Environmental Engineering Reference
In-Depth Information
carbonate minerals, carbon isotopic exchange
between mineral and dissolved phases, and
oxidation of organic matter (Wigley
et al
.,
1978
;
Plummer
et al
.,
1994
). A variety of models have
been developed to correct
14
C ages of ground-
water on the basis of these reactions (Plummer
et al
.,
1994
; Kalin,
2000
). Stable carbon isotope
ratios (δ
13
C) often are helpful in identifying
different sources of carbon contributing to
groundwater DIC.
If ages are estimated at two points along a
groundwater flow path, groundwater velocity
along that flow path can be calculated by divid-
ing the distance between the points by the dif-
ference in calculated ages between the points.
Recharge was estimated with
14
C concentrations
by Vogel (
1967
) as described in Cook and Herczeg
(
1998
). The reduction in
14
C concentration from
82.2 pmc at the 11 m depth to 74.4 pmc at the
67 m depth in an unconfined aquifer in Holland
corresponded to a groundwater age of 821 years
at the 67 m depth. Assuming an aquifer thick-
ness of 70 m, a porosity of 0.4, and
14
C apparent
ages, a recharge estimate of 0.1 m/yr (Equation
(
7.17
)) provided the best fit to the data (Cook
and Herczeg,
1998
). McMahon
et al
. (
2004
) used
14
C data to calibrate a groundwater flow model
for a section of the central High Plains aquifer
in Kansas; simulated results based on steady
recharge rates of 0.8 to 8 mm/yr throughout the
Holocene agreed well with measured apparent
groundwater ages.
and groundwater flow was simulated with a
MODFLOW model (McDonald and Harbaugh,
1988
). Travel times to observation wells in the
basin were simulated by using particle track-
ing in MODPATH (Pollock,
1994
) and com-
pared with
14
C ages. MODFLOW and MODPATH
were calibrated by using nonlinear regression
methods in the UCODE parameter-estimation
program (Poeter and Hill,
1998
) to minimize
differences between measured and simulated
ages. Total recharge to the basin was estimated
to be 2.15 m
3
/s, with the Rio Grande contribut-
ing 0.78 m
3
is The total recharge rate is con-
siderably lower than the previous recharge
estimate of 4.86 m
3
/s obtained from a basin-
wide groundwater flow model calibrated with
heads and discharge estimates alone. The dra-
matic revision of recharge rates underscores
the value of tracers in hydrologic studies.
7.3.3 Historical tracers
Chlorofluorocarbons
Chlorofluorocarbons (CFCs) are organic com-
pounds that are entirely anthropogenic in
origin. They have been manufactured since
the 1930s and used primarily as propellants
for aerosol cans, as refrigerants, and as sol-
vents. Although nontoxic and noncarcinogenic,
CFCs have been shown to contribute to deple-
tion of stratospheric ozone. Hence, worldwide
production of CFCs was banned in industrial
countries by the year 2000 and in developing
countries by the year 2010 under the Montreal
Protocol on Substances that Deplete the Ozone
Layer signed in 1987 and amended in 1990 and
1992. Production of CFCs in the United States
was halted on January 1, 1996 (Plummer and
Busenberg,
2000
).
High atmospheric concentrations, low
analytical detection limits, and generally
conservative natures are features that make
CFCs useful as hydrologic tracers. Dating
groundwater with CFCs was first proposed by
Thompson
et al
. (
1974
), Schultz
et al
. (
1976
), and
Thompson and Hayes (
1979
). A brief overview
of the use of CFC concentrations in ground-
water to estimate recharge is provided here.
More detailed descriptions can be found in
Cook and Solomon (
1997
) and Plummer and
Example: Middle Rio Grande Basin,
New Mexico
The primary sources of recharge in the
Middle Rio Grande Basin (7900 km
2
area) in
New Mexico are losing ephemeral streams
along the mountain front, underflow from
mountain blocks, and losing reaches of the
Rio Grande. Hydrochemistry data were used
to develop a conceptual understanding of
recharge sources and processes (Plummer
et
al
.,
2004
).
14
C ages were determined for 200
groundwater samples. The age data were
used to calibrate a groundwater flow model
for the purpose of estimating recharge rates
(Sanford
et al
.,
2004
). Little correction of
14
C
ages was required for this siliciclastic system,
