Degradation rates of CFC-11, CFC-12 and CFC-113 in anoxic shallow aquifers of Araihazar, Bangladesh

A. Horneman, M. Stute, Peter Schlosser, W. Smethie, N. Santella, D. T. Ho, B. Mailloux, E. Gorman, Y. Zheng, A. van Geen

Research output: Contribution to journalArticle

28 Citations (Scopus)

Abstract

Chlorofluorocarbons CFC-11 (CCl3F), CFC-12 (CCl2F2), and CFC-113 (CCl2F-CClF2) are used in hydrology as transient tracers under the assumption of conservative behavior in the unsaturated and saturated soil zones. However, laboratory and field studies have shown that these compounds are not stable under anaerobic conditions. To determine the degradation rates of CFCs in a tropical environment, atmospheric air, unsaturated zone soil gas, and anoxic groundwater samples were collected in Araihazar upazila, Bangladesh. Observed CFC concentrations in both soil gas and groundwater were significantly below those expected from atmospheric levels. The CFC deficits in the unsaturated zone can be explained by gas exchange with groundwater undersaturated in CFCs. The CFC deficits observed in 3H/3He dated groundwater were used to estimate degradation rates in the saturated zone. The results show that CFCs are degraded to the point where practically no (< 5%) CFC-11, CFC-12, or CFC-113 remains in groundwater with 3H/3He ages above 10 yr. In groundwater sampled at our site CFC-11 and CFC-12 appear to degrade at similar rates with estimated degradation rates ranging from ~ 0.25 yr- 1 to ~ 6 yr- 1. Degradation rates increased as a function of reducing conditions. This indicates that CFC dating of groundwater in regions of humid tropical climate has to be carried out with great caution.

Original languageEnglish (US)
Pages (from-to)27-41
Number of pages15
JournalJournal of Contaminant Hydrology
Volume97
Issue number1-2
DOIs
StatePublished - Apr 4 2008
Externally publishedYes

Fingerprint

Chlorofluorocarbons
CFC
Aquifers
aquifer
Degradation
degradation
Groundwater
groundwater
Gases
Soils
1,1,2-trichloro-1,2,2-trifluoroethane
rate
soil gas
vadose zone
Hydrology
tropical environment
phreatic zone
gas exchange

Keywords

  • Bangladesh
  • CFCs
  • Degradation rates
  • Groundwater dating
  • Tritium/He

ASJC Scopus subject areas

  • Environmental Chemistry
  • Water Science and Technology

Cite this

Degradation rates of CFC-11, CFC-12 and CFC-113 in anoxic shallow aquifers of Araihazar, Bangladesh. / Horneman, A.; Stute, M.; Schlosser, Peter; Smethie, W.; Santella, N.; Ho, D. T.; Mailloux, B.; Gorman, E.; Zheng, Y.; van Geen, A.

In: Journal of Contaminant Hydrology, Vol. 97, No. 1-2, 04.04.2008, p. 27-41.

Research output: Contribution to journalArticle

Horneman, A, Stute, M, Schlosser, P, Smethie, W, Santella, N, Ho, DT, Mailloux, B, Gorman, E, Zheng, Y & van Geen, A 2008, 'Degradation rates of CFC-11, CFC-12 and CFC-113 in anoxic shallow aquifers of Araihazar, Bangladesh', Journal of Contaminant Hydrology, vol. 97, no. 1-2, pp. 27-41. https://doi.org/10.1016/j.jconhyd.2007.12.001
Horneman, A. ; Stute, M. ; Schlosser, Peter ; Smethie, W. ; Santella, N. ; Ho, D. T. ; Mailloux, B. ; Gorman, E. ; Zheng, Y. ; van Geen, A. / Degradation rates of CFC-11, CFC-12 and CFC-113 in anoxic shallow aquifers of Araihazar, Bangladesh. In: Journal of Contaminant Hydrology. 2008 ; Vol. 97, No. 1-2. pp. 27-41.
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AU - Stute, M.

AU - Schlosser, Peter

AU - Smethie, W.

AU - Santella, N.

AU - Ho, D. T.

AU - Mailloux, B.

AU - Gorman, E.

AU - Zheng, Y.

AU - van Geen, A.

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N2 - Chlorofluorocarbons CFC-11 (CCl3F), CFC-12 (CCl2F2), and CFC-113 (CCl2F-CClF2) are used in hydrology as transient tracers under the assumption of conservative behavior in the unsaturated and saturated soil zones. However, laboratory and field studies have shown that these compounds are not stable under anaerobic conditions. To determine the degradation rates of CFCs in a tropical environment, atmospheric air, unsaturated zone soil gas, and anoxic groundwater samples were collected in Araihazar upazila, Bangladesh. Observed CFC concentrations in both soil gas and groundwater were significantly below those expected from atmospheric levels. The CFC deficits in the unsaturated zone can be explained by gas exchange with groundwater undersaturated in CFCs. The CFC deficits observed in 3H/3He dated groundwater were used to estimate degradation rates in the saturated zone. The results show that CFCs are degraded to the point where practically no (< 5%) CFC-11, CFC-12, or CFC-113 remains in groundwater with 3H/3He ages above 10 yr. In groundwater sampled at our site CFC-11 and CFC-12 appear to degrade at similar rates with estimated degradation rates ranging from ~ 0.25 yr- 1 to ~ 6 yr- 1. Degradation rates increased as a function of reducing conditions. This indicates that CFC dating of groundwater in regions of humid tropical climate has to be carried out with great caution.

AB - Chlorofluorocarbons CFC-11 (CCl3F), CFC-12 (CCl2F2), and CFC-113 (CCl2F-CClF2) are used in hydrology as transient tracers under the assumption of conservative behavior in the unsaturated and saturated soil zones. However, laboratory and field studies have shown that these compounds are not stable under anaerobic conditions. To determine the degradation rates of CFCs in a tropical environment, atmospheric air, unsaturated zone soil gas, and anoxic groundwater samples were collected in Araihazar upazila, Bangladesh. Observed CFC concentrations in both soil gas and groundwater were significantly below those expected from atmospheric levels. The CFC deficits in the unsaturated zone can be explained by gas exchange with groundwater undersaturated in CFCs. The CFC deficits observed in 3H/3He dated groundwater were used to estimate degradation rates in the saturated zone. The results show that CFCs are degraded to the point where practically no (< 5%) CFC-11, CFC-12, or CFC-113 remains in groundwater with 3H/3He ages above 10 yr. In groundwater sampled at our site CFC-11 and CFC-12 appear to degrade at similar rates with estimated degradation rates ranging from ~ 0.25 yr- 1 to ~ 6 yr- 1. Degradation rates increased as a function of reducing conditions. This indicates that CFC dating of groundwater in regions of humid tropical climate has to be carried out with great caution.

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