Environmental Engineering Reference
In-Depth Information
recharge, and they can provide valuable infor-
mation on evaporation rates and flow proc-
esses in the unsaturated and saturated zones.
However, attempts to use unsaturated-zone
stable isotope concentration profiles to quan-
tify drainage rates have met with little success.
Concentrations of these isotopes in water, which
are usually given as ratios of a standard and are
referred to as del values, depend on tempera-
ture. Because of this temperature dependence,
seasonal variations in del values of precipita-
tion are common (Gat,
1996
). If these variations
persist in infiltrated water as it moves down-
ward through the unsaturated zone, then the
depths of the cyclical patterns can be used with
the profile method (Equation (
7.1
)) to estimate
drainage rates. The best results (i.e. the most
discernible variations in del values through the
soil profile) have been obtained in temperate
climates where drainage through the unsatu-
rated zone occurs during a large part of the
year (Barnes and Allison,
1988
). But even under
apparently ideal conditions, patterns of del val-
ues with depth do not always occur (Sharma
and Hughes,
1985
), perhaps because of molecu-
lar diffusion. Regions where drainage occurs
only in a single season would not be expected
to show much va r iat ion i in del va lues w it h i in t he
unsaturated zone. Detailed sampling and sta-
ble-isotope analysis of precipitation is required
if one were to use stable-isotope concentration
profiles to estimate drainage rates.
Del values are also affected by evaporation,
due to fractionation, once precipitation arrives
at land surface. In arid regions, evaporation of
soil water can produce a unique isotope signa-
ture; subsequent infiltration events may trans-
port that front downward, and the front could
serve as an event marker for estimating drain-
age rates when using the profile method (Barnes
and Allison,
1983
).
is a good tracer because, being contained in a
water molecule, it is conservative and because
concentrations can be accurately measured.
Tritium was widely used as an unsaturated-zone
tracer in the 1970s and 1980s in all climatic
regions. Tritium use in unsaturated zone stud-
ies since about 2000, however, has largely been
limited to arid and semiarid regions. Because
of relatively thin unsaturated zones and high
drainage rates in more humid regions, tritium,
chlorine-36, and other historical tracers pro-
duced by nuclear testing have been flushed
from unsaturated zones in many areas.
Tritium has a half-life of 12.32 years (Lucas
and Unterweger,
2000
) and is naturally pro-
duced by cosmic-ray neutrons interacting with
nitrogen in the upper atmosphere. Subsurface
production of
3
H is low (Lehmann
et al
.,
1993
).
Natural atmospheric
3
H concentrations prior
to nuclear testing in the 1950s were estimated
from vintage wines at 3 to 6 tritium units
(TU) for Europe and North America (Kaufman
and Libby,
1954
) and at 1 to 3 TU for southern
Australia (Allison and Hughes,
1977
). (One tri-
tium unit is equivalent to one tritium atom in
10
18
hydrogen atoms or 0.118 becquerels per
liter.) Atmospheric tritium concentrations at
Ottawa, Canada, increased from these levels to
more than 400 TU during above-ground nuclear
testing (International Atomic Energy Agency,
1983) that began in 1952 and peaked in 1963
(
F ig u r e 7.1
). Because most nuclear tests were
conducted in the northern hemisphere,
3
H con-
centrations were much greater in the northern
than in the southern hemisphere: 3278 TU at
Ottawa, Canada in 1963 vs. 38 TU at Kaitoke,
New Zealand in 1964 (Solomon and Cook,
2000
).
Information on
3
H input in precipitation is avail-
able from a worldwide network of measure-
ment stations; online data are maintained by
the International Atomic Energy Agency (http://
www-naweb.iaea.org/napc/ih/IHS_resources_
ISOHIS2.html; accessed April 1, 2009).
Pore water for tritium analysis can be
obtained from soil cores or, in humid regions,
from suction lysimeter or other pore water
samplers. Azeotropic or vacuum distillation
is required to collect water from soil cores
(Hendr y,
1983
; Ingraham and Shadel,
1992
). Few
7.2.3 Historical tracers
Tritium
The distribution of bomb-pulse tritium (
3
H)
in the unsaturated zone can be used to esti-
mate drainage rates with any of the methods
described in
Section 7.2
, the tracer-profile, peak-
displacement, or mass-balance method. Tritium
