Environmental Engineering Reference
In-Depth Information
commercial, academic, or government labora-
tories are capable of doing the water extraction
and tritium analysis. Tritium can be analyzed
by radiometric methods (liquid scintillation or
gas proportional counting) or by helium-3 (
3
He)
ingrowth (Clarke
et al
.,
1976
; Schlosser
et al
.,
1989
). Radiometric methods require about
20 mL of sample and have a detection limit of
about 6 TU. Electrolytic enrichment of samples
can reduce that detection limit to about 0.1 TU;
however, the enrichment process requires
much larger volumes of water, up to hundreds
of milliliters (Ostlund and Dorsey,
1977
). The
3
He ingrowth method of measuring
3
H concen-
trations involves degassing of the water sam-
ple under vacuum and sealing and storing the
water in a special gas-tight container for time
periods ranging from 1 month to 1 year, result-
ing in detection limits down to 0.1 TU or less
(Clarke
et al
.,
1976
). The
3
He ingrowth method
for analyzing
3
H in unsaturated pore-water
samples should not be confused with the
3
H/
3
He
groundwater dating method (
Section 7.3.3
).
Dating groundwater with
3
H/
3
He requires iso-
lation of the
3
He from the atmosphere, which
only occurs below the water table (Solomon
et al
.,
1992
).
The presence of
3
H levels greater than about
0.5 TU in a water sample generally indicates
that some of the sample is from post-1952 pre-
cipitation (assuming maximum prebomb
3
H
of 7 TU and a residence time of 55 years or 4.5
half lives). Many studies have estimated unsat-
urated zone drainage rates on the basis of
depth of bomb-pulse tritium (Dincer
et al
.,
1974
;
Aranyossy and Gaye,
1992
; Heilweil
et al
.,
2006
;
Lin and Wei,
2006
). The tracer-profile method
(Equation (
7.1
)) and peak-displacement method
(Equation (
7. 3
)) can be applied by using the
depth of the peak concentration or the center
of mass (Equation (
7. 2
)). In each case, Δ
t
is set
equal to the time between peak atmospheric
concentration of
3
H (1963-4) and sampling time.
Heilweil
et al
. (
2006
) applied the profile method
on the basis of depths of peak concentrations in
11 boreholes in southwestern Utah. Gvirtzman
and Magaritz (
1986
) used differences in
3
H con-
centrations in irrigation water (summer) and
rainfall (winter) to identify 14 years of recharge
cycles in an unsaturated zone tritium-depth
profile at a site in Israel. The peak-displacement
method was applied for each of the 14 concen-
tration peaks. Normally, this method requires
sampling at two different times. However, a
single sampling date sufficed for this study
because of the seasonal variability of tritium
input at land surface; in effect, 14 tracer experi-
ments were analyzed with a single sampling
date. The average vertical velocity for all peaks
was 0.7 m/yr, and drainage was estimated as 8%
of precipitation and irrigation.
The use of tritium in the mass-balance
approach requires modification of Equation (
7.6
)
to account for radioactive decay (Allison
et al
.,
1994
; Cook
et al
.,
1994
; Heilweil
et al
.,
2006
):
∞
∞
∑
∫
uz
∆
(7.12)
D
=
C
()()
z
θ
z dz
/
w C
te
λ
−
i
i
pi
i
=
1
0
where
C
uz
(
z
) is now tritium concentration at the
time of sampling,
i
refers to number of years
prior to the time of sampling,
w
is equal to pre-
cipitation during year
i
(
P
i
) divided by long-term
mean annual precipitation,
C
Pi
is tritium con-
centration in precipitation
i
years prior to the
time of sampling, Δ
t
is 1 year, and λ is the decay
constant for tritium (0.0565 yr
-1
). The decay cor-
rection allows concentrations to be expressed
relative to the time of sampling. The mass-bal-
ance equation for tritium has been expressed in
an alternative form by Heilweil
et al
. (
2006
):
M
=
UZ
PPT
D
P
(7.13)
M
where
M
UZ
is
3
H mass in the unsaturated zone
at the sampling site, equal to the numerator
on the right-hand side of Equation (
7.12
);
M
PPT
is
3
H mass input to the site from precipitation,
equal to the summation in the denominator of
Equation (
7.12
) with each term multiplied by
P
i
, and
P
is mean annual precipitation. The dif-
ference between
3
H mass in precipitation and
the unsaturated zone represents
3
H and, there-
fore, water that was evapotranspired. Heilweil
et al
. (
2006
) found that the tritium mass-balance
drainage estimates were less than estimates
determined with the tritium profile method,
perhaps because of seasonal variability in tri-
tium concentration in precipitation.
