Page 246 - Hydrogeology Principles and Practice

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Groundwater quality and contaminant hydrogeology 229

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Continued
6.5

aeration, flocculation and settlement, and passive techniques, such
as constructed wetlands, inorganic media passive systems (Fig. 4)
and subsurface flow, bacterial sulphate reduction systems. Unless
the metal loadings are particularly high, in which case active treat-
ment can be a cost-effective solution in the long term, passive treat-
ment is increasingly the preferred option (Gandy & Younger 2003).
In practice, mine water remediation should allow for active treat-
ment of discharges for the first decade or two, followed by long-term
passive treatment after asymptotic pollutant concentrations are
attained (Wood et al. 1999).

Fig. 4 (right) Limestone filter installation sited upstream of Cwm
Rheidol, west Wales, to precipitate metals contained in ochreous
acid mine drainage resulting from the flushing of ferrous sulphate
and sulphuric acid (the products of oxidation of pyrite and
marcasite during dry-working of mines) and issuing from the
abandoned mine adits and spoil heaps in the Ystumtuen area.
The acidified water dissolves heavy metals such as Pb, Zn, Cd
and Al which enter the river system, the effect of which has been
detected 16 km downstream of the mined area with a pH as low
as 2.6 (Fuge et al. 1991).

alternative to petroleum-based decreasing solvents 1960s, both TCE and PCE have begun to be replaced
in the metal processing industry. Until about 1970, by the less toxic 1,1,1-trichloroethane (TCA) and
trichloroethene (TCE) and tetrachloroethene (PCE) 1,1,2-trichlorotrifluoroethane (Freon 113).
were predominantly used, the latter also in dry clean- Hydrocarbons include petrol, aviation fuel, diesel
ing applications. TCE and PCE degrade extremely and heating oils. As a group, their physical character-
slowly, and some of the degradation products may be istics are variable, particularly that of viscosity; but all
more toxic, soluble and mobile than the parent com- have a density less than water, and a heterogeneous
pounds. For example, tetrachloroethene can be pro- composition dominated by pure hydrocarbons. In
gressively de-halogenated, first to trichloroethene, the context of groundwater, regulation is aimed prim-
then to dichloroethene, and finally to carcinogenic arily at taste and odour control (guideline values
vinyl chloride. From the mid-1970s, concern was are given in Appendix 9). Sources of contamination
expressed about the potentially carcinogenic effects include oil storage depots, cross-country oil pipelines,
of TCE, PCE and carbon tetrachloride (CTC) at trace service stations, tanker transport and airfields.
level concentrations in drinking water, and the WHO In a further survey of the Birmingham Triassic
−1
−1
set guide values of 70 µgL for TCE, 40 µgL for sandstone aquifer in the English Midlands (Rivett
PCE and 2 µgL −1 for CTC (Appendix 9). Since the et al. 1990), almost half of the 59 supply boreholes