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

Syllabus Date Reading Lecture HW Prof Tu 1/22 Orientation, Principles of tracer applications MS,PS Th 1/24 Surface water hydrology 101 PS Tu 1/29 Groundwater hydrology 101 Groundwater flow HW1 MS Th 1/31 Groundwater transport Tu 2/5 Dyes, particles, ions, and other deliberate tracers HW2 Th 2/7 Stable isotopes Tu 2/12    HW3 Th 2/14   3H Tu 2/19 3H/3He

Radiocarbon dating (14C) Syllabus Date Reading Lecture HW  Prof Th 2/21    CFCs, SF6, SF5CF3 PS Tu 2/26 Carbon Isotopes (14C, 13C)  HW4 MS Th 2/28 85Kr, 39Ar Tu 3/4 Radiocarbon dating (14C) HW5 Th 3/6 Noble gases (incl Rn) Tu 3/11 Noble gas thermometry

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)

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.

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.

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

Delay in release into atmosphere Mean life time [ys] (atm) CFCs: properties Boiling Point [oC] Use Delay in release into atmosphere Mean life time [ys] (atm) CFC 11 23.8 Aerosol spray cans; Solvent; Foam blowing agent » 6 months; » 6 months; 6 months to 20 ys 57 - 105 mean: 74 CFC 12 -29.8 Refrigerators Air conditioners Foam blowing agent 4 – 12 years, 4 – 12 years, 6 months to 20 ys 67 – 333 mean: 111 CFC 113 47.6 Cleaning solvent for manufacturing processes and electronic components 136-195 CCl4 76.8 Cleaning agent

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.

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

CFCs: life cycle

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

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

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

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

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

CFC 11 in the atmosphere Plummer et al., 2000

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

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

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.

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.

Tracer age comparison Delmarva Delmarva New Jersey

CFC sources Prather et al.

CFC 11 in the atmosphere

CFC 11 in the atmosphere

CFC 11 in the atmosphere

CFC 11 in the atmosphere

CFC 11 degradation

CFC 11 degradation

CFC 11 degradation

CFC 11 degradation

CFC 11 degradation

CFC 11 degradation

CFC 11 degradation

CFC 11 degradation Horneman et al., in press

CCl4

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

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

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

Atm. SF6 concentrations Maiss and Brenninkmeijer, EST, 1997

SF6 ages

SF6 sources Maiss and Brenninkmeijer, EST, 1997

Atm. SF6 variability

SF6 age bias

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

SF5CF3 vs SF6

SF5CF3 vs SF6

SF5CF3 vs SF6

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