Environmental Engineering Reference
In-Depth Information
Ground surface
Figure 6.4
Water-table rise
in an observation well due to air
entrapped between the water
table and an advancing wetting
front (
H
d
is initial depth to
saturated zone, m is thickness
of infiltrating saturated front,
and water-level rise in well,
ΔH
= P
a
m/(H
d
- m)
ρ
g
, where
P
a
is
atmospheric pressure,
ρ
is density
of water, and g is acceleration due
to gravity) (Todd,
1980
; reprinted
with permission of John Wiley &
Sons, Inc.).
m
Saturated zone resulting
from infiltrating rainfall
H
d
Zone of
compressed air
∆
H
Capillary
fringe
Water table
sufficient recent rainfall to induce a water-level
rise. Also, von Asmuth
et al
. (
2008
) developed a
time series approach for decomposing a series of
water-level fluctuations into partial series that
represent the influence of individual stresses,
such as evapotranspiration, atmospheric pres-
sure fluctuations, river- and lake-level fluctua-
tions, groundwater pumping, and precipitation.
The approach allows the effects of all stresses
that do not contribute to recharge to be removed
from water-level records. In the absence of an
automated approach, manual inspection and
correction of water-level data benefit from side-
by-side comparison with data on precipitation,
barometric pressure, temperature (to check for
snowmelt), and nearby streamflow.
Other mechanisms
Other mechanisms can induce fluctuations in
water levels in unconfined aquifers. In gen-
eral these mechanisms are more easily iden-
tified or are less frequently encountered than
those already discussed. Temperature vari-
ations affect groundwater levels due to freeze-
thaw action and the temperature dependency
of surface tension, air solubility, and air dens-
ity. Pumping of wells and natural or induced
changes in surface-water elevations can greatly
affect groundwater levels. Earthquakes and
ocean tides can also affect groundwater levels.
Changes in groundwater flow into or out of the
study area, due to processes occurring in adja-
cent areas, can produce water-level fluctuations
(Dickinson
et al
.,
2004
). Additional information
on these processes can be found in Freeze and
Cherry (
1979
) and Todd and Mays (
2005
).
6.2.2 Specific yield
Meinzer (
1923
) defined specific yield of a rock
or soils as the ratio of (1) the volume of water
which, after being saturated, it will yield by
gravity to (2) its own volume. Specific yield has
traditionally been represented by the formula:
Correcting water-level data
Tamura
et al
. (
1991
) and Toll and Rasumssen
(
2007
) described computer programs for remov-
ing the effects of barometric fluctuations and
Earth tides from water-level records. Crosbie
et
al
. (
2005
) presented an algorithm for applying
the WTF method to eliminate rises attributed
to air entrapment and check that there was
S
=−
ϕ
S
(6.4)
y
r
where ϕ is porosity and
S
r
is specific retention
(the volume of water retained by the rock per
