Environmental Engineering Reference
In-Depth Information
drainage rates through the unsaturated zone
in the Southern High Plains in the United
States (Scanlon
et al
.,
2007
) and in the Murray
Basin in Australia (Jolly
et al
.,
1989
). Both loca-
tions are characterized by high chloride con-
centrations under native vegetation, which
indicate no drainage during the last 10 000
to 30 000 years in the Southern High Plains
and drainage rates less than 0.1 mm/yr in the
Murray Basin. Downward displacement of the
high chloride concentrations under increased
recharge beneath cropland and pastures is
used to estimate the change in recharge rates.
In the case of the Southern High Plains, pis-
ton displacement of the preexisting chloride
allows recharge rates to be estimated with
the CMB method, Equation (
7.9
). The chloride
bulge was displaced by a distance of 7.7 m
(
F ig u r e 7.6 a
). The average chloride concentra-
tion in the flushed portion of the profile is
5.3 mg/L (Scanlon
et al
.,
2007
). An average
water flux of 24.4 mm/yr was estimated by
using mean annual precipitation of 461 mm/yr
and an effective chloride concentration in pre-
cipitation of 0.28 mg/L. The peak displacement
method can also be applied. Assuming that the
change in land use occurred 75 years before
samples were obtained, an apparent velocity
of 103 mm/yr is calculated. With an average
volumetric water content of 0.19, Equation (
7. 3
)
provides a drainage estimate of 19.5 mm/yr.
The chloride concentration peak was
displaced downward by about 2.7 m at a
site in the Murray Basin (
F ig u r e 7.6 b
) over a
period of 9 years since native vegetation was
removed and replaced with dryland agricul-
tural crops (Jolly
et al
.,
1989
). Chloride con-
centrations under the native vegetation were
fairly uniform at about 10 000 mg/L at depths
greater than 2 m. Applying the CMB method
(
Equation 7.9
) with average rainfall of 370 mm
and an effective chloride concentration in
precipitation of 2.3 mg/L produces an average
annual drainage estimate of about 0.08 mm.
The peak displacement method was used to
examine the drainage rate in the cleared field.
The 2.7 m movement of the chloride peak over
9 years corresponds to an apparent velocity of
0.3 m/yr. At an average volumetric water
0
(a)
5
10
15
20
Native
Cleared
25
30
0
500
1000
1500
2000
2500
0
(b)
5
10
Native
Cleared
15
0
10 000
20 000
Chloride (mg/L)
Figure 7.6
Comparison of chloride profiles beneath
native vegetation and cropland at sites in (a) the southern
High Plains, Texas (Scanlon
et al
.,
2007
) and (b) the Murray
Basin, Australia (reprinted from
Journal of Hydrology
, v. 111,
Jolly
et al
. (
1989
), Figures 1 and 2, copyright (1989), with
permission from Elsevier).
content of 0.15, this velocity corresponds to a
drainage rate of 45 mm/yr.
Stable isotopes of oxygen and hydrogen
The stable isotopes of oxygen (
18
O) and hydro-
gen (
2
H or D, deuterium) are valuable tracers in
many types of hydrologic studies (Kendall and
McDonnell,
1998
). They can be used in some
regions to identify the source and timing of
