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The Use of Sulphur Hexafluoride as a Conservative Tracer in Saturated Sandy Media
Author(s) -
Wilson Ryan D.,
Mackay Douglas M.
Publication year - 1993
Publication title -
groundwater
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.84
H-Index - 94
eISSN - 1745-6584
pISSN - 0017-467X
DOI - 10.1111/j.1745-6584.1993.tb00842.x
Subject(s) - tracer , sulfur hexafluoride , detection limit , bromide , chemistry , ionic bonding , electron capture detector , gas chromatography , analytical chemistry (journal) , chromatography , radiochemistry , ion , inorganic chemistry , nuclear physics , physics , organic chemistry
Sulphur hexafluoride (SF 6 ), a volatile, nontoxic, inorganic compound, behaved identically to bromide in a laboratory column tracer experiment with saturated sandy media. A simple system was used to achieve an SF 6 injection concentration of approximately 7 mg/1, roughly 25% of its solubility. While this system requires more equipment and time than that involved in preparing ionic tracer solutions, certain benefits are realized. The detection limit for SF 6 was approximately 5 μg/l using a simple headspace extraction technique and a gas Chromatograph with an electron capture detector. Thus, the tracer could be followed for a three order of magnitude decrease in concentration. For similar resolution, bromide had to be injected at a concentration over 1000 mg/1 (typical detection limit approximately 0.5 mg/1). When the required higher concentrations of the ionic tracers may lead to significant geochemical changes or density effects, SF 6 may be the preferable tracer. Furthermore, unlike ionic tracers, the highly volatile SF 6 may be readily removed from water. High volatility does, however, require that some care be taken to limit the potential loss of mass during injection, sampling, and analysis. Since others are developing procedures for analyzing SF 6 in ground water with extremely low detection limits (on the order of 1 ng/1), it appears likely that measurable concentrations could range over six to seven orders of magnitude. This range is considerably wider than typically possible with ionic tracers, and may offer an important advantage in some applications.