Abstract :
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.
