Exploring hydrofluorocarbons as groundwater age tracers

Szeminárium Exploring hydrofluorocarbons as groundwater age tracers

Karl B. Haase

US Geological Survey Groundwater Dating Laborator
Kapcsolattartó Dr. Molnár Mihály
Időpont 2015-02-10 11:00
Helyszín Debrecen, Bem tér 18/c XII. épület 3. emelet
Esemény leírása

Groundwater dating tracers are an essential tool for analyzing hydrologic conditions in groundwater systems. Commonly used tracers for dating post-1940s groundwater include sulfur hexafluoride (SF6), chlorofluorocarbons (CFCs), 3H-3He and other isotopic tracers (85Kr, delta2H and delta18O isotopes, etc). Each tracer carries a corresponding set of advantages and limitations imposed by field, analytical, and interpretive methods. Increasing the number available tracers is appealing, particularly if they possess inert chemical properties and unique temporal emission histories from other tracers. Among the likely candidates identified are HCFC-22 and HFC-134a. Both compounds have unique atmospheric histories and chemical properties compared to other tracers, making them appealing for inclusion lumped parameter mixing models and in situations where SF6 or the CFCs are present in elevated concentrations from non-atmospheric sources such that they cannot be used for dating purposes. To measure these compounds we developed an analytical method that uses an atomic emission detector (GC-AED) in place of an ECD. In contrast to the ECD, the AED is a universally sensitive, highly linear, elementally specific detector, with sensitivities reaching into the low pico-gram range. The GC-AED system is capable of detecting concentrations cca. 200 fM (HCFC-22) and cca. 100 fM (HFC-134a), corresponding to piston flow ages of 54 and 18 yr, respectively, with a typical uncertainty of cca. 1 yr. Comparisons between H(C)FCs, CFCS, and SF6 typically show broad disagreement between traditional dating methods and the H(C)FCs, indicating they are generally subject to loss processes, and may not be suitable for use as groundwater age tracers.

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