Geochemical investigations and interim recommendations for priority abandoned mine sites, BLM lands, upper Animas River 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 U. S. Bureau of Land Management (BLM) in the Upper
Animas River watershed. Results of chemical analyses of dump materials, leachates of
those materials, and surface waters draining mines or dumps provide indications of where
acid is generated or consumed, and metal concentrations below mines or dumps.
Information on sites previously identified as needing reclamation is reviewed and
available geochemical information is used to rank 26 sites into four classes of priority for
reclamation.
Although there are more than a thousand mining sites (productive mines and
prospects) on BLM lands in the Upper Animas River watershed study area, the majority
are very small (less than about 70 cubic yards of dump material), are more than 2 miles
from a major stream, or so inaccessible as to prohibit reclamation. In the summers of
1997 and 1998 approximately 200 sites were observed and more than 100 of these that
appeared to have the potential to geochemically impact the watershed were examined
more carefully and sampled. Building upon the prior work of the BLM and associated
agencies, this work attempted to identify the most significant sources of mine-related
contamination and to rank those sites as to priority for reclamation. These most
significant mining areas have been examined within a geologic framework and were
evaluated by multiple criteria, including tendency to generate acid and release toxic
metals, observed damage to vegetation, potential to release metals based on leach tests,
and likelihood of transport into streams of the watershed. No single measurable
parameter, such as metal concentration, can be used to rank the sites. Rather, subjective
estimates are required to evaluate combinations or interactions among several parameters.
The most subjective estimate, while ranking feasibility of reclamation, is the relative
amounts of naturally occurring metals and acidity and mine-related contamination at each
mining area. Mitigation of natural contributions at mines or unmined areas is beyond the
scope of my studies, but should be considered when planning reclamation. Available
information for the 26 priority sites is adequate for ranking, but at some sites additional
information on groundwater conditions is needed for a more complete site evaluation.