The mine drainage from the spoil tip of a Bavarian pitch coal mine – a history of unmonitored natural attenuation

For more than 60 years, since the abandonment of coal mining in the Hausham colliery, mine drainage
from the site percolates an associated spoil tip over a distance of several hundred meters, before
discharging into a nearby lake – the Loidlsee. Generally, coal mine drainage is often characterized by
poor water quality, e.g. low pH values, high concentrations in dissolved toxic metal ions and high salinity,
posing environmental risks for receiving streams. With this in mind, a closer examination of this
apparently unmonitored coal mine discharge was carried out in summer 2020. A coupled elemental and
isotopic approach was applied to reconstruct the origin and chemical evolution of mine drainage. Water
samples were taken along the mine drainage and from surface waters in the close vicinity of the mine
drainage. In addition, solid samples from secondary phases along the mine drainage were analysed. At
the discharge location the mine water has a pH-value around 6.5, an electrical conductivity of
2,710 μS/cm and high concentrations of sulphate (> 1,000 mg/l) and iron (2,8 mg/l) and elevated
contents of nickel. These values are significantly reduced over the course of the mine water flow,
providing the water to enter lake Loidlsee environmentally non-hazardous. Decreasing values are linked
to (1) the formation of secondary iron precipitates, (2) the formation of secondary calcium carbonates
along the mine drainage and (3) mixing with a different groundwater source along the flow path. 34S/32S
isotopic ratios of dissolved SO42- point to the oxidation of sulphidic minerals such as pyrite. δ2HH2O and
δ18OH2O values of the mine drainage differ significantly from those of the surface waters which i