Metal Sources

Metal loads in a mineralized area can enter a stream from various natural and mining-related sources that could include surface waters flowing through mines or mine wastes, natural acid rock drainage from altered pyritic outcrops, or other metal-rich surface or ground-water inflow (fig. 8). Sources of metal entering a stream commonly are identified by the incremental downstream changes in metal load determined by synoptic sampling and streambed quantification at multiple locations. Instantaneous loads used in synoptic-load studies are calculated as the product of the metal concentration and streamflow at the time of sampling. For typical loading studies, load profiles can illustrate how the magnitude of loads varies, and thereby, indicate the specific reach or inflow where metal loads substantially increase or decrease. However, for this study, metal-load profiles were not developed because the metal concentrations in Miller Creek generally were uniform and near the analytical minimum reporting level, and the chloride tracer used to determine streamflow did not come to full equilibrium in the lower reaches of the stream. Given the low concentrations throughout the study reach, loads entering Miller Creek can generally be assumed to be small.

Figure 8.  Possible sources of metals to Miller Creek, Montana:  A.  The Black Warrior Mine located in the upper study reach.  B.  A natural  ferricrete deposit adjacent to Miller Creek, near site 1,850.  C.  Mine waste at Miller Creek dump 1.  D.  Left-bank inflow (site 6,465), with aluminum staining near the middle of the study reach.  This inflow had the highest total-recoverable copper concentration of any site sampled in the Miller Creek watershed during this study.  Photographs by D.A. Nimick. (click here for jpg file)

The few sources contributing metal load to Miller Creek can be directly inferred from concentration profiles (fig. 6). The metal concentration data indicate that inflow from the Black Warrior Mine causes minor increases of lead and zinc concentrations for a limited distance along the mainstem. Several mid-basin inflows (sites 5,945, 6,465, and 6,750) having moderately elevated copper concentrations also cause minor increases of copper concentrations in the mainstem. Substantial concentration increases in Miller Creek were not observed in the upper reach where faults traverse the watershed, near ferricrete deposits, or near the Miller Creek dumps 1 and 2.

Although complete load profiles were not constructed, selected inflow loads were calculated to indicate the magnitude of load input relative to the mainstem load at the inflow site. Three left-bank surface inflows (sites 5,945, 6,465, and 6,750) upstream from site 7,120 (SW-2) were likely the only substantial sources of copper to Miller Creek (fig. 6). The combined total-recoverable copper load from the three inflows was about 123 µg/s. The total-recoverable copper load in Miller Creek at site 7,120 was about 128 µg/s. Thus, these three inflows collectively accounted for about 96 percent of the mainstem load at site 7,120. This instantaneous rate of transport is equivalent to about 0.02 pound of copper discharged per day by Miller Creek. This load is relatively minor with respect to the mainstem water volume, as can be seen by the minor effect on mainstem copper concentrations (fig. 6; increase in mainstem from 1 to 5 µg/L). The total-recoverable copper load in Miller Creek is negligible when compared with the maximum total-recoverable loads of other streams in the district. For example, copper loads determined during metal loading studies were about 70,000 µg/s in Fisher Creek (Kimball and others, 1999) and about 48,800 µg/s in Daisy Creek (Nimick and Cleasby, 2001).

The three surface inflows (sites 5,945, 6,465, and 6,750) where copper concentrations were moderately elevated had calcium concentrations ranging from 4.8 to 6.2 mg/L. These calcium concentrations were substantially lower than in the majority of the other samples collected in the Miller Creek watershed. Concentrations in other samples typically were higher than 30 mg/L (table 2). The lower calcium concentrations probably indicate that the three inflows flow through different geologic units or originate from a different source than the rest of the water draining to Miller Creek. These inflows may represent diffuse drainage from an area where anomalous copper concentrations were identified in soil samples. This area is in the vicinity of and to the northwest of adit M-25 (fig. 2). Adit M-25 on Henderson Mountain (fig. 2), developed in the Henderson Mountain rhyodacite porphyry upgradient from the inflows, is another possible source of the elevated copper concentrations detected in these inflows. One sample collected at site M-25 in August 1990 had a similar calcium concentration (4 mg/L) as the three inflows. Water from the adit at site M-25 also had an elevated copper concentration (560 µg/L; URS Operating Service Inc., 1998). The load from the adit at the time the sample was collected was about 210 µg/s.

Elevated concentrations of total-recoverable lead in the Black Warrior Mine inflow (site 25; 15 µg/L) and a right bank seep downstream from the Miller Creek dumps 1 and 2 (site 2,645; 6 µg/L) were the only notable sources of lead identified during this study. The combined loading from both sources was about 2 µg/s. Total-recoverable lead concentrations in Miller Creek were above the minimum reporting level at only three sites, with the highest mainstem concentration (5 µg/L) occurring at site 190 just downstream from the Black Warrior Mine inflow. The lead load at site 190 was only about 3.4 µg/s, or 0.0006 pound of lead per day.

The Black Warrior Mine inflow (site 25), a right-bank inflow (site 2,645), and the same three left-bank inflows that had elevated copper concentrations (sites 5,945, 6,465, and 6,750), contributed small loads of dissolved and total-recoverable zinc (combined load of about 40 µg/s) to Miller Creek (fig. 6). The small load from the Black Warrior Mine inflow slightly increased zinc concentrations in Miller Creek (from 8 to 26 µg/L) only because streamflow in Miller Creek downstream from the Black Warrior Mine inflow was not large enough to dilute zinc from this source. Total-recoverable zinc loads that entered Miller Creek downstream from this site were not large enough to substantially change zinc concentrations in Miller Creek. Downstream from East Fork Miller Creek (site 3,205), dissolved and total-recoverable zinc values in Miller Creek did not exceed 3 µg/L, indicating that no significant sources of zinc from this area were affecting concentrations in Miller Creek during this study. The three left-bank inflows (which contributed about 28 µg/s of zinc) had little effect on zinc concentrations in the creek because the streamflow in Miller Creek near these inflows was large enough to dilute the small zinc loads.

From the concentration profiles and selected inflow loads, it appears metal loading to Miller Creek during this low-flow study was sufficiently small to not cause any sharp increases in mainstem metal concentrations. In addition, substantial differences between mining-affected areas and areas influenced by the local geology could not be readily determined.

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