Investigation and Documentation of Acidic Mining Exploration Drill Cuttings at the Pebble Copper– Gold Mineral Prospect, Southwest Alaska

During exploration drilling of the Pebble copper–gold–molybdenum (Cu–Au–Mo) deposit, drilling
wastes were disposed of directly on the landscape or passed through unlined sumps prior to disposal.
The ore and host rock are rich in sulfides, which weather to sulfuric acid with consequent metal
leaching. Oxidized cuttings were visually evident, and confirmed with laboratory and field testing to
have a pH of 2.7–4.3. At these sites, Cu and Mo exceeded or were at the high end of the natural
background. With one exception, Cu was in the range of 545 mg/kg to 4865 mg/kg. Dead vegetation
was observed at all sites with drill cuttings on the surface. Dead vegetation was also observed on
sump soil covers, unrelated to drilling waste. Sites where vegetation had not re-established were from
four to thirteen years old. The potential impact to surface and groundwater was not determined.
Understanding the source and extent of damage from cuttings could lead to better site management.
Drill cuttings sump contents need further sampling to determine the rate of chemistry changes, and
whether groundwater downgradient from sumps is being impacted. Finally, further studies should be
conducted to determine whether there are impacts from sulfidic drill waste deposited in isolated water
bodies, such as glacial kettles. Research sites should be sampled before and after drilling to ensure
clean control samples.