Environmental Engineering Reference
In-Depth Information
0
be obtained by associating a chloride concentra-
tion with each component of the water-budget
equation:
(a)
5
(7. 8)
PC
+
Q
C
=
Q
C
+
DC
+
M
sw
sw
sw
sw
p
on
on
off
off
uz
app
where
C
P
,
C
sw
on
,
C
sw
off
, and
C
uz
are chloride con-
centrations in precipitation, surface-water flow
onto the column, surface-water flow off of the
column, and unsaturated zone pore water at the
base of the column.
M
app
is the rate of chloride
mass application to the column in dry form and
accounts for additional sources of chloride that
can be natural (e.g. atmospheric dry deposition)
or related to human activity (e.g. agricultural
chemicals or road salt). A number of assump-
tions are inherent in Equation (
7. 8
) including
no change in the amount of chloride stored
within the column, no plant uptake of chloride,
no additional sources of chloride in the column
(such as dissolution of rocks), no subsurface
sinks of Cl (such as by adsorption), and verti-
cal downward movement of water and chloride
within the column. Equations (
7.7
) and (
7. 8
) are
readily modified to account for other terms that
contribute or remove water or chloride from
the column.
The CMB estimate of drainage is obtained by
rearranging Equation (
7. 8
):
10
15
20
0
10 000
20 000
30 000
0
(b)
3
6
9
12
(7.9)
D
= +
(
PC
Q
C
−
Q
C
−
M
)/
C
sw
sw
sw
sw
P
on
on
off
off
app
uz
15
0
500
1000
1500
2000
2500
3000
Application of Equation (
7.9
) requires measure-
ments of chloride concentrations in precipita-
tion, surface water, and unsaturated-zone pore
water, along with an estimate of dry-source
application rates. The large number of terms
in Equation (
7.9
) complicates application of the
CMB method in areas where many of the terms
are nonnegligible. The method is most often
applied to estimate natural drainage in arid and
semiarid regions where irrigation, runon, and
runoff are negligible and the chloride mass-
balance equation can be simplified to:
Chloride (mg/L)
Figure 7.5
Subsurface distribution of chloride beneath
native vegetation at sites in (a) the Murray Basin, Australia
(reprinted from
Journal of Hydrology
, v. 111, Cook
et al
.
(
1989
), Figure 3, copyright (1989), with permission from
Elsevier); and (b) the Chihuahuan Desert, West Texas
(reprinted from
Journal of Hydrology
, v. 128, Scanlon (
1991
),
Figure 2, copyright (1981), with permission from Elsevier).
where
P
is precipitation,
Q
sw
on
is surface flow
onto the column (including irrigation),
Q
sw
off
is surface flow off of the column,
D
is drain-
age out of the bottom of the column (potential
recharge), and it is assumed that there is no
subsurface flow through the sides of the col-
umn and that steady-state conditions exist, so
there is no change in water storage within the
column. A chloride mass-balance equation can
C P
+
M
PC
*
(7.10)
p
app
p
D
=
=
C
C
uz
uz
where
C
p
* is effective chloride concentration
(the sum of total wet and dry chloride deposi-
tion rate divided by precipitation).
