Geochemical investigations and interim recommendations for priority abandoned mine sites on U.S.D.A. Forest Service lands, Mineral Creek watershed, San Juan County, Colorado

Field observations, sampling of mine dumps and mine drainage waters, and
laboratory studies of dump materials have been made at mining areas deemed to be on
public lands administered by the USDA Forest Service in the Mineral Creek watershed.
Results of chemical analyses of dump materials, leachates of those materials, and of
surface waters draining mines or dumps provide indications of where acid is generated or
consumed, and what metals are mobilized below mines or dumps. Information on 25
sites is reviewed and reclamation priorities are ranked into four classes (high, medium,
low priority, or no work required).
The western side of the upper Animas watershed (the Mineral Creek watershed)
has a history of mining and prospecting for about 130 years. The intensity of miningrelated
disturbance is higher than in most parts of the San Juan Mountains region, but
actually is much less than the eastern half of the watershed (US BLM lands) and none of
the mines moved millions of tons of rock and ore as in some of the eastern mines. The
majority of the roughly one thousand mining sites on the USFS lands are very small (less
than 100 tons or 70 cubic yards of dump material), are more than 2 miles from a major
stream, or are so inaccessible as to prohibit reclamation. Twenty five sites have been
considered by others to have significant size and potential for significant environmental
degradation. These most significant mining areas were evaluated by multiple criteria,
including tendency to generate acid or liberate toxic metals, observed acidic pH or dead
vegetation (?kill zones?) below dumps or adits, potential mobility of metals, and
likelihood of transport into streams of the watershed. In the author?s opinion, no single
measurable parameter, such as metal concentration, is reliable for ranking significance or
feasibility of reclamation. Rather, subjective estimates are required to evaluate
combinations of, or interactions among, several parameters. The most subjective
estimate in ranking feasibility of reclamation is the amount of natural and mine-related
contamination at each mining area. Mitigation of natural contributions at mines or
unmined areas is beyond the scope of these Abandoned Mine Lands (AML)
investigations, but must be considered when planning reclamation. Available
information for the 25 problem sites is adequate for ranking, but at some sites additional
information on groundwater conditions is needed for a more reliable ranking and
evaluation of reclamation methods.