Metal loading assessment of a small mountainous sub-basin characterized by acid drainage -- Prospect Gulch, upper Animas River watershed, Colorado

strongly affected by natural acidity from pyrite weathering.
Metal content in the water column is a composite of multiple
sources affected by hydrologic, geologic, climatic, and anthropogenic
conditions. Identifying sources of metals from various
drainage areas was determined using a tracer injection approach
and synoptic sampling during low flow conditions on September
29, 1999 to determine loads. The tracer data was interpreted
in conjunction with detailed geologic mapping, topographic profiling,
geochemical characterization, and the occurrence and
distribution of trace metals to identify sources of ground-water
inflows. For this highly mineralized sub-basin, we demonstrate
that SO4, Al, and Fe load contributions from drainage areas that
have experienced historical mining?although substantial?are
relatively insignificant in comparison with SO4, Al, and Fe
loads from areas experiencing natural weathering of highlyaltered,
pyritic rocks.
Regional weathering of acid-sulfate mineral assemblages
produces moderately low pH waters elevated in SO4, Al, and
Fe; but generally lacking in Cu, Cd, Ni, and Pb. Samples
impacted by mining are also characterized by low pH and large
concentrations of SO4, Al, and Fe; but contained elevated dissolved
metals from ore-bearing vein minerals such as Cu, Zn,
Cd, Ni, and Pb. Occurrences of dissolved trace metals were
helpful in identifying ground-water sources and flow paths. For
example, cadmium was greatest in inflows associated with
drainage from inactive mine sites and absent in inflows that
were unaffected by past mining activities and thus served as an
important indicator of mining contamination for this environmental
setting.
The most heavily mine-impacted reach (PG153 to PG800),
contributed 8% of the discharge, and 11%, 9%, and 12% of the
total SO4, Al, and Fe loads in Prospect Gulch. The same reach
yielded 59% and 37% of the total Cu and Zn loads for the subbasin.
In contrast, the naturally acidic inflows from the Red
Chemotroph iron spring yielded 39% of the discharge and 54%,
73%, and 87% of the SO4, Al, and Fe loads; but only 4% of the
total Cu and 30% of the total Zn loads in Prospect Gulch.
Base flow from the Prospect Gulch sub-basin contributes
about 4.8 percent of the total discharge at the mouth of Cement
Creek; compared with sampled instream loads of 1.8%, 8.8%,
15.9%, 28%, and 8.6% for SO4, Al, Fe, Cu and Zn, respectively.
Water-shed scale remediation efforts targeted at reducing loads
of SO4, Al, and Fe at inactive mine sites are likely to fail
because the major sources of these constituents in Prospect
Gulch are predominantly discharged from natural sources.
Remediation goals aimed at reducing acidity and loads of Cu
and other base metals, may succeed, however, because changes
in pH and loads are disproportionately greater than increases in
discharge over the same reach, and a substantial fraction of the
metal loading is from mining-impacted reaches. Whether remediation
of abandoned mines in Prospect Gulch can be successful
depends on how goals are defined?that is, whether the objective
is to reduce loads of SO4, Al, and Fe; or whether loads of
Cu and other base metals and pH are targeted.