Environmental Engineering Reference
In-Depth Information
of SF
6
is estimated to be 0.054 ± 0.009 pptv
(Busenberg and Plummer,
2000
). Atmospheric
concentrations of SF
6
have increased about 7%
per year from background levels in about 1960
to more than 4 pptv in the late 1990s (
Figure
7. 2
). The long residence time of SF
6
in the atmos-
phere (estimated to range from 1935 to 3200
years) results in uniform atmospheric concen-
trations. Concentrations of SF
6
in groundwater
can be used to date water from 1970 to the pre-
sent in a manner similar to that described for
CFCs. Prior to 1970, SF
6
concentrations were
extremely low making it very difficult to date
the water. Ratios of SF
6
to CFCs have also been
used to date groundwater.
As with CFCs, the measured SF
6
concentra-
tion, [
SF
6meas
], includes the mass of SF
6
due to
equilibrium with the atmosphere at the time
of recharge, entrapped excess air, contami-
nant sources, and losses to biodegradation and
adsorption:
Estimated errors in age from underestimation
of excess air by 1 cm
3
STP/kg water range from 1
to 2.5 years. Local anthropogenic sources of SF
6
in groundwater have not been reported, prob-
ably because SF
6
is not generally contained in
household products. Ho and Schlosser (
2000
),
however, found high atmospheric SF
6
concen-
trations in the vicinity of New York City. SF
6
seems resistant to both aerobic and anaerobic
biodegradation and does not adsorb signifi-
cantly to organic matter (Wilson and Mackay,
1996
; Busenberg and Plummer,
2000
).
Analysis of SF
6
concentrations in ground-
water is by gas chromatograph with electron
capture detector; accuracy is 1 to 3% (Busenberg
and Plummer,
2000
). Samples are collected in
1 L glass bottles. After the well to be sampled
is purged, the outflow line from the pump is
inserted into the bottom of the sample bottle,
and the bottle is allowed to overfill. It is recom-
mended that at least 2 L of water be allowed to
overflow before screwing the cap onto the bot-
tle. There should be no head space, and after
drying, the bottle cap should be secured with
electrical tape.
SF
6
concentrations were measured in
groundwater samples from multiple wells at a
field site in Locust Grove, Maryland, where con-
centrations of other tracers were also measured
(CFCs,
3
H/
3
He,
85
Kr) (Busenberg and Plummer,
2000
). SF
6
ages agreed to within 3 years with
those based on CFC-11 and CFC-113 in 80 to 85%
of analyses. Discrepancies resulted largely from
the inability to date groundwater older than
1970 with SF
6
and problems with dating post-
1993 water with CFCs because of flattening of
the CFC growth curve.
SF SF SF SF SF SF
(7.26)
where subscripts are as defined for Equation
(
7. 25
) and the term [
SF
6terr
] accounts for natu-
ral subsurface (terragenic) sources of SF
6
. [
SF
6eq
]
is the concentration needed for age dating, so
the other interfering terms on the right side of
Equation (
7. 26
) must be evaluated. Although SF
6
solubility varies markedly with temperature
(3.5% per °C), the rapid increase in atmospheric
SF
6
concentration with time results in low sen-
sitivity of ages to uncertainties in recharge
temperature. For example, a 1 to 2°C uncer-
tainty in recharge temperature would result
in an error of less than 0.5 years in calculated
age. Similarly, calculated ages are only slightly
sensitive to recharge elevation; an uncertainty
of ±300 m would result in an age uncertainty
of ±5 years.
If entrapped excess air is not accounted for,
SF
6
concentrations will be overestimated, and
groundwater ages will be underestimated. Errors
in estimated ages from excess air are potentially
greater for SF
6
than for CFCs because of the lower
solubility of SF
6
relative to CFCs. The Henry's
Law constant for SF
6
is 55- and 13-times lower
than those for CFC-11 and CFC-12, respectively.
[
] [
=
][
+
][
+
][
+
][
+
]
6meas
6eq
6exc air
6cont
6loss
6terr
Tritium and tritium/helium-3
Many studies have used
3
H concentrations in
groundwater to estimate recharge rates. The
tracer-profile, peak-displacement, and mass-
balance methods, as described for unsaturat-
ed-zone application in
Section 7.2
, can also be
applied with tritium concentrations in ground-
water. These methods invoke the assumption
of vertical groundwater flow for some distance
below the water table and require measurement
of the vertical profile of tritium concentrations
