Summary and Conclusions

A metal-loading study was conducted during August 24-27, 1999, to quantify and identify the principal sources of metal loads to Daisy Creek and to examine the downstream transport of these metals into the Stillwater River. Water-quality and aquatic conditions in Daisy Creek have been affected by acid rock drainage derived from waste rock and adit discharge at the McLaren Mine as well as from natural weathering of pyrite-rich altered and mineralized rock that makes up and surrounds the ore zones in the New World Mining District. Knowledge of the main sources and transport pathways of metals and acid can aid resource managers in planning and conducting effective and cost-efficient remediation activities.

The study reach included virtually all of Daisy Creek and a 850-ft reach of the Stillwater River downstream from the confluence of Daisy Creek. Metal loads in the mainstem were quantified from streamflow data determined by tracer injection and water-quality data determined from synoptic samples. Loads contributed by surface inflows were determined from these data as well as supplemental streamflow measurements made using conventional methods. Downstream changes in metal loads in the stream then were attributed to sources along the stream as well as to instream geochemical reactions. These sources included visible surface inflows and diffuse subsurface inflows.

The metals cadmium, copper, lead, and zinc have concentrations sufficiently elevated to be of concern for aquatic life in Daisy Creek and the Stillwater River. Copper is the most important of these toxic metals, with a maximum dissolved concentration of nearly 5,800 mg/L measured in Daisy Creek during this study. Metal concentrations increased sharply in the short reach between the tracer-injection site and a site 611 ft downstream, where the highest concentrations measured in Daisy Creek occurred. Acidic, right-bank (mined side) inflows in this reach had dissolved concentrations as high as 20.6 μg/L cadmium, 26,900 μg/L copper, 76.4 μg/L lead, and 3,000 μg/L zinc. These inflows resulted in maximum dissolved concentrations in Daisy Creek of 5.8 μg/L cadmium, 5,790 μg/L copper, 3.8 μg/L lead, and 848 μg/L zinc. Left-bank inflows in this upstream reach consistently had low concentrations (<1 μg/L cadmium, <21 μg/L copper, <1 μg/L lead, and <4 μg/L zinc), similar to the values in Daisy Creek at the tracer-injection site. Downstream from mainstem site 611, concentrations of metals decreased to the end of the study reach.

Significant copper loading to Daisy Creek only occurred in the reach upstream of mainstem site 5,475. Sources included right-bank surface inflows and sub­surface inflow; copper loads in left-bank surface inflows were virtually nonexistent. The most significant metal loading (71 percent of the total copper loading in the study reach) occurred between mainstem sites 270 and 611. About 53 percent of the total load was contributed by the five right-bank inflows in this reach. Four of these inflows drain the manganese bog that is on the right bank between mainstem sites 270 and 460. Just downstream, inflow site 481, which heads in the southern part of the McLaren Mine, contributed the single largest amount of copper, or about 33 percent of the total copper load in the study reach.

Copper loading from subsurface inflow is substantial, contributing 46 percent of the total dissolved copper load to Daisy Creek. Most of this subsurface copper loading occurred in the reach of Daisy Creek downstream from the reach that received surface loading.

Flow through the shallow subsurface is an important copper-transport pathway from the McLaren Mine and surrounding altered and mineralized bedrock to Daisy Creek during base-flow conditions. The pH and metal concentrations in the subsurface flow probably varied in response to the varying amounts of alteration and buffering capacity in the rocks along different subsurface flow paths. Unfortunately, little is known about the source of acid and copper in this subsurface flow. Possible sources include the mineralized rocks of Fisher Mountain upgradient of the McLaren Mine area, the surficial waste rock at the mine, and the underlying bedrock, which hosts both the McLaren ore deposit and the surrounding altered rock.

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