Environmental Engineering Reference
In-Depth Information
Age dates determined by other gas tracers tend
to be more sensitive to excess air concentra-
tions. Recharge temperature and the amount of
excess air can be estimated by measuring con-
centrations of noble gases, such as neon, argon,
and krypton, and solving a set of nonlinear
equations (Aeschbach-Hertig
et al
.,
1999
). Other
approaches to determining these unknowns
involve measurement of dissolved nitrogen and
argon (Heaton and Vogel,
1981
).
Contamination is a critical issue that limits
the use of CFCs in some areas. Sources of CFC
contamination in groundwater include efflu-
ent from septic tanks and from leaking sewer
lines, industrial waste water, and recharge
from rivers that have effluent from sewage
treatment plants (Plummer and Busenberg,
2000
). Local contamination of air is also asso-
ciated with industrial and residential regions.
Contamination is obvious when CFC concen-
trations exceed concentrations in equilibrium
with normal hemispherically averaged air. The
presence of contaminant sources of CFCs pre-
cludes their use for age dating, although CFCs
can be useful for tracking the movement of
groundwater contamination plumes (Schultz
et al
.,
1976
).
CFCs are stable in groundwater under aero-
bic conditions. The stability of CFCs has been
confirmed by concurrence of groundwater ages
calculated from CFC-11, CFC-12, CFC-113, and
other dating methods (Dunkle
et al
.,
1993
; Szabo
et al
.,
1996
; Ekwurzel
et al
.,
1994
). However, CFC
degradation occurs under anaerobic conditions.
The different CFC compounds are not equally
susceptible to degradation. CFC-11 and CFC-113
are generally degraded under sulfate reducing
and methanogenic conditions, whereas CFC-12
is quasi-stable (Plummer and Busenberg,
2000
;
Cook
et al
.,
1995
). Adsorption can also result in
reduction of CFC concentrations and overesti-
mation of groundwater ages; however, Plummer
and Busenberg (
2000
) suggested that adsorption
does not seem to be important for CFC-11 and
CFC-12 in most groundwater systems. Few data
are available on adsorption of CFC-113.
Samples of groundwater for CFC analyses
must be isolated from contact with the atmos-
phere. Simple 125 mL glass bottles are used for
sampling. After purging the well, the sample
bottle is placed in a 2 to 6 L beaker. The outlet
line of the pump is placed in the bottom of the
bottle, the bottle is allowed to overflow into the
beaker, and the beaker is allowed to overflow
as well. After at least 2 L has flowed out of the
beaker, a cap (rinsed in the beaker) is screwed
onto the bottle while still submerged. Once
capped, the bottle is removed from the beaker
and dried, and the cap is secured with electrical
tape. Five samples are required from each well
(US Geological Survey CFC lab, http://water.usgs.
gov/lab; accessed April 10, 2009).
Recharge rates of 0.05 to 0.45 m/yr were esti-
mated on the basis of CFC-12 concentrations
in samples obtained from several unconfined
aquifers in the eastern United States (Böhlke,
2002
). Recharge rates were calculated assuming
a linear increase in age with depth (Equation
(
7.1
)) near the water table and an exponential
increase with depth (Equation (
7.17
)) for profiles
at greater depth (
F ig u r e 7.10
). At the Sturgeon
Falls site in Ontario, Canada, Cook
et al
. (
1995
)
used CFC-12 data to estimate average recharge
at 130 mm/yr, a value in good agreement with
that predicted with a groundwater flow model.
High organic matter content and anaerobic con-
ditions at that site resulted in degradation of
CFC-11. Retardation of CFC-113 was also noted.
CFC data have also been used to calibrate
groundwater flow models (Reilly
et al
.,
1994
;
Portniaguine and Solomon,
1998
). Hydraulic
heads are sensitive to the ratio of recharge
flux to hydraulic conductivity, whereas ages
are sensitive to the ratio of recharge flux to
porosity. Combining head and age data in an
inverse model (
Section 3.5
) allows estimation of
recharge flux and other hydraulic parameters
(Portniaguine and Solomon,
1998
).
Sulfur hexafluoride
The use of sulfur hexafluoride (SF
6
) to date
groundwater and estimate recharge is similar
to the use of CFCs described in the previous
section. Although primarily of anthropogenic
origin (electrical insulator), SF
6
also exists nat-
urally in some igneous, metamorphic, and sedi-
mentary rocks and in hydrothermal fluids. The
natural background atmospheric concentration
