The release occurred within the mine site from two different locations. The primary release point was an eight inch line used to inject the paste backfill underground. This release point is approx. 100 yards upgradient from the 1690 portal (Fig. 1, sitrep). The secondary release occurred from a valve inside the Tank 32B Pump House (Fig. 1, sitrep) located on the mill bench.
Time/Date of Incident
Pogo employees discovered the spill at 7:30 a.m. on May 7, 2015 and reported the spill to ADEC at 1:47pm.
"Paste backfill" contains 1-3 parts-per-million cyanide and has a pH of 10-12. It consists of milled tailings mixed with cement to create a paste, and it is disposed by backfilling mined-out tunnels. When in place underground, the paste backfill cures forming concrete, which offers support to the underground workings. Pogo estimates a total spill volume of 90,000 gallons, of which an estimated 36,000 gallons was released onto the 1690 portal bench.
Pogo believes a ruptured eight inch paste backfill line was the cause of the primary release.