Determination of chemical-constituent loads during base-flow and storm-runoff conditions near historical mines in Prospect Gulch, upper Animas River watershed, southwestern Colorado

Prospect Gulch is a major source of iron, aluminum, zinc, and other metals to
Cement Creek. Information is needed to prioritize remediation and develop strategies for
cleanup of historical abandoned mine sites in Prospect Gulch. Chemical-constituent
loads were determined in Prospect Gulch, a high-elevation alpine stream in southwestern
Colorado that is affected by natural acid drainage from weathering of hydro-thermally
altered igneous rock and acidic metal-laden discharge from historical abandoned mines.
The objective of the study was to identify metal sources to Prospect Gulch. A tracer
solution was injected into Prospect Gulch during water-quality sampling so that loading
of geochemical constituents could be calculated throughout the study reach. A
thunderstorm occurred during the tracer study, hence, metal loads were measured for
storm-runoff as well as for base flow. Data from different parts of the study reach
represents different flow conditions. The beginning of the reach represents background
conditions during base flow immediately upstream from the Lark and Henrietta mines
(samples PG5 to PG45). Other samples were collected during storm runoff conditions
(PG100 to PG291); during the first flush of metal runoff following the onset of rainfall
(PG303 to PG504), and samples PG542 to PG700 were collected during low-flow
conditions.
During base-flow conditions, the percentage increase in loads for major
constituents and trace metals was more than an order of magnitude greater than the
corresponding 36 % increase in stream discharge. Within the study reach, the highest
percentage increases for dissolved loads were 740 % for iron (Fe), 465 % for aluminum
(Al), 500 % for lead (Pb), 380 % for copper (Cu), 100 % for sulfate (SO4), and 50 % for
zinc (Zn). Downstream loads near the mouth of Prospect Gulch often greatly exceeded
the loads generated within the study reach but varied by metal species. For example, the
study reach accounts for about 6 % of the dissolved-Fe load, 13 % of the dissolved-Al
load, and 18 % of the dissolved-Zn load; but probably contributes virtually all of the
dissolved Cu and Pb. The greatest downstream gains in dissolved trace-metal loads
occurred near waste-rock dumps for the historical mines. The major sources of trace
metals to the study reach were related to mining. The major source of trace metals in the
reach near the mouth is unknown, however is probably related to weathering of highly
altered igneous rocks, although an unknown component of trace metals could be derived
from mining sources The late-summer storm dramatically increased the loads of most dissolved and
total constituents. The effects of the storm were divided into two distinct periods; (1) a
first flush of higher metal concentrations that occurred soon after rainfall began and (2)
the peak discharge of the storm runoff. The first flush contained the highest loads of
dissolved Fe, total and dissolved Zn, Cu, and Cd. The larger concentrations of Fe and
sulfate in the first flush were likely derived from iron hydroxide minerals such as jarosite
and schwertmanite, which are common on mine dumps in the Prospect Gulch drainage
basin. Peak storm runoff contained the highest measured loads of total Fe, and of total
and dissolved calcium, magnesium, silica and Al, which were probably derived from
weathering of igneous rocks and clay minerals in the drainage basin.