COPPER CONCENTRATIONS IN WATER AND SEDIMENT FOLLOWING THE AUGUST 2015 GOLD KING MINE RELEASE

The August 2015 Gold King Mine release sent 3 million gallons of acidic, metal-rich water flowing down Cement Creek and the Animas River in southern Colorado, creating an orange plume of contamination that extended into the San Juan River in northern New Mexico. Photographs and videos of the orange plume were prevalent in the media in the days following the release, and the incident has refocused attention on the impact of mining on water quality. Despite the visual impact of the release noted in the media, short and long term effects of the release may be hard to discern given the highly mineralized nature of the Silverton Caldera, were mining activities have adversely affected water quality for decades. 

A review of water and sediment data collected before, during, and after the release suggests that Cu may be a key constituent used to determine the effects of the release. Data collected during the USGS Abandoned Mine Lands Initiative (1996-2001) and an October 2012 synoptic study indicate that the Gold King Mine is the largest source of Cu in the Upper Animas watershed, for example, and Cu concentrations should be elevated downstream following the release. This hypothesis is borne out by water-quality samples collected at the mouth of Cement Creek during the second week of the release, which document increases in metal loads when compared to historical data (2009-2014) collected under similar low-flow conditions. Although this increase in load is evident for most metals, it is especially pronounced for Cu. 

Increased pH downstream of the Cement Creek mouth in the Animas River leads to the sorption of Cu onto colloidal iron oxides, transforming Cu from the dissolved phase to a colloidal form that aggregates and settles to the streambed. Bed sediment samples collected at USGS gage 09359020 (Animas River below Silverton, CO) after the release suggest increased concentrations of As, Cr, Cu, and V when compared to data collected prior to the release (September 2014). Sediment concentrations of Cd, Fe, Mn, Ni, Ti, and Zn were lower following the release, whereas Ag, Al, and Pb concentrations were relatively unchanged. Of all the metals considered, the largest increase in bed sediment concentration is for Cu, further supporting the idea that copper serves as an effective marker for the release.