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226 Chapter Six

BO X
Mine water pollution
6.5

Mine water pollution is a widespread problem in present and former The study of long-term changes in the quality of polluted mine water
mining districts of the world with numerous cases of severe water discharges from abandoned underground coal workings in the
pollution having been reported from base metal mines, gold mines Midland Valley of Scotland shows that mine water pollution is most
and coal mines. In the United Kingdom, concern centres on aquatic severe in the first few decades after a discharge begins (the ‘first
pollution from the major coalfields and most of the base metal ore flush’), and that the largest systems settle down to a lower level of
fields, such as those of Cornwall, upland Wales, northern England pollution, particularly in terms of iron concentration, within 40 years
and Scotland. A general conceptual model for sources of mine water (Wood et al. 1999). As shown in Fig. 2, long-term iron concentrations
−1
−1
pollution, transport pathways for soluble contaminants and poten- of less than 30 mg L are typical, with many less than 10 mg L . In
tially sensitive receiving waters at risk of contamination is shown in the Scottish coalfield, low pH values do not generally persist due to
Fig. 1. Mining activities contribute significantly to the solute load of the rapid buffering of localized acidic waters by carbonates.
receiving surface waters and aquifers. To illustrate, the contribution The Durham coalfield of north-east England (Fig. 3) was one of
of sulphide mineral weathering associated with mine sites to the the first coalfields in the world to be commercially exploited and has
sulphate ion load is estimated at 12% of the global fluvial sulphate left a legacy of acid mine discharge as the mines have closed. The
flux to the world’s oceans (Nordstrom & Southam 1997). Associated worked Coal Measures comprise Carboniferous strata of fluvio-
with this weathering flux are dissolved metal ions (for example Fe, lacustrine and fluvio-deltaic facies. High-sulphur coals, which might
Zn and Al), sulphate and, in the case of pyrite, acidity. When dis- be expected to be prolific generators of acid mine water, are associ-
charging into the wider environment, acid mine drainage can coat ated with shale bands of marine origin (Younger 1995). In the more
stream beds with orange precipitates of iron hydroxides and oxyhy- easterly districts of the coalfield, the Coal Measures are uncon-
droxides (‘ochre’) as well as white aluminium hydroxide deposits formably overlain by Permian strata including the Magnesian lime-
(Gandy & Younger 2003). stone, an important public supply aquifer. Mining ceased in the
The oxidative weathering and dissolution of contaminant source exposed coalfield to the west of the Permian scarp in the early
minerals such as pyrite and sphalerite associated with abandoned 1970s and the deep mines beneath Permian cover on the coast
deep or opencast coal and metal mines and surface spoil heaps is closed in 1993. Following closure, pumping water from the coastal
described by the following two equations: mines ceased and the water table has begun to rebound. In the far
west of the coalfield, uncontrolled discharges of acid mine drainage
2+
FeS (s) + 7/2O (aq) + H O → Fe + 2SO 2− + 2H + occurs. The water quality of the five most significant discharges is
2
4
2
2
(pyrite weathering) eq. 1 given in Table 1.
Remediation technologies for the treatment of acidic mine
2+
ZnS(s) + 2O (aq) → Zn + SO 4 2− (sphalerite weathering) eq. 2 waters can be divided into active techniques, such as alkali dosing,
2

Fig. 1 Schematic diagram of contaminant sources, transport pathways and potential receiving waters in mining environments. A
distinction is made between juvenile contamination arising from active weathering of sulphide minerals above the water table where
oxygen ingress occurs, and vestigial contamination that accumulates as secondary mineral precipitates of metal ions and sulphate that
arise from sulphide mineral weathering in dewatered void spaces within mine environments. After Banwart et al. (2002).