The Use of Sulphur Hexafluoride as a Conservative Tracer in Saturated Sandy Media

Abstract

AbstractSulphur hexafluoride (SF6), 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 SF6 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 SF6 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, SF6 may be the preferable tracer. Furthermore, unlike ionic tracers, the highly volatile SF6 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 SF6 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.