Monitoring the Post-Remediation Recovery of Aquatic Biota in the Boulder River Watershed, Jefferson County, Montana

The Boulder River watershed in Jefferson County, Montana, contains elevated concentrations of arsenic (As), cadmium (Cd), copper (Cu), lead (Pb), and zinc (Zn) in water, sediment, and aquatic biota from historic mining activity.  Abandoned mine and stream channel reclamation and remediation projects were conducted in several affected tributary streams from 1997 to 2005.  To assess potential recovery, we investigated three types of parameters 1.) measured concentrations of As, Cd, Cu, Pb, and Zn in water, biofilm and macroinvertebrates, 2.) assessed macroinvertebrate assemblage and aquatic habitat quality 3.) conducted in situ survival experiments with juvenile cutthroat trout.  The methods and sites were similar to the initial assessment of the site in 1997 and the first monitoring effort in 2003.  When compared to previous years, biofilm and invertebrate metal concentrations significantly decreased at many tributary sites in 2009.  Biofilm collected from the lower Boulder River contained significantly lower concentrations of all metals measured when compared to previous years. Metal concentrations in biofilm and invertebrates remained significantly elevated when compared to upstream reference sites for at least one metal at multiple sites.  Invertebrate assemblages remains impacted at several downstream locations (as defined by number of families/site), but appear to be recovering compared to samples collected in 2003.  Survival of cutthroat trout during in situ experiments was still reduced at 2 of 5 sites, but was improved at 2 of 5 sites compared to experiments conducted in 1998 and 1999.  However, higher stream discharge and dilution of mine effluent may have contributed to the higher survival in 2009 when compared to previous years.