The August 2015 uncontrolled release of water from the Gold King Mine, in the headwaters of the Animas River in southwestern Colorado, brought renewed attention to the issue of water-quality degradation caused by abandoned draining mines. One remediation technique that has been used in Colorado to limit effluent from draining mine tunnels is installation of structural bulkheads. Long-term (1990 to present) water quality at the mouth of Cement Creek, a tributary to the Animas River, shows complicated and inconsistent changes in metal concentrations during a period when multiple bulkheads were installed to limit drainage from abandoned mine tunnels. For example, copper concentrations increased in 2005 and decreased to pre-2005 levels in 2010, whereas manganese (Mn) and zinc (Zn) concentrations both increased beginning in 2003–05 and remained at elevated levels through at least 2016. The timing and magnitude of these concentration changes are likely related to the changing configuration of mine pools and groundwater flow paths caused by installation of multiple bulkheads. The large extent of underground mine workings affecting the Cement Creek watershed complicates understanding how bulkhead installation changes groundwater flow paths, mine-pool configuration, and water quality. In contrast to Cement Creek, in Lake Fork Creek, a tributary to the upper Arkansas River, concentrations of Mn and Zn generally decreased (2010–17) following installation of one bulkhead (2009) at the Dinero mine tunnel. In addition, in Lake Fork Creek after remediation, metal concentrations generally decreased relative to specific conductance indicating a fundamental shift in the geochemistry of the source water for metals to the creek. This shift was possibly caused by suppression of the acid-rock generating reaction due to increased size and depth of the mine pool and consequent limits on the supply of dissolved oxygen to submerged, reactive minerals. Underground mine workings in the Dinero area are much less extensive than in the Cement Creek watershed, likely leading to the better water-quality result and understandable water-quality changes after bulkhead installation. Analysis of these two contrasting sites provides information on how water quality is influenced by the changing configuration of underground mine pools and groundwater flow paths resulting from bulkhead installation and can inform future remediation efforts.

Water quality changes following remediation of abandoned draining mines using structural bulkheads, upper Animas River, and upper Arkansas River Watersheds, Colorado, USA

2019 Spring National Meeting

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