Page 231 - Soil and water contamination, 2nd edition

218 Soil and Water Contamination

according to a bivariate Gaussian distribution with a mean of zero and standard deviation s of

2 D t and 2 D t for longitudinal and transverse dispersion, respectively (see above).
x y

11.4 MULTI-FLUID FLOW
In some contamination problems two or more fluids are involved. Examples are groundwater
and organic liquids (dense non-aqueous phase liquids (DNAPLs ) and light non-aqueous
phase liquids (LNAPLs)) in the saturated zone, or air, water, and organic liquids in the
unsaturated zone. Problems involving transport of organic liquids are more complex than
transport of dissolved chemicals. The most important features of multi-fluid flow include the
differences in densities of the fluids, their limited miscibility, and the interference with one
another during flow. If the organic liquid is less dense than water (LNAPLs), the liquid tends
to float on the water; if it is denser than water (DNAPLs), the liquid tends to sink. Figure
11.13 illustrates the behaviour of DNAPLs and LNAPLs in the unsaturated and saturated
zones.
When two immiscible fluids are in contact with a solid surface, one fluid will
preferentially spread over the solid surface at the expense of the other fluid. The fluid that
is most spread over the solid surface is called the ‘wetting fluid’, whereas the other fluid
the ‘non-wetting fluid’. The relative degree to which a fluid will spread on or coat the solid
surface is called the ‘wettability’ of the fluid. The wettability depends on the properties of the
fluids and the solid surface and reveals itself in the contact angle, i.e. the angle subtended
by the liquid–liquid interface and the solid surface (Figure 11.13). A fluid that produces a
contact angle of less than 90° is wetting relative to water and a fluid that produces a contact
angle of more than 90° is non-wetting relative to water. Fluids with contact angles ranging
between 70° and 110° are considered neutrally wetting (Anderson, 1986). In general, NAPLs

LNAPL DNAPL

e
adose zone
a
n
o
s
d

z
V V
o
Vadose zone e
a
a
t
u
S Saturated zone e
r
Saturated zone
t

d
z
o
n
e
o
r
Groundwater r
G Groundwater
t
a
w
d
u
n
e
i
d
r
c
e
fl flow direction
flow direction n
o

w
i
o
t
e
p
e
m

y
e
l
a
Impermeable layer r
b
e
a
I Impermeable layer
l
r
m
NAPL Dissolved contaminant plume Vapour 6642 6642 6642
Figure 11.13 Behaviour of LNAPLs and DNAPLs in the subsurface environment.
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