Throughout the western United States, numerous sources of legacy acid mine drainage (AMD) are the result of turn-of-the-century mining of metal sulfide ores and continue to impact local fresh-water systems.  Two such point sources (Gregory Incline and National Tunnel) enter the North Fork of Clear Creek (NFCC) in the City of Blackhawk in central Colorado.  Compared to an upstream reference site, these inputs result in elevated concentrations of metals (e.g., Cu, Fe, Zn) in the water column of NFCC, in the sediment pore waters, and on the stream substrate.  We have developed a novel field method to examine the contamination and recovery rates of the impacted sediment.  Standardized trays of river stones and cobble were deployed in a heavily-contaminated section of stream to allow for metal deposition.  The contaminated trays and clean control-rock trays were then deployed at the upstream reference site.  Temporal sampling  and subsequent chemical analysis of residual rock coatings allowed us to examine the kinetics of sediment recovery <i>in situ</i>.  Two tray-deployment methods, favoring deposition of either primarily fine-grained precipitates or a combination of coarser and fine-grained sediments, were used. The extent of substrate recovery (i.e., return of metals to control concentrations) depended on the nature of the contaminant-deposition process. Contaminated trays were also sampled for use in simulated recovery experiments in the laboratory, in which we “leached” the contaminated rocks with upstream water from the reference site in a static-renewal, <i>ex-situ</i> analysis.  This allowed for quantitative determination of metals released from the contaminated sediment.  Combining these approaches with stream water chemistry data, geochemical modeling, and in-stream aquatic toxicity testing provides a more integrated picture of the fate, transport and bioavailability of metals in a recovering AMD system.

Methods for simulating the restoration and recovery of acid mine drainage-impacted stream sediment

249th National Meeting (2015)

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