Environmental Engineering Reference
In-Depth Information
The mass balance of a conservative tracer can be used with a potentially reactive
tracer to identify a possible reaction and estimate its rate. The mass-balance
equation for a reactive tracer can be written as
Δ
ð
C
w
V
Þ=Δ
t
¼
C
P
P
þ
C
Si
S
i
C
w
S
o
þ
ð
G
o
Þ þ
C
Gi
G
i
þ
C
of
O
f
þ
R
xn
(3.53)
where
R
xn
(kg/m
3
/s) is the rate of reaction per volume. If all other terms in Eq.
3.53
are known from solving the water-budget equation and the mass balance equation
for a conservative tracer, then
R
xn
can be determined as the residual of Eq.
3.53
. For
example, Heagle et al. (
2007
) solved the water-budget equation with the mass
balance equations for naturally occurring chloride and sulphate to estimate the
rate of sulphate reduction in a prairie wetland in Saskatchewan, Canada.
3.10.4 Final Remarks
The tracer mass-balance approach provides a useful tool for estimating the water-
budget components that are difficult to measure directly. Unlike other methods for
estimating groundwater flow, the mass-balance method evaluates the flow averaged
over the entire wetland, while giving no information about the spatial distribution of
groundwater recharge or discharge within the wetland. Therefore, it is beneficial to
use this method in combination with other methods that give local values of flow,
such as a seepage meter or mini-piezometer (see Sect.
3.8
). The mass-balance
method provides a constraint on the possible range of total groundwater flow,
whereas a local-scale method is useful for delineating areas of focused recharge
or discharge, which may have significant influence on the distribution of wetland
flora and fauna (e.g., Rosenberry et al.
2000
).
3.11 Estimation of Errors
As introduced in Sect.
3.2
, a wetland water budget can be written as the change in
wetland volume per time (plus residual) equal to the sum of all inputs and losses
(Eq.
3.1
). If all of the hydrological components are measured as accurately as
possible, it is almost certain that the sum of those components will not equal the
change in volume in the wetland over an accounting period.
R
in Eq.
3.1
can be
disturbingly large relative to
V
for some water budgets. Error stems from (1) incor-
rect measurement of a parameter (instrument error), (2) misapplying point
measurements to specific areas or volumes of a wetland (a common but often
neglected error), and (3) misinterpreting the hydrologic setting, usually by not
Δ
