Environmental Engineering Reference
In-Depth Information
Obtaining a single value for
K
that is representative of an entire wetland is
possible where wetland stage can be changed rapidly, either by pumping wetland
water elsewhere or by altering wetland stage with a control structure. As an
example, water was pumped from a 60-m-diameter wetland in Florida until wetland
stage was lowered 0.3 m and the recovery of wetland stage was recorded following
the end of the pumping period (Wise et al.
2000
). Because the recovery occurred
during a time of minimal rainfall, and the rate of recovery was much larger than
potential effects of rainfall or evapotranspiration, recovery of wetland stage follow-
ing pumping was attributed to seepage from groundwater. By measuring the
vertical hydraulic gradient,
i
v
, between wetland stage and several piezometers
installed within the wetland basin, carefully measuring wetland bathymetry to
obtain a good estimate of
A
for each increment of wetland stage, and knowing the
amount of water pumped from the wetland (
Q
), Darcy's law can be manipulated to
calculate the vertical component of hydraulic conductivity,
K
v
, of the wetland
sediments:
K
v
¼
Q
v
A
=
ðÞ
(3.36)
Once
K
v
is known,
i
v
can be monitored with measurements of piezometers installed
in the wetland and
G
i
or
G
o
can be determined depending on whether
i
v
is indicating
upward or downward flow potential.
3.8.2 Direct Seepage Measurements
Most devices or methods for quantifying exchange between groundwater and surface
water are based on indirect measurements. For example, hydraulic gradient and
hydraulic conductivity are determined using the segmented-Darcy approach, but the
actual quantity of interest is the flux across the sediment-water interface. A seepage
meter is an instrument that directly measures flow across the sediment-water interface
between groundwater and surface water. Although several early versions developed
in the 1950s and 1960s were unwieldy and quite complex (listed and described
in Carr and Winter
1980
), the meter generally in use since the mid 1970s, the
“half-barrel” seepage meter, is very simple and inexpensive. The device consists of
an open-ended seepage cylinder placed on the bed to which an attached plastic bag is
used to record the time-averaged rate of flow (Lee
1977
). The open-ended cylinder
isolates a portion of the bed, commonly 0.25 m
2
, and all flow across the bed area
covered by the cylinder is routed to (or from, depending on the direction of flow) the
attached plastic bag (Fig.
3.25
). By recording the volume contained in the bag at the
times of emplacement and removal, the volumetric seepage rate is determined:
V
t
1
V
t
2
Q
¼
(3.37)
t
1
t
2
