Nature and practical implications of heterogeneities in the geochemistry of zinc-rich, alkaline mine waters in an underground F–Pb mine in the UK

Abstract

Water pollution arising from base metal sulphide mines is problematic in many countries, yet the hydrogeology of the subsurface contaminant sources is rarely well-characterized. Drainage water pumped from an active F–Pb mine in northern England has unusual chemistry (alkaline with up to 40 mg.l −1 Zn) which profoundly impacts the ecology of the receiving watercourse. Detailed in-mine surveys of the quantity and quality of all ground water inflows to the mine were made. These revealed major, temporally persistent heterogeneities in ground water quality, with three broad types of water identified as being associated with distinct hydrostratigraphic units. Type I waters (associated with the Firestone Sill aquifer) are cool (<10°C), Ca–HCO 3–SO 4 waters, moderately mineralized (specific electrical conductance (SEC)≤410 μS.cm −1) with <4 mg.l −1 Zn. Type II waters (associated with the Great Limestone aquifer) are warmer (≈15°C), of Ca–SO 4 facies, highly mineralized (SEC≤1500 μS.cm −1) with ≤40 mg.l −1 Zn. Type III waters (in the deepest workings) are tepid (>18°C), of Ca–HCO 3–SO 4 facies, intermediately mineralized (SEC≤900 μS.cm −1) with ≤13 mg.l −1 Zn, and with significant Fe (≤12 mg.l −1) and Pb (≤8 mg/l). Monotonic increases in temperature and Cl − concentration with depth contrast with peaks in total mineralization, SO 4 and Zn at medium depth (in Type II waters). Sulphate, Pb and Zn are apparently sourced via oxidation of galena and sphalerite, which would release each metal in stoichiometric equality with SO 4. However, molal SO 4 concentrations typically exceed those of Pb and Zn by 2–3 orders of magnitude, which mineral equilibria suggest is due to precipitation of carbonate “sinks” for these metals. Contaminant loading budgets demonstrate that, although Type II waters amount to only 25% of the total ground water inflow to the mine, they account for almost 60% of the total Zn loading. This observation has important management implications for both the operational and post-abandonment phases of the mine life cycle.