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Science for Watershed Decisions on Abandoned Mine Lands: Review of Preliminary Results, Denver, Colorado, February 4-5, 1998

What Streams are Affected by Abandoned Mines?—Characterization of Water Quality in the Streams of the Boulder River Watershed, Montana

By David A. Nimick¹ and Tom E. Cleasby²

Water is the link connecting toxic metals derived from abandoned mine lands (AML) to aquatic biota and, therefore, is an integral aspect of the assessment of environmental effects associated with AML. Water quality was characterized by sampling streams throughout the Boulder River watershed, comparing trace-element concentrations to aquatic-life standards and to concentrations in reference streams, and estimating annual loading of metals from the main streams in the watershed.

Characterization of watershed water quality was based on samples collected during 1996-97 for this study and during 1989- 96 by previous investigators from 113 sites during low-flow conditions and from 56 sites during high-flow conditions. pH values were near-neutral to alkaline everywhere except in isolated circumstances where acid discharge from AML affects small streams. In comparison to five reference sites, trace-element concentrations in many stream reaches were elevated, particularly in water from Jack Creek, Uncle Sam Gulch, lower Cataract Creek, High Ore Creek, and the Boulder River downstream from Cataract Creek. Longitudinal concentration profiles indicate that the primary sources of metals are three AML complexes located in the headwaters of Jack Creek, Uncle Sam Gulch, and High Ore Creek.

Cadmium, copper, lead, and zinc were the trace elements that most commonly occurred at concentrations exceeding aquatic-life standards. Montana chronic aquatic-life standards for one or more of these metals were exceeded at all sampling sites between the three main AML complexes and the confluence of the Boulder and Jefferson Rivers. Concentrations of chromium, mercury, nickel, and silver typically were less than minimum reporting levels. Concentrations of arsenic, which is pervasive in both mined and unmined areas, typically were much lower than the chronic aquatic-life standard (190 micrograms per liter). During spring runoff, cadmium and zinc concentrations decreased, whereas lead concentrations increased. Cadmium and zinc are predominantly dissolved (using 0.45-micrometer filtration) and presumably are diluted by high flow. Lead is primarily in the particulate phase, and its concentration increases as higher flows carry more sediment.

Annual loads of metals from Basin, Cataract, and High Ore Creeks and at two sites on the Boulder River upstream and downstream of these tributaries were estimated using water-quality data for 12 sample sets and flow estimates derived from the continuous streamflow record for a nearby gage on the Boulder River. Although the three tributaries combined contributed only 33 percent of the annual streamflow at the downstream Boulder River site, they contributed 41-89 percent of the cadmium, copper, lead, and zinc loads; Cataract Creek contributed the largest loads of these metals. About one quarter of the total-recoverable arsenic, copper, lead, and zinc at the downstream site came from the channel or unsampled sources. These increased loads may reflect the erosion and transport of contaminated fluvial deposits along the Boulder River.

¹U.S. Geological Survey, Drawer 10076, Federal Building, Helena, MT 59626 (dnimick@usgs.gov)

²U.S. Geological Survey, Drawer 10076, Federal Building, Helena, MT 59626 (tcleasby@usgs.gov)

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