Page 335 - Soil and water contamination, 2nd edition

322 Soil and Water Contamination

left. The tracer has dispersed both longitudinally and vertically. It should be emphasised that
in reality, dispersion occurs in three dimensions; the lateral dispersion cannot be shown in
the two-dimensional model in Figure 17.11. Leachate migration should thus be considered
as a three-dimensional plume developing in a three-dimensional aquifer.
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Figure 17.12 shows a vertical cross-section of observed Cl concentrations in a leachate
plume in a relatively homogeneous, sandy aquifer from the Vejen landfill in Denmark
(Lyngkilde and Christensen, 1992). Obviously, the elevated chloride concentrations
-
originate from the landfill. Since Cl is a conservative substance, the plume is only
-
affected by advection and dispersion . Figure 17.12 shows that the Cl plume is diluted in
downgradient direction and that the dilution is much greater longitudinally than vertically.
This phenomenon of a greater longitudinal than transverse dispersion , also in the horizontal
plane, is typical for many landfill leachate plumes (Christensen et al., 2001), so most leachate
plumes are not much wider than the landfill.
Rivett et al. (2001) performed a controlled field experiment to study the development
of the dissolved plume from a continuous, block-shaped multi-component DNAPL source.

This experiment was carried out in a 10 m thick unconfined sandy aquifer located in
Ontario, Canada . The DNAPL source contained trichloromethane (TCM), trichloroethene
(TCE ), and perchloroethene (PCE ). TCM behaved essentially conservatively and was barely
degraded or adsorbed. Figure 17.13 shows the development of the TCM plume in time.
In this experiment the longitudinal dispersion was also much greater than the transversal
dispersion, just as in the landfill leachate plume described above. This caused the TCM
plume to be long and narrow. Apart from the effect of dispersion, the TCM plume exhibits
some density-induced sinking. Figure 17.13 shows that the zone of maximum concentrations
is below the groundwater flow line through the contaminant source. Because the TCM
solution was denser than the surrounding, uncontaminated groundwater, the plume was
diverted slightly downwards

Distance from landfill (m)
0 100 200 300
> 1000
40 500-1000
B1 B2 B3 B4 B5 B6 B7 B8 B9
250-500
38
100-250
36
50-100
Distance above Sea Level (m) 32 <35
34
30-50
Water level
30
Landfill
28
Clay
26
24
22
20
6642 6642 6642

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Figure 17.12 Cl concentrations in a longitudinal, vertical cross-section in the Vejen landfill leachate plume.
Adapted from Lyngkilde and Christensen (1992).

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