Environmental Engineering Reference
In-Depth Information
Busenberg (
2000
) and at the US Geological
Survey CFC lab website (http://water.usgs.gov/
lab; accessed April 10, 2009).
CFCs that are used to date young (less than
about 60 years in age) groundwater include
CFC-11 (CCl
3
F), CFC-12 (CCl
2
F
2
), and CFC-113
(C
2
Cl
3
F
3
). Atmospheric concentrations of CFCs
(
F ig u r e 7. 2
), estimated from production and
release data before 1977 (McCarthy
et al
.,
1977
)
and from a global measurement network since
then (Cunnold
et al
.,
1994
), increased from the
1930s until the mid- to late-1990s, when con-
centrations began declining. Low CFC degrad-
ation rates result in long residence times in
the atmosphere (approximately 44 years for
CFC-11, 180 years for CFC-12, and 90 years for
CFC-113) and relatively homogeneous concen-
trations within each hemisphere (Plummer and
Busenberg,
2000
).
CFC concentrations can be detected to
0.3 picogram per kilogram (pg/kg) of water by
using a purge and trap method with a gas chro-
matograph equipped with an electron capture
detector. Concentrations of CFCs in air during
recharge are calculated from those in ground-
water according to Henry's Law:
Several processes can affect the measure-
ment of CFC concentrations and calculation of
groundwater ages. These include the entrap-
ment of excess air as percolating water moves
past the water table, other sources of CFCs (such
as from a sewage plume), and loss of CFCs by
degradation or adsorption. The measured con-
centration of a CFC in groundwater, [
CFC
meas
], is
influenced by all of these sources and sinks:
[
CFC
][
=
CFC
][
+
CFC
][
+
CFC
][
−
CFC
]
meas
eq
exc air
cont
loss
(7. 25)
where the bracketed terms are concentrations
and the subscripts eq, exc air, cont, and loss
refer to CFC mass due to equilibrium with the
atmosphere, entrapped excess air, contamin-
ation, and losses related to microbial degrad-
ation and adsorption, respectively (Plummer
and Busenberg,
2000
). [
CFC
eq
] is the concen-
tration needed for calculating groundwater
age, and to determine that concentration, the
magnitude of other terms on the right side of
Equation (
7. 25
) must be evaluated. Solubilities
of CFCs decrease with increasing temperature.
An uncertainty of ±2°C in recharge tempera-
ture would result in a ±3 year uncertainty in
age for water recharged prior to 1990. For water
recharged after 1990, CFC ages are much more
sensitive to recharge temperature because
atmospheric concentrations have changed lit-
tle since then. Overestimation of recharge tem-
peratures underestimates apparent ages. [
CFC
eq
]
is also a function of barometric pressure, which
can be estimated from recharge elevation.
Most groundwater is supersaturated with
air (Busenberg and Plummer,
2000
). The excess
air (i.e. dissolved air concentrations in excess
of equilibrium solubility) is generally attrib-
uted to the entrapment of air during recharge.
Excess air concentrations are typically less than
3 cm
3
STP/kg of water (Wilson and McNeill,
1997
),
but concentrations as high as 18 cm
3
STP/kg
have been reported (Glynn and Busenberg,
1996
). If excess air is ignored, dissolved gas con-
centrations will be overestimated and ground-
water ages will be underestimated. For water
recharged prior to 1990, the effect of excess
air on CFC concentrations is negligible, but
for younger water the effect can be important.
K
(7. 23)
Pa
= /
C
i
i
i
where
i
refers to the
i
th
CFC compound,
Pa
i
is
the partial pressure of air in equilibrium with
water,
C
i
is concentration in water, and
K
i
is
Henry's Law constant. Values for
K
i
are depend-
ent on recharge temperature (temperature at
the water table at the time of recharge), sal-
inity, and pressure (Warner and Weiss,
1985
;
Bu and Warner,
1995
). The mixing ratio,
x
i
, is
determined from the partial pressures:
(7. 2 4)
x
=
Pa
/(
P
−
P
)
i
i
vapor
where
P
and
P
vapor
are total atmospheric and
water-vapor pressure, respectively, and
x
i
is in
units of volume of CFC per unit volume of air.
The mixing ratio of the CFC is used with the
atmospheric growth curve (
F ig u r e 7. 2
) to deter-
mine the time of recharge or recharge date. The
age of the water is the difference between sam-
pling and recharge dates. Independent ages can
be calculated for each of the CFC compounds
and also for ratios of different CFCs.
