Summary

Miller Creek is a tributary of Soda Butte Creek east of Cooke City in south-central Montana near the northeast corner of Yellowstone National Park. Mining for gold, copper, and silver began in the late 1800s and continued through the early 1900s, resulting in prospects and mining wastes that are sporadically distributed throughout the watershed. Metal-loading studies have been a useful tool in characterizing water quality in historical mining areas. Thus, a study was conducted during August 28-31, 2000 to characterize metal concentrations and identify possible sources contributing metal to Miller Creek during low flow. Water-quality samples were collected at 55 sites throughout the watershed and streambed-sediment samples were collected at 16 sites. The study reach was about 15,000 ft in length, which comprised almost the entire length of Miller Creek.

Water-quality data collected during this study indicate that the water in Miller Creek is not substantially affected by mining disturbances or the weathering of naturally occurring mineralized rocks. Values for pH were near neutral to slightly basic in all samples and metal concentrations were mostly low, with many concentrations below the analytical minimum reporting level. In Miller Creek, total-recoverable concentrations of copper, lead, and zinc were less than the State of Montana chronic aquatic-life criteria in all samples, with the exception of one lead value that exceeded this criteria. This site, just downstream from the Black Warrior Mine inflow, also had the highest zinc concentration measured in Miller Creek. These slightly elevated concentrations in the mainstem originated from the Black Warrior Mine inflow, which had the highest total-recoverable lead (15 µg/L) and zinc (140 µg/L) values measured during this study.

In the upper part of Miller Creek, total-recoverable copper concentrations ranged from <1 to 3 µg/L. Three left-bank inflows had elevated total-recoverable 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, which were less than the chronic aquatic-life criterion of 9.3 µg/L.

Similarly, metal concentrations in East Fork Miller Creek were typically less than the analytical minimum reporting level. At the seven sites sampled in the East Fork drainage, 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 and one total-recoverable zinc value of 2 µg/L. Both of these values were from surface inflows to East Fork.

Leachable lead and zinc concentrations (4,500 and 4,200 ppm, respectively) in the streambed-sediment sample collected from the Black Warrior Mine inflow were the highest measured during this study and were about 20 times greater for lead and 11 times greater for zinc than the streambed-sediment samples collected in Miller Creek immediately upstream from this inflow. Although leachable metal concentrations were elevated in the streambed sediment at the Black Warrior Mine inflow, lead and zinc concentrations at the mainstem site just downstream from this inflow were only slightly elevated compared to the concentrations in the mainstem upstream from this inflow. The streambed-sediment sample collected at the mouth of East Fork Miller Creek had about the same leachable lead concentration and slightly lower zinc concentration compared to adjacent mainstem sites.

Leachable-copper concentrations in streambed-sediment samples from the mainstem peaked near mid-basin at site 7,120 and remained elevated at similar concentrations throughout the rest of the study reach. The sample collected at the mouth of the Black Warrior Mine inflow had moderately elevated leachable copper concentration (330 ppm), but no apparent effect on copper concentration in the mainstem immediately below. East Fork Miller Creek had leachable-copper concentrations in the streambed sediment (320 ppm) similar to the Black Warrior Mine inflow, but this value was only slightly higher than the concentrations at adjacent mainstem sites.

Compared to guidelines established by the Canadian Council of Ministers of the Environment, risks to aquatic biota associated with the leachable-metal concentrations found in the streambed sediments appear to be basin-wide rather than related to a particular source. Exceedances of the lower interim sediment quality guidelines for copper and zinc were observed in every sample collected during this study, with several values above the upper guideline, the probable effect level. Leachable-lead concentrations in all streambed-sediments samples exceeded the upper probable effect level. According to the guidelines, values exceeding the upper guidelines are expected to adversely affect the aquatic biota and values between the lower and upper guidelines are associated with occasional adverse effects. The guidelines typically are used as references for concentrations of elements in bulk sediment, whereas the results from this study are associated with the sieved fraction finer than 2mm. Higher metal concentrations typically are associated with the smaller size fraction and comparison to bulk sediment guidelines is intended only to provide a general reference to the exposure risk from the streambed sediment in Miller Creek.

Substantial load increases in Miller Creek were not observed near faults that traverse the watershed, near ferricrete deposits, or near the Miller Creek dumps 1 and 2. The small loads entering Miller Creek during this study were delivered primarily from the Black Warrior Mine inflow, a right-bank inflow just downstream from Miller Creek dumps 1 and 2, and three left-bank inflows near mid-basin. The three left-bank surface inflows near mid-basin were the only substantial sources of copper loading to Miller Creek. The combined total-recoverable copper load from the three inflows was about 123 µg/s, which accounted for about 96 percent of the total-recoverable copper load (128 µg/s) in Miller Creek at site 7,120. The Black Warrior Mine inflow, a right-bank inflow, and the same three left-bank inflows that had elevated copper concentrations, contributed small loads of dissolved and total-recoverable zinc to Miller Creek. The Black Warrior Mine inflow and a right-bank seep downstream from Miller Creek dumps 1 and 2 were the only sources of lead identified during this study. The combined loading from both sources was small, being just over 2 µg/s. Total-recoverable lead concentrations in Miller Creek were above the minimum detection level at only three mainstem sites, with the highest mainstem concentration (5 µg/L) occurring at site 190 just downstream from the Black Warrior Mine inflow.

Metal loading to Miller Creek during this low-flow study was relatively small and had minor effect on metal concentrations in the mainstem. Substantial differences between mining-related areas and areas influenced by local geology could not be readily determined.

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