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234 Chapter Six

Fig. 6.20 Temporal variation of concentration profiles of (a) chloride, (b) total organic carbon (TOC) and (c) pH in the unsaturated zone of
the Triassic sandstone aquifer at the Burntstump landfill site, Nottinghamshire, 1978–1987. After Williams et al. (1991).

was completed in 1977 after which the chloride front metres wide, corresponding to the width of the
effectively stagnated as a result of the effectiveness landfill; and restricted to less than 1000 m in length as
of the completed cap in limiting infiltration to very a result of attenuation processes within the leachate
low values. During an 8-year period of stagnation plume, although potentially longer if contamination
until 1986, a persistent decrease in the TOC : Cl ratio occurs in fissured or fractured material. According to
occurred in the unsaturated zone indicating the con- Christensen et al. (2001), the infiltrating leachate cre-
tinuous removal of organic carbon. Early surveys of ates a sequence of redox zones in the groundwater,
organic compounds at the site showed the presence with methanogenic conditions close to the landfill
of phenols (absent post-1984), readily degradable and oxidized conditions at the outer boundary of the
volatile fatty acids (absent post-1977), mineral oils plume. The anaerobic zones are driven by microbial
and halogenated solvents; the latter remaining at utilization of dissolved organic matter in the leachate
levels of up to 50 µgL −1 beneath the landfill. The in combination with reduction of oxidized species in
results showed that the high buffering capacity of the aquifer, particularly iron oxides, which provides
the Chalk is conducive to microbial metabolism to substantial redox buffering by reducing iron oxides
explain the decrease in TOC : Cl ratios with time and and precipitating reduced iron species. Other import-
the disappearance of readily degradable organic com- ant attenuating mechanisms include dilution, ion
pounds. However, the organic solvents remained as exchange, complexation and precipitation, such that
persistent contaminants (Williams et al. 1991). heavy metals are not normally considered a major
Below the water table, the general shape of a land- groundwater pollution problem in leachate plumes.
+
fill leachate plume is determined by the advective- The attenuation mechanism for NH is not well
4
dispersive nature of groundwater flow in the aquifer, understood but probably involves anaerobic oxida-
the amount of recharge from the leachate mound tion of this persistent pollutant in landfill leachate.
developed below the landfill and the increased dens- Xenobiotic organic compounds in leachate are not
ity of the leachate (Fig. 6.15). In most cases, the extensively attenuated by sorption processes, but
leachate plumes are relatively small: a few hundred there is increasing evidence that many organic com-