The practicalities of using CFCs and SF6 for groundwater dating and tracing |
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| Authors: | WG Darling DC Gooddy AM MacDonald BL Morris |
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| Institution: | 1. British Geological Survey, Maclean Building, Wallingford OX10 8BB, UK;2. British Geological Survey, Murchison House, Edinburgh EH9 3LA, UK |
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| Abstract: | Knowledge of groundwater residence time is important in understanding key issues in the evolution of water quality, whether this occurs due to water–rock interaction or simply by mixing or contamination. The build-up in the atmosphere of the trace gases chlorofluorocarbons (CFCs) and sulphur hexafluoride (SF6) from the middle of the last century offers a convenient way of dating waters up to ∼60 a old. The gases are well-mixed in the atmosphere so their input functions are not area-specific as is the case with 3H. While any one of these trace gases can in principle provide a groundwater age, when two or more are measured on water samples the potential exists to distinguish between different modes of flow including piston flow, exponential flow and simple end-member mixing. As with all groundwater dating methods, caveats apply. Factors such as recharge temperature and elevation must be reasonably well-constrained. Primarily for SF6, the phenomenon of ‘excess air’ also requires consideration. Primarily for the CFCs, local sources of contamination need to be considered, as do redox conditions. For both SF6 and the CFCs, the nature and thickness of the unsaturated zone need to be factored into residence time calculations. However, as an inexpensive dating method, the trace gases can be applied to a wide range of groundwater problems where traditional age indicators might once have been used more sparingly. Examples include tracing flowlines, detecting small modern inputs in ‘old’ waters, and pollution risk assessment. In the future, with the main CFCs already declining in the atmosphere, new anthropogenic trace gases are likely to take their place. |
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