Examining the Potential Impacts to Wetlands and Water Bodies Due to Mineral Exploration, Pebble Copper-Gold Prospect, Southwest Alaska

Most mining projects do not make it past the exploration stage. However, there is little information in
the literature about the impacts of mineral exploration drilling on natural waters. A copper-goldmolybdenum
mining deposit in Alaska was heavily explored until 2012 and partially reclaimed;
however, full reclamation of drill sites remained incomplete in 2016. Copper is sub-lethally toxic to
salmon, a highly-valued resource in this area. Of 109 sites inspected, nine sites had confirmed
impacts due to un-reclaimed drill-holes or drill waste disposal practices. At seven sites artesian waters
at the drill stem resulted in surface water or sediment elevated in aluminum, iron, copper, or zinc with
neutral pH. Copper concentrations at artesian sites were <0.4, 0.7, 2, 7, 15, 76, and 215 μg/L; the
latter four exceed water quality criteria in the state of Alaska. Drilling waste is known to have been
disposed of in ponds and unlined sumps. At one of five ponds sampled, copper declined from 51 to 8
μg/L over nine years, however at the only sump area with historical data, copper increased from 0.3 to
1.8 μg/L at a downgradient wetland spring over five years. This research identifies contaminant types
and sources and can be used to guide future ecotoxicity studies and improve regulatory oversight.
The results of this investigation provide direction on the sources and types of contamination. Future
research should sample pond sediment, where contaminants from drill waste may sequester, and
investigate drill waste sumps for evidence of leaching. It would be in the public interest for state
regulators to conduct water, soil, or sediment sampling during inspections to monitor environmental
changes where drill holes are not fully and permanently reclaimed.