MINERALOGICAL AND CHEMICAL COMPOSITION OF CEMENT CREEK AND ANIMAS RIVER BEDLOAD SEDIMENT

The August 5, 2015 Gold King release resulted in the downstream flow of visually distinct, turmeric-colored turbid water that is perhaps the most lasting impression of the event for most people. A study was carried out on bedload sediment collected from Cement Creek and the Animas River to characterize the mineralogy, chemistry, and solubility of material deposited during this event. Mineralogy by quantitative X-ray diffraction showed that fine-grain sediment (<63µm) was 35 to 47 percent of clay minerals, primarily illite, with 12 to 25 percent amorphous iron oxyhydroxides and/or oxyhydroxysulfates, and approximately 1 percent goethite in all samples. Potassium end-member jarosite was 1.7 to 6.1 percent of the sediment mineralogy. Scanning electron miroscopy was used to determine that lead was primarily substituted into jarosite. However, no suspected end-member plumbojarosite was observed. Arsenic was primarily associated with goethite and amorphous iron minerals. The bedload sediment included elevated concentrations of arsenic (54 to 89 mg As kg^-1) and lead (730 to 2130 mg Pb kg^-1). While the total arsenic is elevated, it was not soluble in either deionized water or acidic bioaccessability leachates. Sediment collected from the Animas River above the City of Durango water intake had elevated bioaccessability for lead, as well as cadmium, copper and zinc. The bedload sediment deposited during the 2015 event is similar to material deposited from chronic acid mine drainage as reported in 2007¹. While the release of August 2015 resulted in distressing images due to suspended sediment, the chronic mine drainage impacting the larger Animas River is an ongoing outcome of historical mining in this region.

¹http://pubs.usgs.gov/pp/1651/downloads/Vol2_combinedChapters/vol2_chapE12.pdf