Environmental Engineering Reference
In-Depth Information
direct physical evidence of the forces required by plants to
remove water from the capillary zone and fringe. It should
also be noted that no water will enter the augered borehole,
at least at first. Augering deeper will define the bottom of the
capillary zone when water enters the borehole from the
water table.
The height of the capillary zone can be estimated by using
a more quantitative approach than a hand auger. The height,
H
, that a liquid column of water will obtain is described by
H
¼
2
T
cos
y=r
gr
(4.10)
where
T
is the surface tension (N/m),
y
is the angle of contact
is the density of the
liquid (kg/m
3
),
g
is the acceleration of a body due to gravity
(m/s
2
), and
r
is the radius of the capillary tube (m). For
example, relative to sea level,
T
is 0.0728 J/m
2
,
between the surface and the liquid,
r
is 20
y
is 1,000 kg/m
3
, and
g
is 9.8 m/s
2
; therefore, a
1-m diameter well can allow water to obtain a height of
about 1.4
(0.35 rad),
r
10
5
m, or 0.014 mm, at sea level, which
would be impossible to measure relative to other processes
that affect this surface. If the well diameter were decreased
to 1 cm, the water would rise 1.4 mm; if the well diameter
were decreased to 0.1 mm, the water would rise 14 cm. The
weight of the liquid column of water, therefore, is propor-
tional to the square of the tube diameter. This equation can
be used to calculate the height that water would rise above
the water table using the largest effective porosity that is
known for a particular site's soil or sediment.
An interesting and perhaps counterintuitive observation
of contaminant transport processes that occur in the unsatu-
rated zone is that the maximum transport rate, regardless of
soil type, is near 13 m/d (Nimmo 2007). This indicates that
rate-limiting processes that affect the speed of falling objects
in the atmosphere may be similar to those in the pore spaces
of the unsaturated zone.
Fig. 4.10
The total porosity,
n
of saturated sediment is the sum of
the specific yield;
S
y
or water removed by gravity, and specific retention;
S
r
or water that remains after gravity flow.
4.4.3 Specific Yield and Specific Retention
Because the water-table surface can fluctuate across the
thickness of the aquifer, it follows that, based on porosity,
the fluctuation could provide a direct indication of the
change in water volume. However, this is not the case;
because of gravity, only part of the fluctuating water is
actually available for removal. This volume of water that
drains from a water-table aquifer by gravity is called specific
yield (
S
y
, Fig.
4.10
); the balance left behind that is retained
on media by tension against gravity is called specific reten-
tion,
S
r
. Hence, previously saturated sediments can essen-
tially exhibit capillary fringe characteristics after water
removal, and this provides a more useful definition of poros-
ity. That is, total porosity,
n
similar to wilting point and field capacity commonly used
by plant physiologists and soil scientists, as discussed in
Chap. 3.
4.4.4 Confined Aquifers
The Darcy column experiment discussed previously does not
represent water-table conditions, as first might be imagined.
Rather, it represents confined aquifer conditions, because the
water levels in the tubes rise above the top of the elevated
column. In other words, in the field, water levels in wells
¼
S
y
+
S
r
. These terms are
Search WWH ::
Custom Search