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WATER RESOURCES RESEARCH, VOL. 43, W10423, doi:10.1029/2006WR005854, 2007

Transport and degradation of chlorofluorocarbons (CFCs) in the pyritic Rabis Creek aquifer, Denmark

K. Hinsby

Geological Survey of Denmark and Greenland, Copenhagen, Denmark


Anker L. Højberg

Geological Survey of Denmark and Greenland, Copenhagen, Denmark


P. Engesgaard

Department of Geography and Geology, University of Copenhagen, Denmark


K. H. Jensen

Department of Geography and Geology, University of Copenhagen, Denmark


F. Larsen

Institute of Environment and Resources, Technical University of Denmark, Denmark


L. N. Plummer

U. S. Geological Survey, Reston, USA


E. Busenberg

U. S. Geological Survey, Reston, USA


Abstract

Vertical profiles of the chlorofluorocarbons CFC-11, CFC-12, and CFC-113 penetrating aerobic and anaerobic parts of a shallow sandy aquifer show that the CFC gases are degraded in the <1 m thick transition zone from aerobic to anaerobic groundwater in a pyritic sand aquifer at Rabis Creek, Denmark. Two-dimensional solute transport simulations with either zero-order or first-order degradation in the anaerobic zone corroborate this interpretation. The transport model was previously calibrated against detailed tritium profiles in the same wells. First-order degradation is found to best match the observed CFC profiles yielding an approximate half-life of a few months for CFC-11. Degradation is not as clearly recognized for CFC-12 and CFC-113, but it may occur with rates corresponding to a half-life of a few years or more. Data indicate a geochemical control of the CFC concentration gradient at the redox front and that denitrification and denitrifiers are not of major importance for the observed CFC degradation. The responsible mechanism behind the observed degradation is not known but we suggest that reductive dehalogenation by surface-bound Fe(II) on pyrite possibly enhanced by the presence of Fe(III)-bearing weathering products (green rust) may be a plausible mechanism. The observed data and the performed simulations confirm the potential application of the CFC gases as age-dating tools in the aerobic part of the investigated aquifer, but also that CFC data must be analyzed carefully before it is used as a dating tool in reducing aquifers because degradation may have occurred. The use of multiple or alternative tracers should be considered in anaerobic environments.

Received 22 December 2006; accepted 3 August 2007; published 20 October 2007.

Keywords: Dating; CFCs; transport; degradation; pyrite; anaerobic.

Index Terms: 1832 Hydrology: Groundwater transport; 1849 Hydrology: Numerical approximations and analysis; 1847 Hydrology: Modeling; 1843 Hydrology: Land/atmosphere interactions (1218, 1631, 3322).

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Citation: Hinsby, K., A. L. Højberg, P. Engesgaard, K. H. Jensen, F. Larsen, L. N. Plummer, and E. Busenberg (2007), Transport and degradation of chlorofluorocarbons (CFCs) in the pyritic Rabis Creek aquifer, Denmark, Water Resour. Res., 43, W10423, doi:10.1029/2006WR005854.