Environmental Engineering Reference
In-Depth Information
Complete drainage of soil water, which is assumed in laboratory methods, may
take up to several weeks. Since such assumptions do not reflect the dynamic water-
table conditions in the field, it is preferable to use field methods to estimate
S
y
.
Several methods have been proposed based on conducting an aquifer pumping test
and interpreting the data. However, pumping tests require installation of a network
of observation wells and the interpretation is strongly influenced by model
assumptions, such as the boundary conditions or time scale of soil-water drainage
(Healy and Cook
2002
).
An alternative to aquifer pumping tests is the water-balance approach, where
Δ
S
sub
over a given time period is estimated from careful measurements of other
water balance components:
Δ
S
sub
¼
P
þ
O
f
E
þ
S
i
S
o
þ
G
i
G
o
(3.48)
Since it is usually impossible to measure all components in Eq.
3.48
, periods are
chosen so that some of the hydrologic components are negligible and can be
omitted. For example, if a wetland does not have surface water input and output,
overland flow is negligible, and the magnitude of net groundwater input (
G
i
G
o
)
is expected to be much smaller than that of net atmospheric input (
P
E
), then
plotting
Δ
h
WT
observed in response to rainfall events against the amount of net
precipitation (
P
E
) may yield a linear relation between
P
E
(
Δ
S
sub
) and
Δ
h
WT
. The slope of this linear relation is equal to
S
y
. This type of approach is
commonly used in wetland studies (e.g., Gerla
1992
; Rosenberry and Winter
1997
).
Considerable uncertainty and discrepancy is noted in values of
S
y
estimated
using different methods. Field-based methods generally give smaller values than
laboratory methods, presumably because laboratory methods usually allow a long
time for complete drainage of soil samples compared to the time scale of field
processes (Healy and Cook
2002
). Therefore, investigators must be aware of the
time scale of processes under investigation, as well as the assumptions associated
with the definition of
S
y
.
3.10 Use of Conservative Tracers
In many low-gradient wetlands with extensive areas of vegetation it can be difficult
to quantify several of the terms in the wetland water budget. In these situations,
water chemistry may provide a separate or perhaps better estimate of some of the
water-budget terms. In Sect.
3.8.3
we discussed the procedure of writing two
equations, one a water-budget equation (Eq.
3.39
) and the other a mass-balance
equation for a conservative tracer (Eq.
3.40
), for the purpose of determining either
G
i
or
G
o
. These equations can be rearranged and this procedure can be used to solve
for any of the water-budget components, not just
G
i
or
G
o
. Here we discuss further
the characteristics of a conservative tracer, assumptions associated with this
method, and procedures for proper sample collection.
