Environmental Engineering Reference
In-Depth Information
500
3.5
Figure 7.1 Chlorine-36
fallout based on Zerle et al .
( 1997 ) and atmospheric
tritium concentrations from
Ottawa, Canada (International
Atomic Energy Agency, http://
www-naweb.iaea.org/napc/ih/;
accessed April 1, 2009), decay
corrected to 2008.
3 H
36 Cl
3
400
2.5
300
2
1.5
200
1
100
0.5
0
0
1940
1950
1960
1970
1980
1990
2000
2010
(Rangarajan and Athavale, 2000 ; Wang et al .,
2008 ), in which case bromide (Br), Cl, and fluor-
escent dyes are commonly used. In other cases,
use of applied chemicals as tracers may be sec-
ondary to other purposes (e.g. fertilizers and
pesticides can be used to track water movement
in the subsurface).
Ideal chemical and isotopic tracers are
highly soluble in water, behave conservatively
in the environment (i.e. they are chemically
inert with no subsurface sources or sinks), and
can be measured accurately and inexpensively.
In addition, in the case of applied tracers, back-
ground concentrations in the system should be
low. There is no perfect tracer that can be used in
all studies. Theoretically, however, any chemi-
cal or isotope could be used as a tracer. 3 H is a
useful tracer because it is part of the water mol-
ecule. Inorganic anions, such as Cl and Br, often
are used as tracers because they are usually not
affected by adsorption or other geochemical or
biochemical processes.
The nature of tracer application is import-
ant and in part determines the manner in
which subsurface tracer concentrations are
interpreted. Mass application rates can be rela-
tively constant over time. Many studies have
relied on the assumption that the atmospheric
deposition rate of Cl has been constant over
the last several thousand years (Scanlon, 1991 ;
Phillips, 1994 ). Tracer application can occur at a
single point in time, as is usually the case with
applied tracers; this is referred to as a pulse
application. Tracer application can also vary
in time, such as with gases like CFCs, SF 6 , and
krypton-85 ( 85 Kr) whose atmospheric concentra-
tions continually change. CFC concentrations
increased from the 1930s through the mid-to-
late 1990s, after which gradual declines have
been observed ( F ig u r e 7. 2 ). Whether a tracer
application can be considered a single pulse is
a somewhat relative question depending on the
time lag between application and sampling. As
previously mentioned, 3 H and 36 Cl were released
to the atmosphere in the 1950s and early 1960s.
More than 40 years later, these isotopes are com-
monly thought of as occurring as a pulse input
(the term bomb-pulse tritium frequently appears
in the hydrologic literature), even though the
application period extended for more than a
decade. It is conceivable that by 2050, atmos-
pheric concentrations of CFCs will have dimin-
ished to a point whereby hydrologists will treat
their introduction to a hydrologic system as a
pulse in time.
Tracer methods have some important
attributes that need to be considered in light
of the goal of a recharge study. To clarify ter-
minology, estimates of vertical flux within the
unsaturated zone are referred to as potential
recharge or drainage because the term recharge ,
by definition, is reserved only for water that
 
Search WWH ::




Custom Search