Environmental Engineering Reference
In-Depth Information
in groundwater. Vertical flow in the saturated
zone is a more tenuous assumption than ver-
tical flow in the unsaturated zone and, there-
fore, requires closer scrutiny. The assumption
is likely to be more valid for shallower sam-
pling depths (
F ig u r e 7.9
) or in highly permeable
aquifers. Knott and Olimpio (
1986
) identified
the peak tritium concentration at a depth of
about 36 m below the water table in an uncon-
fined sand and gravel aquifer on Nantucket
Island, MA. An apparent vertical velocity of
1.89 m/yr was calculated, and Equation (
7.1
) was
applied with a porosity of 0.36 to derive an aver-
age annual recharge rate of 0.68 m/yr (68% of
annual precipitation). Water levels measured in
piezometers confirmed a downward head gra-
dient. Robertson and Cherry (
1989
) also applied
the profile method with tritium concentrations
in groundwater at the Sturgeon Falls study site
in Ontario. Peak tritium concentrations were
found at depths of 8 to 12 m; assuming a porosity
of 0.35, an average recharge rate of 150 mm/yr
was calculated with Equation (
7.1
). The validity
of the vertical flow assumption at this site was
confirmed by results from a groundwater-flow
model (Robertson and Cherry,
1989
).
As the elapsed time since peak atmospheric
tritium concentration increases, tritium con-
centrations in groundwater become less useful
for estimating recharge because dispersion and
radioactive decay make it difficult to determine
the location of the bomb-pulse concentration
peak, particularly in the southern hemisphere
where bomb tritium levels were about an order
of magnitude lower than those in the northern
hemisphere (Allison and Hughes,
1977
).
The tritium/helium-3 (
3
H/
3
He) method for
determining apparent groundwater ages does
not require information on atmospheric tritium
concentrations, nor does it require detailed pro-
filing to identify the location of the bomb-pulse
tritium concentration peak. The use of
3
H/
3
He to
date water is briefly summarized here; detailed
descriptions can be found in Solomon and Cook
(
2000
). Tritium (
3
H) decays by beta emission to
the noble gas isotope,
3
He.
3
He that is generated
by decay of tritium in a water sample is referred
to as
tritiogenic
helium-3 (
3
He
trit
). Knowledge of
the rate of
3
H decay allows the age of a water
sample,
t
, to be determined from the concentra-
tion ratio of
3
He
trit
to
3
H (Schlosser
et al
.,
1989
):
1
[
He
]
3
trit
(7. 27)
t
=
ln
+
1
λ
[
H
]
3
where
λ
is the
3
H decay constant; [
3
He
tri
t
] and
[
3
H
] are
3
He
trit
and
3
H concentrations, respect-
ively; and [
3
He
tri
t
] is expressed in equivalent
tritium units (1 cm
3 3
He STP per kg of water
= 4.0177 × 10
11
TU). Equation (
7. 27
) relies on
the assumptions that groundwater is isolated
from the atmosphere, that there is no loss of
3
He
trit
through diffusion into the unsaturated
zone, and that there is no hydrodynamic dis-
persion or mixing. Estimated losses of
3
He
by diffusion to the unsaturated zone range
from 1% for a recharge rate of 150 mm/yr to
20% for a recharge rate of 30 mm/yr (Solomon
and Cook,
2000
). Therefore, the reliability of
the recharge rates decreases with decreasing
recharge rates.
Determining the concentration of tritiogenic
helium-3, [
3
He
trit
], is not always straightforward.
As with other dissolved gases, there can be mul-
tiple sources of
3
He in groundwater. [
3
He
trit
] can
be calculated in a manner similar to that used
for CFCs and SF
6
(Solomon and Cook,
2000
):
He He He He He
(7.28)
[
3
] [
=
3
][
+
3
][
+
3
][
+
3
]
meas
trit
eq
exc air
terr
where [
3
He
meas
] is
3
He concentration measured in
the water sample and subscripts for the other
terms are analogous to those in Equations (
7. 25
)
and (
7. 26
). Concentrations of
3
He in equilibrium
solubility with the atmosphere depend on the
temperature and to a lesser degree on salin-
ity of the water and the ambient pressure dur-
ing recharge. Recharge temperature, [
3
He
exc air
],
and [
3
He
terr
] can be estimated by measuring
concentrations of noble gases, as with CFC dat-
ing (Aeschbach-Hertig
et al
.,
1999
). Additional
details on segregating the different sources of
3
He in groundwater are provided in Schlosser
et al
. (
1988
,
1989
). The solubility of
3
He is much
less than that of CFCs; therefore,
3
H/
3
He ages
are more sensitive to excess air than CFC ages.
Although
3
H decay begins in the unsaturated
zone,
3
He is only preserved in the saturated
zone; therefore,
3
H/
3
He age is zero at the water
