Use of SF6 to label drilling air in unsaturated,fractured rock studies: risk of over-purging

Sulfur hexafluoride was injected as a tracer gas into the air stream during air-drilling of a borehole in the unsaturated zone of a partially-welded, fractured tuff at Apache Leap, Arizona. One-meter intervals were later isolated at multiple depths and pumped to purge drilling air from each interval. The volume of air purged (at 1 atm, 20 °C), ranged from a low of 0.3 m³ in an unfractured interval, to a high of 252 m³ in a highly fractured interval. The concentration of SF₆ remained high throughout the purge in all intervals and often increased over time. Measurements of δ¹³C, ¹⁴C and CO₂ concentration indicated that atmospheric air was eventually drawn into several of the intervals in spite of the fact that SF₆ concentrations remained high. Possible explanations include mixing of atmospheric air drawn through fractures with partially-purged matrix air, and delayed removal of SF₆ relative to atmospheric gases due to adsorption of SF₆ within the tuff matrix, dissolution into pore water, or diffusion from dead-end pores with restricted openings. In this system, following a long delay between drilling and purging, it was found that the risk of contamination from surface air by over-purging was substantially greater than the risk of contamination from residual drilling air by under-purging.