Water

Water-quality data collected during this study indicate that metal concentrations (except for iron) were mostly low, with many values below the analytical minimum reporting levels (table 2). Values for pH were all nearly neutral to slightly basic (range from 7.1 to 8.5). These results indicate the water in the Miller Creek watershed is mostly unaffected by mining disturbances or natural weathering of the mineralized rocks common elsewhere in the mining district.

Profiles of copper, lead, and zinc concentrations in the mainstem sites are shown on figure 6. To evaluate the potential biological exposure risks from these metals, the State of Montana aquatic-life criteria for chronic and acute toxicity (Montana Department of Environmental Quality, 1999) also are shown on this figure. Based on historical samples collected at SW-2 and SW-5 (URS Operating Services, Inc., 1998), a hardness of 100 mg/L CaCO3 was used to calculate these hardness-dependent criteria. Except for the total-recoverable lead concentration in Miller Creek (site 190) just downstream from the Black Warrior Mine inflow, all mainstem concentrations of copper, lead, and zinc in Miller Creek were less than the chronic aquatic-life criteria. Water at site 190 had the highest mainstem concentration of total-recoverable lead (5 µg/L) and zinc (26 µg/L). These slightly elevated lead and zinc values, relative to site 0, presumably result from the Black Warrior Mine inflow (site 25), which had the highest total-recoverable lead (15 µg/L) and zinc (140 µg/L) values measured during this study.

Figure 6.  Concentration profiles of dissolved and total-recoverable copper, total-recoverable lead, and dissolved and total-recoverable zinc in water samples collected in the Miller Creek watershed, Montana, August 30, 2000.  Aquatic-life standards from Montana Department of Environmental Quality (1999) are applicable to total-recoverable concentrations.  Standards have been adjusted for a hardness of 100 mg/L CaCO3. (click here for pdf file)

In the upper part of Miller Creek (upstream from site 6,450), total-recoverable copper concentrations ranged from 1 to 3 µg/L. Three left-bank inflows (sites 5,945, 6,465, and 6,750) near site 6,450 had elevated copper concentrations (29 to 63 µg/L) that slightly increased concentrations in Miller Creek. From site 6,450 to the downstream end of the study reach, total-recoverable copper concentrations in Miller Creek ranged from 3 to 6 µg/L. These slightly elevated copper concentrations in the mainstem were less than the chronic aquatic-life criterion of 9.3 µg/L.

Similarly, metal concentrations in East Fork Miller Creek were mostly less than the minimum reporting level (table 2). At the six sites sampled upstream from the mouth of the East Fork (site 3,205), all copper, lead, and zinc concentrations were equal to or less than 1 µg/L, except for one dissolved copper value of 5 µg/L at site E700 and one total-recoverable zinc value of 2 µg/L at site E280. Both of these values were from surface inflows to East Fork.

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