Environmental Engineering Reference
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volumetric water content (0.15) produces a
d r a i n a g e es t i m ate of 69 m m / y r. T The d i f fe r e nc e
in results of the two methods can perhaps be
explained by the different time intervals over
which the drainage estimates are made. The
peak-displacement method is averaged over
9 years, whereas the tracer-profile method
is averaged over 25 years (1988) and 34 years
(1997). The fact that average annual precipi-
tation for 1989 through 1997 (about 390 mm)
was slightly higher than the long-term aver-
age could also contribute to the difference.
zone, and chloride is extracted with deionized
water, as described in Section 7.2.1 . A total of
1 to 10 mg of chloride is required for the ana-
lysis. In areas of low chloride concentrations
(which usually are areas of high drainage rates),
hundreds of grams of soil may be required to
provide sufficient amounts of chloride for 36 Cl
analysis. The analysis is done by accelerator
mass spectrometry; the cost for chemical prep-
aration of the extract sample and 36 Cl analysis
can be up to $1000 per sample. Currently (2010),
there are three laboratories where 36 Cl can be
analyzed: Purdue University (http://www.phys-
ics.purdue.edu/primelab/; accessed April 2,
2009), Lawrence Livermore National Laboratory
(https://cams.llnl.gov/; accessed April 2, 2009),
and the Australian National University (http://
wwwrsphysse.anu.edu.au/nuclear/group.php?-
id=6; accessed April 2, 2009).
Drainage rates can be estimated from the
depth distribution of 36 Cl by using the meth-
ods described above for tritium (Scanlon, 1992 ;
Liu et al ., 1995 ; Tyler et al ., 1996 ). Prych ( 1998 )
compared drainage estimates based on 36 Cl
measurements (using the profile method and
penetration depth of the center of mass) with
those based on the chloride mass balance; the
estimates based on 36 Cl were higher by a fac-
tor of about 10 at sites in eastern Washington
State. The reason for this discrepancy is not
clear. Cook et al . ( 1994 ) compared drainage
estimates from 36 Cl mass-balance and chlor-
ide mass-balance methods at sites in southern
Australia; they found variability in results, but
no consistent trend between estimates by the
two methods. The surprising penetration of
bomb-pulse 36 Cl to depths exceeding 400 m at
Yucca Mountain, Nevada, has been attributed to
preferential flow through faults or fractures in
the volcanic rock (Campbell et al ., 2003 ).
Chlorine-36
Chlorine-36 (half-life of 301 000 years) is pro-
duced naturally in the atmosphere by cosmic-
ray spallation of 36 Ar and neutron activation
of 35 Cl (Bentley et al ., 1986 ). 36 Cl was also intro-
duced into the atmosphere by neutron activa-
tion of 35 Cl as a result of nuclear weapons testing
between 1952 and 1958 in the Pacific Ocean
( F ig u r e 7.1 ). As a tracer, 36 Cl is similar to 3 H in
several respects; both have bomb-pulse sources,
both are conservative in nature, and data inter-
pretation is similar. Much of the discussion on
tritium in the previous section is pertinent for
36 Cl as well. There are some important differ-
ences, though. 36 Cl is nonvolatile and its move-
ment in the subsurface is restricted to liquid
phase water movement, whereas tritium can
move in the liquid and vapor phases (Scanlon,
1992 ). Tritium may be taken up by plant roots
and returned to the atmosphere via evapotrans-
piration; plants roots tend to reject 36 Cl and
chloride in general. The peak in atmospheric
36 Cl concentration occurred in 1957, about 6
years prior to that of tritium. Tritium has been
much more widely used than 36 Cl in hydrologic
studies, perhaps because historically there have
been few laboratories capable of analyzing for
36 Cl. Sample preparation and analysis of 36 Cl are
also expensive (Scanlon, 1992 ).
The 36 Cl concentration-depth profile must
be measured to locate the depth of the peak
or center of mass; the soil-water content depth
profile also must be determined. Sampling pro-
cedures for 36 Cl are similar to those for chloride.
Soil samples are usually collected through-
out the interval of interest in the unsaturated
7.2.4 Applied tracers
Applied tracers are generally used in areas of
relatively high rates of infiltration and drain-
age because tracer movement is evaluated over
fairly short times (from days to one or more
years). Tracers are usually applied on land sur-
face as a single pulse, and they are transported
downward through the unsaturated zone by
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