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Groundwater quality and contaminant hydrogeology 233

position processes that take place during the break- organic compounds such as volatile fatty acids are
down of putrescible materials contained in domestic being degraded at rates faster than they are pro-
wastes. Three phases of decomposition are recog- duced. The less aggressive nature of this leachate is
nized. In the first phase, aerobic decomposition rapidly associated with substantial reductions in concentra-
uses the available oxygen in the wastes. The reaction tions of Fe and Ca being solubilized from the wastes.
is common with septic systems (eq. 7 in Table 6.8) Concentrations of conservative determinands such
with the process usually lasting for up to 1 month, in as chloride remain stable or even increase, demon-
which time significant quantities of CO and some H strating that dilution is of little importance in the
2 2
+
are produced. In the second phase, anaerobic and fac- results obtained. High concentrations of NH are
4
ultative organisms (acetogenic bacteria) hydrolyse also maintained, demonstrating the continuing bio-
and ferment cellulose and other putrescible mater- logical activity and rapid acetogenic processes occur-
ials, producing simpler, soluble compounds such as ring to decompose the solid wastes (Robinson 1989).
volatile fatty acids. This phase (represented by eq. 3 In the event of leachate contamination of the
in Table 6.8) can last for several years producing an unsaturated zone below a landfill site, attenuation
acidic leachate (pH of 5 or 6) high in biological oxy- of the leachate by physical, chemical and biological
−1
gen demand (BOD) of greater than 10,000 mg L , processes is possible. However, the extent of penetra-
+ −1
and NH in the range 500–1000 mg L . The aggress- tion of organic and other components of landfill
4
ive leachate assists in the dissolution of other waste leachate in an aquifer will depend on its buffering
components, such that the leachate can contain high capacity leading to the development of favourable
levels of Fe, Mn, Zn, Ca and Mg. Gas production con- conditions for microbial degradation. This buffer-
sists mainly of CO with lesser quantities of CH ing effect is shown in Figs 6.20 and 6.21 for two con-
2 4
and H . The third phase experiences slower-growing trasting landfill sites located above sandstone and
2
methanogenic bacteria that gradually consume sim- Chalk aquifers, respectively. In the Nottinghamshire
ple organic compounds, producing a mixture of CO Triassic sandstone aquifer at the Burntstump munici-
2
and CH gases which is released as landfill gas that pal landfill site, sequential profiling of the unsaturated
4
can be recovered and used as an energy source. This zone shows a downward migration of a conservative
phase of methanogenesis (eqs 6a and 6b in Table 6.8) chloride front over a 9-year period (Fig. 6.20). A sim-
will continue for many years, if not decades, until ilar movement is observed for the organic compounds
the landfill wastes are largely decomposed and atmo- as indicated by the TOC (total organic carbon) val-
spheric oxygen can once more diffuse into the landfill. ues, mainly attributable to total volatile acids. A zone
Leachates produced during this last phase are char- of reduced pH values migrated downwards consist-
acterized by relatively low BOD values. However, ently with the TOC, with the lower edge of the low
+
NH continues to be released and will be present pH zone corresponding closely with the leading edge
4
at high values in the leachate. Inorganic substances of the TOC front. The profiles for TOC and pH are
such as Fe, Na, K, sulphate and chloride may continue interpreted to result from the combination of the
to dissolve and leach from the landfill for many years. low buffering capacity of the sandstone due to its
These various phases of landfill decomposition limited carbonate content, the high concentrations
are represented in the analyses of leachate given in of organic acids, and the dissolution of CO generated
2
Table 6.7. The landfill cell at Compton Bassett was within the wastes, all of which depress the pH of
filled over a 2- to 3-year period from late 1983 during the interstitial water at the leachate front. Hence, at
which time waste was filled to a depth of 15–20 m, this site, there is significant penetration of the organic
compacted, capped with clay and restored to grass- and other components of leachate due to the low
land in 1987. Table 6.7 demonstrates rapid changes buffering capacity and the persistence of conditions
in leachate composition from a strongly acetogenic unfavourable to microbial degradation at low pH
to methanogenic state during a 12-month period (Williams et al. 1991).
from October 1985. The change is accompanied by a By contrast, the landfill site at Ingham, Suffolk, in
rise in pH from acid to slightly alkaline conditions, eastern England and situated on the Cretaceous
but more obviously by steep declines in BOD and Chalk aquifer gave the sequential profiles shown in
chemical oxygen demand (COD) concentrations as Fig. 6.21. Reinstatement of the site to agricultural soil