1. Dating Old Groundwaters

2. Stable Isotopes and Paleogroundwaters

3. The Environmental Radioisotopes Isotope Half-life Dating range (years) 3 H 12.43 <50 14 C 5,7301,000 to 30,000 36 Cl301,000 > 50,000 39 Ar 269 50 to 1000 81 Kr210,000 >50,000 85 Kr 10.72 <50 129 I 15,700,000 >1 million

5. 6 neutrons + 6 neutrons 12 C

6. 14 C 12 C 1 1,000,000,000,000 = 100 percent modern C (pMC) = 10 –12

7. 100 pMC

8. 14 C 14 N Half-life = 5730 years

9. Radiocarbon decay

10. DI 14 C

11. 14 C dating groundwaters generally sample the DIC –CO 2 + H 2 CO 3 + HCO 3 – + CO 3 2– can also separate the DOC (CH 2 O) –humic acid –fulvic acid but DIC 5 to >10 ppm DOC <1 to 2 ppm

12. Typical evolution of 14 C in DIC Soil 14 CO 2 100 pMC Measured Measured DI 14 C 14 C dilution: CaCO 3 + 14 CO 2 2HCO 3 ~ 50 pMC CaCO 3 + 14 CO 2 2HCO 3 ~ 50 pMC 14 C decay < 50 pMC << 50 pMC dilution vs decay?

13. Correcting for 14 C dilution (reaction)

14. Statistical correction (STAT model) for recharge through different soil types (Vogel, 1970): 0.65 – 0.75for karst systems 0.75 – 0.90for fine-grained carbonate soils (loess) 0.90 – 1.00for crystalline rocks (granites)

15. Dilution correction with 13 C

16. Typical evolution of 14 C and 13 C in DIC Soil 14 CO 2 100 pMC Measured Measured DI 14 C 14 C dilution: CaCO 3 + 14 CO 2 2HCO 3 ~ 50 pMC CaCO 3 + 14 CO 2 2HCO 3 ~ 50 pMC 14 C decay < 50 pMC 13 C initial = ? 13 C 13 C final = DIC

17. 13 C mixing ( 13 C model) Pearson (1965) and Pearson and Hanshaw (1970) q= CC C C 13 DIC 13 carb 13 rech 13 carb where: 13 C DIC = measured 13 C in groundwater 13 C rech = 13 C of DIC in recharge (~ –15) 2 13 C carb = 13 C of the calcite being dissolved (~0) Correcting for 14 C-free Carbonate t = –8267 · ln – a 14 C DIC q · a o 14 C atm

18. Fossil groundwater of Southern Oman

19. The aquifers of Southern Oman

20. -60 -40 -20 0 20 -8-6-4-2024 18 O VSMOW 2 H VSMOW Monsoon Interior Plateau G/W Coastal Plain G/W

21. -8 -6 -4 -2 0 2 4 6 8 10 020406080 Sample Tritium (TU) Dhofar Mountains Interior Plateau Coastal Plain Monsoon

23. Radiocarbon ages corrected for carbonate dissolution q = C -C C - C 13 DIC 13 carb 13 rech 13 carb

25. Humic acid CH 2 O DOC in groundwater

27. Oxidation of organic carbon by sulphate reduction Hydrocarbon: CH 4 + SO 4 2- HS - + HCO 3 - + H 2 O Fixed organic carbon: CH 2 O + ½SO 4 2- ½H 2 S + HCO 3 - Hydrocarbon: CH 4 + SO 4 2- HS - + HCO 3 - + H 2 O Fixed organic carbon: CH 2 O + ½SO 4 2- ½H 2 S + HCO 3 - dilutes 14 C pool dilutes 14 C pool lost 13 C and DIC as measures of dilution lost 13 C and DIC as measures of dilution precludes classical age corrections precludes classical age corrections

29. Sulphate reduction 14 C decay DOC + SO 4 H 2 S + DIC Soil 14 CO 2 Measured Measured DI 14 C 14 C dilution - carbonate dissolution and exchange and exchange 13 C q = ?

30. Pluvial