Environmental Engineering Reference
In-Depth Information
This approach was first used to estimate
recharge in a study in Israel (Eriksson and
Khunakasem,
1969
). An inherent assumption in
Equation (
7. 21
) is that all of the chloride within
the aquifer is derived from atmospheric depos-
ition. However, additional chloride sources
can be accommodated by use of Equation (
7.9
).
Estimation of chloride deposition rates are sub-
ject to the same consideration discussed in
Section 7.2.2
.
There are some differences between appli-
cation of the CMB method with groundwater
data as opposed to unsaturated-zone data. The
groundwater CMB method is not sensitive to
mechanisms of flow through the unsaturated
zone, whereas estimates of drainage provided
by the unsaturated zone CMB method cannot
account for flow through preferential flow
paths (Wood,
1999
). Collecting groundwater
samples for chloride analysis requires much
less effort than collecting samples from within
the unsaturated zone. Analytical techniques for
determining chloride concentrations are iden-
tical, ion-specific meter or ion chromatography,
regardless of whether groundwater or unsatur-
ated zone pore water is being analyzed. Large
volumes of groundwater can be collected and
concentrated, thereby reducing analytical detec-
tion limits and extending the maximum rate of
recharge that can be determined with Equation
(
7. 21
). In addition, extensive data on chlor-
ide concentrations in groundwater are avail-
able through the US Geological Survey (http://
waterdata.usgs.gov/nwis/qw; accessed April 9,
2009), the US Environmental Protection Agency
(http://www.epa.gov/storet/; accessed April 9,
2009), and other international, national, state,
and local agencies. These data greatly facilitate
application of this method.
Wood and Sanford (
1995
) used Equation (
7. 21
)
to estimate recharge to the Ogallala aquifer in
the southern High Plains of Texas on the basis
of more than 3000 measurements of chloride
concentrations in groundwater. The average
concentration was 25.2 mg/L. Mean annual pre-
cipitation was 485 mm/yr, and the effective
chloride concentration in precipitation was esti-
mated at 0.58 mg/L. An average recharge rate of
11 mm/yr was calculated. In this same region,
Scanlon and Goldsmith (
1997
) used unsaturated
zone tracer methods and determined that drain-
age was negligible in interplaya areas but could
be as high as 120 mm/yr beneath playas.
Carbon-14
Carbon-14 (
14
C) is a radioactive isotope with a
half life of 5730 years. It is formed naturally
from nitrogen-14 (
14
N) by cosmic-ray bombard-
ment. It was also produced as a result of nuclear
bomb testing in the 1950s and 1960s. Because of
its long half life,
14
C can be used to date a much
wider range of water ages than can be dated
with tritium, CFCs, or SF
6
. The
14
C age of water
is given by:
t
=−
(1 /
λ
) ln([
CC
] /[
])
(7. 22)
14
14
o
where λ is the decay constant for
14
C and [
14
C
]
and [
14
C
o
] are the measured and initial concen-
trations of dissolved
14
C in groundwater, respect-
ively. Equation (
7. 22
) applies if radioactive decay
is the only process occurring and there are no
carbon exchanges.
The scale used to represent
14
C concentra-
tions is percent modern carbon (pmc) with
100 pmc equal to atmospheric
14
C concentra-
tions in 1950. Nuclear bomb testing in the
1950s and 1960s increased
14
C concentrations
up to 200 pmc. Concentrations of
14
C that
exceed 100 pmc represent post-1950s water.
For the purposes of age dating, measurements
of
14
C are made on dissolved inorganic carbon
(DIC), which includes dissolved carbon dioxide
(CO
2
) and bicarbonate (HCO
3
-
). Concentrations
are generally measured by Accelerator Mass
Spectrometer (Zhu,
2000
), which requires about
10 mg of carbon. The amount of water required
for
14
C analysis can be determined from know-
ledge of DIC concentrations.
As with other age-dating tracers, the
14
C age
refers to the age of the
14
C rather than the age
of the water itself. In a simple, open system,
all carbon within an aquifer is derived from
recharge water with atmospheric
14
C concen-
trations. At any point in such an aquifer, water
age is assumed equal to the uncorrected
14
C age.
However, there often are additional sources or
sinks of
14
C that must be taken into account.
These include dissolution and precipitation of
