Metal Sources

Copper is the metal of most concern in Daisy Creek because it occurs at higher concentrations than other metals and can be toxic to aquatic life. Therefore, sources of copper loading are discussed in this section. The sources of the other metals in Daisy Creek, with the occasional exception of lead, are thought to be the same as copper. The magnitude and source of copper loads contributed to subreaches of Daisy Creek from surface and subsurface inflows are presented in table 2.

Click here for Table 2. Sources of dissolved copper to subreaches of Daisy Creek, Montana, August 26, 1999

Copper loading to Daisy Creek was substantial in the reach upstream of mainstem site 5,475 (fig. 15). This reach can be divided into five subreaches on the basis of the different source areas that contribute copper to Daisy Creek (table 2). Dissolved copper loads are discussed here because, in this reach, almost all of the copper load is dissolved. Sources of copper included right-bank surface inflows and subsurface inflow. Left-bank inflows contributed less than 0.02 percent of the entire copper load in this reach.

The upstream subreach (between sites 0 and 270) flows past a small, right-bank hill composed of land­slide or glacial-moraine deposits. This subreach had minor copper loading (461 μg/s) from an inflow on the south side of the hill (inflow site 74) and from stream­side seeps at its base (inflow sites 114 and 161). Sub­surface inflow also contributed a small copper load (151 μg/s) to Daisy Creek (table 2).

The second subreach (between sites 270 and 460) received a substantial copper load (10,100 μg/s). Most of this load came from the four right-bank surface inflows (sites 292, 348, 401, and 411) that originate in the manganese bog adjacent to Daisy Creek. Copper loading from subsurface inflow was small (251 μg/s).

In the third subreach (between sites 460 and 611), one right-bank inflow (site 481), enters Daisy Creek and contributed more copper (16,400 μg/s) to Daisy Creek than all other surface inflows combined (table 7). This inflow drains the southern part of the McLaren Mine (fig. 2), where much of the mine wastes are stockpiled and where substantial unmined mineralized rock remains. Unlike the two upstream sub­reaches, copper loading from subsurface inflow (8,900 μg/s) in this part of Daisy Creek was substantial.

In the fourth subreach (between mainstem site 611 and inflow site 1,700), subsurface inflow contributed almost all the copper loading (7,040 μg/s). The most prominent right-bank inflows (sites 691 and 1,700, which drain the northern part of the McLaren Mine) contributed only 143 μg/s (table 7). The copper loading from these two sites and the other four right-bank inflows was only 245 μg/s. Although inflow site 1,700 was not a significant source of copper to Daisy Creek, the inflow did contribute a relatively large load of dissolved lead (6.11 μg/s), almost as high as the load contributed by inflow site 481 (7.45 μg/s, table 7).

The fifth, and most downstream, subreach (between sites 1,700 and 5,475) is the longest of the five subreaches. Surface inflows to this subreach do not drain the McLaren Mine. Copper loading from surface inflows was negligible (2 μg/s) while subsurface loading (6,210 μg/s) was relatively large, although smaller than in the two previous upstream subreaches.

In summary, the most substantial copper loading to Daisy Creek (71 percent of the total copper load contributed to the entire study reach) occurred between sites 270 and 611, where right-bank inflows originate in the manganese bog and the southern part of the McLaren Mine. About 53 percent of the total load in the study reach was contributed by the five right-bank inflows in this 341-ft reach, with inflow site 481 (33 percent of the total) being the most important. Copper loading to Daisy Creek from all surface inflows downstream from mainstem site 611, including the surface inflows that drain the northern part of the McLaren Mine, was not significant, at least during the low-flow conditions that existed during this study. While surface inflows contributed the most copper to Daisy Creek upstream of site 611, subsurface loading was the only important source of copper for the longer downstream reach between sites 611 and 5,475. Subsurface loading in this reach contributed over half of the total subsurface loading to Daisy Creek and 27 percent of the total load to Daisy Creek.

Although the subsurface inflow to Daisy Creek was not sampled, copper concentrations in the subsurface inflow (table 3) can be calculated from the subsurface inflow rates and copper loads contributed by subsurface inflow. These calculations assumed that one-half of the subsurface inflow came from the right bank and was metal rich; the subsurface inflow from the left bank was assumed to contribute no copper load to Daisy Creek. For the reach between sites 460 and 611, the calculated dissolved copper concentration (26,200 μg/L) in subsurface inflow from the right bank is similar to the concentration in some of the nearby right-bank inflows (for example, sites 292, 348, and 401), indicating that the subsurface pathway feeds both the right-bank surface and subsurface inflows. Calculated copper concentrations for subsurface inflow between sites 611 and 5,475 were lower, but substantially higher than concentrations in surface inflows in that reach. The pH and copper concentrations in the subsurface flow vary spatially, most likely in response to the varying amounts of alteration and buffering capacity in the rocks along different subsurface flow paths, as well as to dilution provided by any deeper ground water flowing from areas of unaltered bedrock.

Click here for Table 3. Calculated concentrations of dissolved copper in subsurface inflow to subreaches of Daisy Creek, Montana, August 26, 1999

Much of the metal-rich subsurface inflow to Daisy Creek probably was acidic. Although pH was not measured in subsurface flow, this hypothesis is supported by the presence of dissolved aluminum in the subsurface inflow. Aluminum is only likely to be dissolved where the pH is less than about 4.5 (Stumm and Morgan, 1996, p. 273).

The copper load entering Daisy Creek between sites 270 and 1,700 as surface and subsurface inflow is derived from the McLaren Mine area. Although this copper load has been divided between surface and sub­surface inflows (table 2), in reality virtually none of this load comes directly from the McLaren Mine to Daisy Creek as surface flow. Almost all of the right-bank channels (except site 1,700) were dry from the McLaren Mine area to within a short distance of Daisy Creek. Flow in these short reaches near Daisy Creek was maintained by subsurface inflow. Determining the source of the subsurface inflow to Daisy Creek and inflow channels is difficult because several possible sources exist within the McLaren Mine area. These 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 that is pyritic. More detailed hydrogeologic information would be needed to determine the importance of each of these sources.

The occurrence of metal-rich subsurface inflow to Daisy Creek upstream and downstream from the tributaries that drain the McLaren Mine area indicates that bedrock to the south and north of the McLaren Mine area apparently is a source of acid rock drainage not related to mining. The small subsurface metal load that discharges to Daisy Creek upstream of site 104 may be derived from the Chimney Rock area (fig. 1). The larger subsurface metal load that discharges to Daisy Creek downstream from site 1,700 may be derived from Fisher Mountain. The ferricrete deposits mapped by Furniss and others (1999) near site 2,334 and dated as 6,490 radiocarbon years before present support the hypothesis that unmined bedrock is one of the current sources of metals to Daisy Creek. However, this subsurface flow has not been directly measured. Monitoring well MW-3 (fig. 2) presumably should intercept this subsurface flow, but water-quality data for samples collected from the well do not support this hypothesis. This shallow well is completed in unconsolidated surficial material and the Wolsey Shale with a screen that extends from 16 to 46 ft below ground surface (Hydrometrics, Inc., 1990). Five samples collected during 1989-90 had pH values greater than 7 and low dissolved-metal concentrations. Cadmium, copper, and lead concentrations were at or less than minimum reporting levels (Michael Cormier, Maxim Technologies, Inc., written commun., 1999). Manganese (230-380 μg/L) and zinc (10-100 μ g/L) concentrations were higher than minimum reporting levels but much lower than would be expected if ground water at this site were affected by acid rock drainage.

Cleanup activities that reduce metal and acid loading from the McLaren Mine area will result in improvements in water quality in Daisy Creek and the Stillwater River. Metal concentrations likely would decrease, and pH values in reaches that are currently acidic likely would increase. However, potential reductions in metal and acid loading and changes in pH and metal concentrations are difficult to predict because the ultimate source of the metals and acid are not well defined. In addition, decreasing copper con­centrations during baseflow conditions to values less than the aquatic-life standards may be impossible because of natural sources of copper in unmined mineralized and altered bedrock. If the assumptions are made that all copper loading upstream of site 1,700 comes from sources at the McLaren Mine and that these sources can be removed or isolated, the copper load (6,210 μg/s, table 2) contributed by subsurface inflow derived from bedrock away from the mine between sites 1,700 and 5,475 would result in a copper concentration in Daisy Creek of about 450 μg/L at site 5,475 compared to the 3,570 μg/L measured in this study (table 6). Because the assumed cleanup activities likely would substantially reduce acid loading, thereby resulting in higher pH values, iron and aluminum colloids would be present and some copper likely would be adsorbed to this material. Therefore, this calculated copper concentration represents a total-recoverable concentration; the dissolved copper concentration would be lower. Downstream, at the mouth of Daisy Creek (site 11,549), the estimated total-recoverable copper concentration would be about 190 μg/L under base-flow conditions compared to the 1,200 μg/L mea­sured during this study. Both calculated copper con­centrations are higher than the acute aquatic-life standard of 13 μg/L (assuming a hardness of 100 mg/L, U.S. Environmental Protection Agency, 1999). Farther downstream, the maximum total-recoverable copper concentration in the Stillwater River at the end of the study reach (site 12,410) would be about 35 μ g/L compared to the 176 μg/L measured in this study. These calculated copper concentrations are based on the copper loading and streamflow conditions that existed during the short period during which this study was conducted. Concentrations would vary to an unknown degree as hydrologic conditions in the drainage basin changed.

The toxicity of copper (and other metals) is dependent on the hardness of the water. If the metals load in Daisy Creek were reduced, hardness values also would be lower because the right-bank and subsurface inflows contributing metals also contribute calcium and magnesium. Therefore, in considering post-cleanup hardness values to use to compute aquatic-life standards, the values in left-bank inflows (generally less than 100 mg/L) or the Stillwater River (58 mg/L) may represent potential post-cleanup values.

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