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Groundwater quality and contaminant hydrogeology 213

BO X
Continued
6.3

Fig. 4 Plan view of the conservative sulphate and organic solute plumes observed after 322 days during the emplaced source tracer
experiment and based on maximum concentrations measured in each multilevel sampling well. Contour values are shown at the base of
the plots, with the minimum value corresponding to the outer plume contour. After Rivett et al. (2001).

significant longitudinal dispersion. TCM was observed to be the experiment, exhibited a retardation factor up to three times lower
most mobile dissolved organic solute, closely followed by TCE. PCE than observed in the previous Stanford-Waterloo natural gradi-
was comparatively more retarded. Background sulphate concentra- ent tracer experiment. Further laboratory and modelling studies
tions in the aquifer restricted sulphate plume detection to a range by Rivett and Allen-King (2003) indicated that sorption of PCE
of two orders of magnitude, such that the lower detection limit for was non-linear and competitive with reduced sorption behaviour
the organic solute plumes caused these plumes to be detected observed in the presence of co-solvents such as TCE.
beyond the conservative sulphate tracer plume. Significantly, plume attenuation due to abiotic chemical reac-
Hence, in the emplaced source tracer experiment, all the organic tions or biodegradation processes was not apparent in the
solute plumes appeared to have relatively high mobility, including emplaced source tracer experiment. The absence of biodegradation
the most retarded solute, PCE. This higher mobility was confirmed was primarily attributed to a lack of additional, easily biodegrad-
by the estimated retardation factors; TCM was essentially conserva- able carbon within the emplaced source plumes, and the prevailing
tive (R = 1.0) and TCE almost so (R = 1.1), much lower than aerobic conditions which did not favour the sulphate-reducing or
d ~
d ~
values estimated for the organic solutes injected in the Stanford- methanogenic conditions required for dechlorination. Although
Waterloo tracer experiment. The sorptive retardation of PCE of attenuation due to biodegradation may be a significant factor at
about 1.4–1.8 (Rivett & Allen-King 2003) was observed to be some sites, the emplaced source experiment demonstrated that dis-
greater than TCE, consistent with its greater hydrophobicity. solved DNAPL plumes can be extremely mobile, persistent and very
Interestingly, PCE, the most retarded solute in the emplaced source capable of causing extensive aquifer contamination.