Environmental Engineering Reference
In-Depth Information
A
as described above is the wetted portion of the wetland bed across which water
flows to enter or leave the wetland. By definition, flow across
A
is perpendicular to
the plane of
A
, the orientation of which ranges from nearly vertical to horizontal at
the wetland bed. Therefore,
A
in Fig.
3.20
is instead oriented vertically and located
at the shoreline of the wetland, which simplifies determination of
i
by allowing it to
be made on a horizontal axis. Flow across this vertical plane is assumed to be equal
to flow across the sediment-water interface of the wetland.
A
is the product of
m
, the shoreline reach associated with the monitoring well,
and
b
, the thickness of the portion of the aquifer that exchanges with the wetland
(Fig.
3.20
). Both
m
and
b
are conceptually simple but often difficult to determine.
The distance,
m
, of a shoreline segment associated with the gradient between a
monitoring well and the wetland is based on how far one can reasonably extrapolate
the hydraulic gradient along the wetland shoreline. Because the monitoring well for
each shoreline segment is usually located at the center of the segment,
m
, and
therefore
A
, as shown in Fig.
3.20
are both half of what they should be because the
figure does not show the half of the shoreline segment that would extend out of the
page. If the shoreline reach is straight and hydrogeological conditions are expected
to be uniform, the shoreline segment corresponding to a monitoring well could be
quite long and the extent relatively easy to determine. If the shoreline is curvilinear
(commonly the case in wetland settings) and hydraulic gradients are expected to
vary substantially along the shoreline reach, then additional monitoring wells
should be installed and shoreline segments should be correspondingly shorter.
The other component needed to determine
A
, and one that often is the most difficult
to estimate, is
b
, the thickness of the vertical plane through which water has to flow
to enter (or leave) the wetland. Water passing through any portion of the aquifer
deeper than
b
will not exchange with the wetland but will instead pass beneath the
wetland. This is shown by the two flowlines that extend beyond the wetland in
Fig.
3.20
. Unless the subsurface geology is known to constrain exchange with the
wetland, or unless the wetland depth extends to the base of the aquifer,
b
has to be
estimated. One common approach is to arrive at a reasonable estimate for
b
through
the use of a simplified groundwater flow model.
The terms of the Darcy-flux method are extrapolated along an entire shoreline
segment, the extent of which is based on what is determined to be reasonable.
The longer the segment, the weaker the assumption that
K
,
A
, and
h/l
indeed are
uniform along the segment. Therefore, the wetland perimeter is divided into several
segments, each of which is associated with a specific monitoring well located near
the wetland. This commonly is referred to as the segmented-Darcy approach, an
example of which is depicted in Fig.
3.21
. Once the hydraulic gradient, shoreline-
segment length, and estimated
K
and
b
are determined for each shoreline segment,
all of the information is available to calculate
Q
for each segment and the entire
wetland. The example shown in Fig.
3.22
is of a flow-through wetland, one that
both receives groundwater discharge and contributes wetland water to the adjacent
groundwater system. This approach allows for estimation of total groundwater
discharge, total recharge of wetland water to groundwater, as well as the net
(
G
i
minus
G
o
) term.
Δ
