New Mexico Geological Society Annual Spring Meeting — Abstracts

The Animas River (Gold King Mine) Spill has been touted as the worst mining related environmental disaster in US history, although was small compared to tailings dam failures elsewhere in the world over the past few years. The estimates are that about 3 million gallons (10 000 tons) of low pH water ponded in the mine workings and 400 tons of metals in solution and as suspended mineral particles, amounts which would normally seep out of the mine over about 6 months, were released in a slug over a period of about two days. With dispersion, the Animas River remained significantly contaminated over a longer period in NW New Mexico.

Although the trend has reversed, the spill took place after years of general improvement to the quality of Animas River water, a result of cessation and mine and mill activity and site by site remediation by various groups, including the Animas River Stakeholders Group. Interestingly the Gold King Mine had not originally been identified as a top priority site for treatment. The earlier assessments had been based on steady state metal and acid loading and had not considered failure of blockages or engineered structures, or changes to hydrology and other conditions at mines. The Gold King Mine was relatively large, its ores and wallrock have high acid-producing capacity, and its hydrology had been affected by remediation work at topographically underlying mines.

The metal load in the Animas will reside largely in stream bed precipitates and sediments, presumably along the whole length to Lake Powell, and has the potential to be chemically and physically remobilised. The potential for acid production in mines, waste rock and tailings will remain so long as pyrite is available for oxidative dissolution. Despite the visual prominence of iron-oxide staining, pyrite dissolution in waste rock is slow relative to historic time frames in the climate of the Mountain West. Estimates of reaction rates from natural acid rock drainage and petrology of historical mine waste imply that an order of magnitude drop in rate of acid production requires of order 1000 years. Rates may be different in mine workings as a result of temperature, dissolution into a near stagnant partially deoxygenated mine pool, and other factors, but order of magnitude estimates of metal flux suggest similar time spans for ‘reactive flushing’ of the Gold King Mine workings by through-flow of meteoric waters.