, and mine water samples were collected over a 1-year period for the analysis of δ2H, δ18O, and 3H, along with the in situ measurement of temperature, specific conductance, pH, dissolved oxygen, and flow. The isotope tracers were used to identify the source waters, unmix mine water as it moved deeper in the mine, and examine flowpaths in and near the acid-generating pyritic zone. The results indicate creek water infiltrating relatively quickly through the anthropogenically-modified pathways and causing the largest amount of acidic water in the upper levels of the mine. Slower, natural pathways associated with faults, fractures, and bedding planes produce mostly neutral waters with the source waters typically originating at higher elevations. Travel times ranged from <1 to 22 years with shorter pathways to the upper levels of the mine and increasing contributions deeper in the mine from pathways containing older, higher-elevation snowmelt. These slower and older inflows were identified by depleted δ18O values, smaller 3H concentrations, the dampening of the variability of the isotope signals, and pH increases. Reduction of infiltration zones near the upper workings of the mine likely will decrease the acidic waters in the upper levels of the mine, but the higher elevation infiltration zones will continue to contribute snowmelt-derived waters at all mine levels.