Environmental Engineering Reference
In-Depth Information
include deploying the bag 1 m or more from the seepage cylinder to minimize local
disturbance during attachment and removal of the bag, increasing the diameter of
bag-connection hardware to improve meter efficiency, and connecting multiple
cylinders to a single bag to reduce measurement time and increase the bed area
represented by each measurement (Rosenberry
2005
). Additional discussion of
modifications and sources of error (and how to minimize them) are presented in
Rosenberry et al. (
2008
).
Seepage meters also have been modified for use in streams (Rosenberry
2008
).
Such a meter would be useful for riparian wetland settings where flow, although
usually relatively slow, could still corrupt seepage measurements made with meters
not modified for use in flowing water. For the typically slow flow velocities
associated with wetland settings, the most important consideration is to place the
bag inside of a bag shelter.
As indicated earlier, many of the errors associated with seepage measurements
can be attributed to problems associated with the seepage bag. Furthermore, any
variability in seepage rate is integrated over the duration of each bag attachment. To
address these concerns, the bag can be replaced with alternate means of quantifying
flow ranging from chemical-dilution methods to heat-pulse flow technology to
mechanical or electromagnetic flowmeters (Rosenberry et al.
2008
). Much finer
temporal resolution is possible with these designs that allow quantification of
processes that would otherwise be impossible with standard designs (Rosenberry
and Morin
2004
; Rosenberry
2011
).
3.8.3 Determining Groundwater Fluxes as the Residual
of a Water Budget
The wetland water budget presented earlier (Eq.
3.1
) can be reordered to solve for
net groundwater exchange:
G
i
G
o
R
¼ Δ
V
=Δ
t
P
þ
ET
S
i
þ
S
o
O
f
(3.39)
where the terms are as described earlier. This is a common approach for determin-
ing net groundwater contribution to lakes, wetlands, or stream reaches where
groundwater fluxes are difficult to determine with more direct measurements.
Hood et al. (
2006
), for example, used Eq.
3.39
to estimate the contribution of
groundwater to an alpine lake. Note that this provides only the net groundwater
exchange (
G
i
G
o
). For wetlands where either
G
i
or
G
o
dominates, determining
the net groundwater contribution may be all that is needed, but for many other
wetland settings determining the net term may not be sufficient. For example, by
only knowing net groundwater exchange, the water residence time cannot be
determined. Fortunately, if we also have a chemical constituent of some sort that
is associated with each of the water terms, then both
G
i
and
G
o
can be determined.
