Environmental Engineering Reference
In-Depth Information
Bomb-pulse
3
H can also provide some insight
into flow processes in the unsaturated zone.
Penetration of bomb-pulse
3
H to about 12 m in
the Negev Desert was attributed to preferen-
tial flow in fractured chalk (Nativ
et al
.,
1995
).
Tritium concentrations beneath playas in the
Texas High Plains provide examples of piston
and preferential flow (Wood and Sanford,
1995
;
Scanlon and Goldsmith,
1997
). The tritium
concentration-depth profile beneath a playa in
the southern part of the Texas High Plains is
smooth and has a single peak (
F ig u r e 7.7
), indi-
cating predominantly piston-like flow. In con-
trast, a profile beneath another playa to the
north contains multiple peaks, the deepest at
21 m, suggesting preferential flow.
0
5
10
15
20
25
30
0 0 0 0 0
100
120
140
160
180
Tr itium (TU)
Figure 7.7
Representative tritium profiles beneath
playas. The smooth, shallow profile, beneath a site in the
southern Texas High Plains, suggests predominantly piston-
like flow (Wood and Sanford,
1995
); the deeper profile,
from a site further north, has multiple peaks suggesting
preferential flow (Scanlon and Goldsmith,
1997
).
Example: Inner Mongolia
Unsaturated-zone tritium concentration pro-
files were determined with soil cores collected
in 1988 and 1997 in a loess-covered region of
Wudan County, Inner Mongolia (Lin and Wei,
2006
). The site was free of vegetation, and mean
annual precipitation was 360 mm. The 0.1 to
0.4 m sample intervals to depths of 15 to 20 m
provide an unusually detailed profile of
3
H con-
centration (
F ig u r e 7. 8
). The profiles are note-
worthy for several reasons: the distinct peaks,
the high concentrations, the fact that tritium
mass (when corrected for decay) is about equal
for the two sampling dates, and the fact that
the 1997 profile shows more dispersion (a fea-
ture that suggests non-piston-type flow).
Peak concentrations were 338 TU at the
6.3 m depth for the 1988 profile and 230 TU at
the 10.4 m depth for the 1997 profile (all values
are decay corrected to 1997). These high
3
H con-
centrations indicate that this region received
relatively high
3
H fallout from nuclear weap-
ons testing. Lin and Wei (
2006
) estimate that
maximum atmospheric tritium concentration
in northern China may have been as high as
4000 TU.
Pore-water velocities were estimated from
the penetration depths of the
3
H peaks at
0.25 m/yr (1988) and 0.3 m/yr (1997). Mean
annual drainage rates of 38 and 45 mm/yr were
calculated by the profile method (Equation (
7.1
))
with an average volumetric water content of
0
5
10
15
1988
1997
20
0
100
200
300
400
Tr itium (TU)
Figure 7.8
Subsurface distribution of tritium in profile
CHN88 sampled in 1988 (peak depth 6.3 m) and profile
CHN97 sampled in 1997 (peak depth 10.4 m) in the Loess
Plateau, Inner Mongolia (reprinted from
Journal of Hydrology
,
v. 328, Lin and Wei (
2006
), Figure 2, copyright (2006), with
permission from Elsevier).
0.15. These fluxes represent 11 to 13% of mean
annual precipitation, a substantial percentage
for semiarid regions.
The peak-displacement method (Equation
(
7. 3
)) can also be used to estimate tracer and
water velocity from the Lin and Wei (
2006
)
data. The peak was displaced 4.1 m over the
course of 9 years, equivalent to a velocity of
about 0.46 m/yr. Multiplying this velocity by
