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Using CFCs and SF6 for groundwater dating : a SWOT analysis

By W.G. Darling, D.C. Gooddy, B.L. Morris and A.M. MacDonald

Abstract

A knowledge of the residence time of groundwater is of importance in\ud understanding key issues in the evolution of water quality. Chlorofluorocarbons (CFCs) and\ud sulphur hexafluoride (SF6) offer a convenient way of dating waters up to ~60 yrs old. In\ud contrast to tritium, these gases are well-mixed in the atmosphere so their input functions are\ud much less problematic. While any one of these gases can in principle provide a groundwater\ud age, when two or more are measured on water samples the potential exists to distinguish\ud between different modes of flow including piston flow, exponential flow and simple endmember\ud mixing. As with all groundwater dating methods, caveats apply. Factors such as\ud recharge temperature and elevation must be reasonably well-constrained. Mainly for SF6, the\ud phenomenon of ‘excess air’ also requires consideration. Mainly for the CFCs, local sources\ud of contamination need to be considered, as do redox conditions. For both SF6 and the CFCs,\ud the nature and thickness of the unsaturated zone need to be factored into residence time\ud calculations. This paper attempts a balanced look at the pros and cons of the trace-gas dating\ud method

Topics: Hydrology
Publisher: CRC Press
Year: 2010
OAI identifier: oai:nora.nerc.ac.uk:10940

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