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Groundwater quality and contaminant hydrogeology 223

Fig. 6.15 Effect of density on the transport of dissolved contaminants in a uniform groundwater flow field. In (a) the contaminant density
(ρ ) is slightly greater than the density of the groundwater (ρ ). In (b) the contaminant density is greater than the groundwater density.
c w

6.3.4 Effects of density and heterogeneity In fractured material, aquifer properties are spa-
tially variable and are often controlled by the orienta-
The processes of advection, dispersion and retardation tion and frequency of fractures. As shown in Fig. 6.17,
all influence the pattern of contaminant distribution when contamination occurs in fractures, there is a
away from a pollution source. Other considerations gradient of contaminant concentration between the
include the effects of contaminant density and, more- mobile groundwater in the fracture and the static
over, the influence of aquifer heterogeneity, both of water in the adjacent rock matrix. Under this condi-
which make the overall monitoring and prediction tion, part of the contaminant mass will migrate by
of the extent of groundwater pollution very difficult. molecular diffusion from the fracture into the pore-
A density contrast between the contaminant and water contained in the rock matrix, so effectively
groundwater will affect the migration of a pollution removing it from the flowing groundwater. Such
plume, with a contaminant denser than water tend- dual-porosity aquifers are notoriously difficult to
ing to sink steeply downwards into the groundwater remediate since the contaminant stored in the matrix
flow field as shown in Fig. 6.15. can gradually diffuse back into the moving ground-
The description of contaminant transport given in water in the fracture, long after the source of contam-
Section 6.3.1 assumes a homogeneous porous mater- ination has been removed.
ial with steady, uniform groundwater flow. This is
a simplification of real situations in nature where
heterogeneities within the aquifer lithology create a 6.4 Sources of groundwater contamination
pattern of solute movement considerably different to
that predicted by the theory for homogeneous mater- The sources of groundwater contamination are, as
ial. If a pollution source contains multiple solutes and shown in Table 6.1, as varied as the range of polluting
occurs within a heterogeneous aquifer containing beds, activities. The purpose of this section is to introduce
lenses and fractures of differing hydraulic conductiv- the principal sources and classes of groundwater con-
ity, then there will be a number of contaminant fronts taminants with respect to urban and industrial con-
and pathways such that the morphology of the result- taminants, municipal and septic wastes, agricultural
ing plume will be very complex indeed (Fig. 6.16). contaminants, and saline intrusion in coastal aquifers.