Isotope Hydrology: CFC, SF 6 dating

1. Isotope Hydrology: CFC, SF6 dating Peter Schlosser, February 21, 2008

2. Syllabus

3. Syllabus

4. Transient Tracers • Tracers: trace substances of natural or anthropogenic origin (stable and radioactive isotopes; chemical compounds. Sometimes toxic or otherwise harmful (contaminants) • Transient tracers: ‘Dyes’with known delivery rates to the environment (e.g.,3H, 3He,CFCs, 129I, SF6, 85Kr) • Radioactive clocks (e.g., 14C,39Ar, 3H/3He) • Special sources (e.g., 18O, Ba, nutrients, Ra isotopes, Rn isotopes) • Deliberately released tracers (e.g., SF6, 3He)

5. CFCs CFCs are anthropogenic compounds. CFC 11: CCl3F (trichlorofluoromethane) CFC 12: CCl2F2 (Dichlorodifluoromethane) They have virtually no natural background. Their industrial production started in the 1930s and 1940s. Between this time and the 1990s, their atmospheric concentration increased, first quasi-exponentially, then quasi-linearly. In the late 1980s, the Montreal protocol led to a drastically reduction in CFC production. Consequently, the atmospheric concentrations of CFCs leveled off. Today, atm. CFC 11 and CFC 113 are decreasing significantly. That of CFC 12 is in a plateau and will drop at a lower rate.

6. CFCs CFCs or chlorofluorocarbons are compounds that are essentially inert in the troposphere. The main compounds used as tracers in natural systems are CFC 11 (CCl3F), CFC 12 (CCl2F2) and CFC 113 CCl2F-CClF2). All compounds have no known natural sources. In the early literature, CFC 11 and CFC 12 were also called chlorofluoromethanes due to their methane-type structure. The trade name of CFCs (DuPond) is Freon (F 11, F 12, F 113). The main use of CFCs is summarized in Table 1.

7. CFCs: find the number To find the number, given the chemical formula: consider the number as consisting of 3 digits: a, b, and c. For 2-digit numbers (e.g., CFC-11) the a digit is zero (e.g., CFC-011). a is the number of carbon atoms minus 1;b is the number of hydrogen atoms plus 1;c is the number of fluorine atoms. For CFCl3: a = the number of carbon atoms (1) minus 1 = 0. b = the number of hydrogen atoms (0) plus 1 = 1 c = the number of fluorine atoms = 1 and, the compound is CFC-011, or CFC-11. Similarly: CCl2F2 is CFC-12 C2Cl3F3 is CFC-113 http://cdiac.ornl.gov/pns/cfcinfo.html

8. CFCs: properties

9. CFCs CFCs are degraded in the stratosphere by photo dissociation. The resulting chlorine radicals contribute to the destruction of the ozone layer and led to the ban of CFC production (Montreal Protocol). The individual CFCs have different atmospheric life times. These were determined during the so-called ALE experiment (Atmospheric Lifetime Experiment; see Cunnold et al.). The life times of the individual CFCs are summarized in Table 1.

10. CFC life cycle • Production • Release to troposphere; lag times • Mixing laterally and vertically • Decomposition in stratosphere • Dissolution in natural waters

11. CFCs: life cycle

12. Fluorocarbon production http://www.afeas.org/img/table/prod2004.png

13. ODP – weighted production http://www.afeas.org/img/table/odp2004.png

14. GWP - weighted production http://www.afeas.org/img/table/gwp2004.png

15. CFC 11 in the atmosphere • Atmospheric CFC concentrations are determined by: • Production rate • Decomposition • Hemispheric and inter-hemispheric mixing • Vertical mixing • Local sources • Determined by measurements and historic industrial production records • Typically well known in clean air • Small hemispheric gradients • Inter-hemispheric gradients are of the order of several percent • Local sources are highly variable and lead to concentrations in excess of clean-air conc. Of the order of several percent to several 10 percent (Oster et al; Ho et al.). http://www.uwmc.uwc.edu/geography/globcat/cfc.html

16. CFC 11 in the atmosphere http://www.uwmc.uwc.edu/geography/globcat/cfc.html

17. CFC 11 in the atmosphere Plummer et al., 2000

18. Chlorine in the atmosphere http://www.uwmc.uwc.edu/geography/globcat/cfc.html

19. CFC 11 in the atmosphere http://www.uwmc.uwc.edu/geography/globcat/CL-agreement-impact.htm

20. CFC ages There are two basic principles for determining the mean residence time of ‘age’ of a groundwater parcel using CFCs: pCFC method: If the CFC concentration of a water sample is measured and the solubility for the temperature and salinity (typically zero) of the water at the time of formation at the groundwater table are known, the CFC concentration in the soil air above the water table can be calculated. The CFC solubilities have been measured by Warner and Weiss and Bu and Warner. The recharge temperature can be determined through N2/Ar temperatures. Comparing the CFC concentration in soil air determined in this way, the age can be estimated by matching this value with the atmospheric CFC concentration curve and reading the age off the graph (time at which the atmospheric concentration and the soil air concentrations determined from the groundwater measurement match.

21. CFC ages There are two basic principles for determining the mean residence time of ‘age’ of a groundwater parcel using CFCs: The ratio of certain CFCs (e.g., CFC 11 and CFC 12) are time dependent, at least over certain time intervals (Fig.). This time dependence can be used to estimate the age of a water parcel. Dating with CFC ratios has been used mainly in oceanography. In hydrology, the ratio dating is more problematic due to the fact that CFC 11 is frequently degraded, especially in anoxic environments.

22. Tracer age comparison Delmarva Delmarva New Jersey

23. CFC sources Prather et al.

24. CFC 11 in the atmosphere

25. CFC 11 in the atmosphere

26. CFC 11 in the atmosphere

27. CFC 11 in the atmosphere

28. CFC 11 degradation

29. CFC 11 degradation

30. CFC 11 degradation

31. CFC 11 degradation

32. CFC 11 degradation

33. CFC 11 degradation

34. CFC 11 degradation

35. CFC 11 degradation Horneman et al., in press

36. CCl4

37. CCl4 degradation in soil Liu, Schlosser, Anid, Santella, Smethie, in prep.

38. SF6 Sulfur hexafluoride (SF6) is the most potent greenhouse gas known. Its atmospheric concentration has increased by 2 orders of magnitude since industrial production started in 1953. Once released into the atmosphere, SF6 will only be removed exceedingly slowly due to its atmospheric lifetime of about 3200 yr. The dominant uses of SF6 are in gas insulated switchgear (GIS) and in blanketing or degassing molten aluminum and magnesium. Maiss and Brenninkmeijer, EST, 1997

39. SF6 From 1978 onward, the rapidly growing global SF6 burden is well-documented by atmospheric observations. The natural background of SF6 is lower than 0.04 ppt. A geographical analysis of SF6 uses suggests that the North American market needs to be better specified. With certain technological efforts, a global reduction of SF6 releases of up to 90% seems feasible, equivalent to 5500 t for the year 1995, and climatically equivalent to 132 million ton of CO2. Maiss and Brenninkmeijer, EST, 1997

40. Atm. SF6 concentrations Maiss and Brenninkmeijer, EST, 1997

41. SF6 ages

42. SF6 sources Maiss and Brenninkmeijer, EST, 1997

43. Atm. SF6 variability

44. SF6 age bias

45. SF5CF3 http://cdiac.ornl.gov/trends/otheratg/sturges/sturges.gif

46. SF5CF3 vs SF6

47. SF5CF3 vs SF6

48. SF5CF3 vs SF6

49. Summary • CFCs can be used effectively for dating of young groundwater in many hydrogeological settings. • CFC measurements require small water samples, can be performed on ‘routine’ GC equipment and are relatively inexpensive • Complications arise from declining atmospheric concentrations, degradation in the soil and in aquifers, and CFC excesses in the atmosphere close to populated regions • SF6 is similar to the CFCs with the advantage of a higher stability in soils and groundwater and a still rising atmospheric concentration. • SF5CF3 might become another valuable tracer