Dating Old Groundwaters

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Dating Old Groundwaters
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Stable Isotopes and Paleogroundwaters
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The Environmental Radioisotopes
Isotope Half-life Dating range
(years) 3H 12.43 lt50 14C
5,730 1,000 to 30,000 36Cl 301,000 gt
50,000 39Ar 269 50 to 1000
81Kr 210,000 gt50,000 85Kr
10.72 lt50 129I 15,700,000 gt1
million
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6 neutrons 6 neutrons
12C
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14C
12C
1
?
1012
1,000,000,000,000

100 percent modern C (pMC)
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100 pMC
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14C
14N
Half-life 5730 years
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Radiocarbon decay
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DI14C
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14C dating groundwaters

• generally sample the DIC
• CO2 H2CO3 HCO3 CO32
• can also separate the DOC (CH2O)
• humic acid
• fulvic acid
• but DIC ? 5 to gt10 ppm
• DOC ? lt1 to 2 ppm

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Typical evolution of 14C in DIC
Soil 14CO2 100 pMC
14C dilution CaCO3 14CO2 2HCO3 50 pMC
14C decay lt 50 pMC
Measured DI14C
ltlt 50 pMC dilution vs decay?
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Correcting for 14Cdilution(reaction)
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Statistical correction (STAT model) for recharge
through different soil types (Vogel, 1970)
0.65 0.75 for karst systems 0.75 0.90 for
fine-grained carbonate soils (loess) 0.90
1.00 for crystalline rocks (granites)
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Dilution correction with 13C
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Typical evolution of 14C and 13C in DIC
Soil 14CO2 100 pMC
d13Cinitial ?
14C dilution CaCO3 14CO2 2HCO3 50 pMC
d13C
14C decay lt 50 pMC
Measured DI14C
d13Cfinal DIC
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Correcting for 14C-free Carbonate
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13
d
d
C mixing (
C model
)
Pearson (1965) and Pearson and Hanshaw (1970)
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13
d
d
-
C
C
DIC
carb
q


d
d
-
13
13
C


C
rech
carb
13
13
d
where
C
measured
C in groundwater
DIC
d
13
d
13
C

C of DIC in recharge ( 15)
rech
2
13
13
d
d
C

C of the calcite being dissolved (0)
carb
a14CDIC
t 8267 ln
q ao 14Catm
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Fossil groundwaterof Southern Oman
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The aquifers of Southern Oman
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20
0
H VSMOW
-20
Monsoon
Interior Plateau G/W
-40
2
Coastal Plain G/W
-60
-8
-6
-4
-2
0
2
4
18
d
O VSMOW
21
10
8
Monsoon
6
4
2
Tritium (TU)
0
Interior Plateau
Coastal Plain
-2
Dhofar
-4
Mountains
-6
-8
0
20
40
60
80
Sample
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Radiocarbon ages corrected forcarbonate
dissolution
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DOC in groundwater
Humic acid CH2O
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Oxidation of organic carbonby sulphate reduction
Hydrocarbon CH4 SO42- HS- HCO3-
H2O Fixed organic carbon CH2O ½SO42- ½H2S
HCO3-

• dilutes 14C pool
• lost d13C and DIC as measures of dilution
• precludes classical age corrections

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Sulphate reduction
Soil 14CO2
d13C
14C dilution - carbonate dissolution and
exchange
d13C
DOC SO4 H2S DIC
d13C
14C decay
q ?
Measured DI14C
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Pluvial