Environmental Engineering Reference
In-Depth Information
6.4.1.1
Transport in Groundwater
Leaking underground storage tanks, old unlined landfills, surface impoundments,
and accidental spills are the main causes for groundwater contamination. There are
two regions in the subsurface that are subject to contamination: the unsaturated
(vadose) zone and the saturated zone (Figure 6.51). Liquid spilled on the surface
gradually migrates downward. The light nonaqueous-phase liquid (LNAPL) floats on
the groundwater table. A dense nonaqueous-phase liquid (DNAPL) sinks and pools
at the bottom of the impervious layer. Contaminants slowly dissolve in the ground-
water. In the zone of solubilization, globules of the organic solvent slowly dissolve
with time. The transport of the contaminant plume in groundwater, therefore, consists
of two processes: dissolution of globules (ganglia) of pure fluid and movement of the
solubilized fraction.
6.4.1.1.1 Solubilization of Ganglia
An NAPL globule trapped in a soil pore will slowly dissolve in the groundwater.
The rate of dissolution will depend on the mass transfer coefficient,
k
L
(m/s),
between the NAPL globule and water under specified flow conditions. The flux is
given by
F
A
=
[A]
. [A]
∗
is the aqueous solubility of the NAPL and
[A] is the concentration in water. Consider the NAPL globule to be approximately
spherical (diameter
D
p
, m), with a volume ratio
k
L
[A]
∗
−
θ
N
(m
3
/m
3
of the medium)—see
Figure 6.52. The area per unit volume of the medium
a
θ
N
/D
p
. Note that both
θ
N
and
D
p
are functions of the distance
x
from the globule in the flow direc-
tion.
a(x
,
t)
=
6
θ
N
(x
,
t)/D
p
(x
,
t)
. Thus the overall concentration change along the
x
-direction is given by (Hunt et al., 1988)
=
6
Soil surface
Landfill
Vadose zone
1
2
Groundwater table
Solute front
3
Saturated
zone
5
Groundwater flow
4
Less permeable strata
FIGURE 6.51
Groundwater contamination from a leaking source.
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