Metal Concentrations

The metals cadmium, copper, lead, and zinc have concentrations sufficiently elevated to be potentially lethal to aquatic life in Daisy Creek and to pose a toxicity risk in part of the Stillwater River (URS Operating Services, Inc., 1998). Copper is clearly the most important of these toxic metals, with maximum dissolved concentrations in Daisy Creek during this study of nearly 5,800 μg/L, compared to maximum concentrations of 848 μg/L for zinc and about 4 to 6 μg/L for cadmium and lead (table 6). These concentrations greatly exceed chronic aquatic-life standards and, except for lead, acute aquatic-life standards (U.S. Environmental Protection Agency, 1999). These metals, as well as aluminum, iron, and manganese, which potentially could control some geochemical reactions in the these streams, were selected for analysis in the synoptic water samples collected on August 26.

Downstream concentration profiles are similar for cadmium, copper, and zinc (fig. 9). Concentrations were near minimum reporting levels at site 0 and increased sharply, reaching the highest levels measured in Daisy Creek at site 611. Acidic, right-bank inflows in this reach had dissolved concentrations as high as 20.6 μg/L cadmium, 26,900 μg/L copper, and 3,000 μg/L zinc. These inflows resulted in dissolved concentrations in Daisy Creek at site 611 of 5.8 μg/L cadmium, 5,790 μg/L copper, and 848 μg/L zinc. Left-bank inflows in this upstream reach consistently had low concentrations (<1 μg/L cadmium, <21 μg/L copper, and <4 μg/L zinc), similar to the values at site 0. Downstream from site 611, concentrations of these metals decreased to the end of the study reach. The large inflows at sites 5,519, 5,671, 11,644, and 11,560 (the Stillwater River), in particular, caused large decreases in concentrations.

Figure 9. Concentrations of cadmium, copper, lead, and zinc in synoptic samples collected in the Daisy Creek and Stillwater River drainage, Montana, August 26, 1999.  Values less than the minimum reporting level (1 μg/L) are plotted as one-half the minimum reporting level. (click here for pdf file)

Dissolved and total-recoverable concentrations were nearly the same for cadmium, copper, and zinc in the upper half of the study reach between mainstem sites 425 and 5,475. These data indicate that these metals were entirely dissolved, which would be expected based on likely solubility controls for water having pH values less than about 4.5. Downstream from inflow sites 5,519 and 5,671, the higher pH in Daisy Creek resulted in changes in the partitioning of these metals between the dissolved and colloidal phases. The total-recoverable concentrations remained high (or unchanged), whereas the dissolved concentrations decreased (fig. 9), indicating an increase in colloidal concentrations. This pattern was most pronounced for copper and less so for cadmium and zinc, which is to be expected on the basis of the relative reactivity of these metals (Benjamin and Leckie, 1981). Amacher and others (1995) observed a similar pattern in metal partitioning in Daisy Creek and suggested that dissolved copper concentrations in Daisy Creek are controlled by dilution in reaches where pH is less than 4.5 and by sorption to iron oxyhydroxides where pH is higher than 4.5. Analysis of sequential extractions of sediment collected from Daisy Creek showed that copper concentrations in sediment were higher in those reaches of Daisy Creek where copper sorption was predicted (Amacher and others, 1995). A similar result was found in Fisher Creek and noted by Kimball, Nimick, and others (1999).

Lead concentrations were relatively low (<5.5 μg/L) throughout the study reach (fig. 9). Dissolved lead concentrations generally were between 2 and 4 μg/L in the reach between mainstem sites 611 and 5,661, where pH values were less than 5. Downstream from mainstem site 5,661, pH values were higher than 5, and the dissolved lead from upstream was converted to colloidal lead and transported downstream. Total-recoverable lead concentrations remained in the 1 to 5 μg/L range downstream to the Stillwater River. All left-bank surface inflows had lead concentrations less than the minimum reporting level (1 μg/L). Many of the right-bank inflows contributed lead to Daisy Creek, with dissolved concentrations generally between 1 and 12 μg/L. The dissolved lead concentration (76.4 μg/L) in surface-inflow site 1,700 was almost seven times higher than in any other inflow. The reason for this relatively high concentration is not known.

Some metals exhibit diel (24-hr) variation in dissolved concentration in streams affected by mine drain­age (Brick and Moore, 1996; T.E. Cleasby and D.A. Nimick, unpubl. data). These studies indicate that, during mid-summer in Montana, maximum concentrations of dissolved manganese and zinc occur in the morning and that minimum concentrations occur in late after­noon or evening. These diel variations in concentration can have the potential to affect the results of metal-loading studies. Therefore, some of the samples collected for chloride analysis at tracer-monitoring sites T-2, T-3, and T-4 were analyzed for selected dissolved metals to determine if diel concentration variations occurred in Daisy Creek and the Stillwater River. Data for sites T-3 and T-4 are shown in figure 10.  

Figure 10.  Diel variation in dissolved concentrations of copper, manganese, and zinc in Daisy Creek (site 9,925, top) and the Stillwater River (site 12,410, bottom) Montana, August 24-27, 1999. (click here for pdf file)

Data for site T-3 show that dissolved copper, manganese, and zinc concentrations exhibit a diel variation in concentration, with maximum values occurring between 0800 and 1000 hours and minimum values between 1900 and 2200 hours. The magnitude of variation was different for these metals, with copper and manganese varying proportionally much less than zinc. Dissolved copper concentrations varied between 34.3 and 55.4 μg/L, an increase of about 62 percent from the minimum value. Dissolved manganese concentrations varied between 477 and 609 μg/L, an increase of about 28 percent from the minimum value. Dissolved zinc concentrations ranged from 44.8 to 112 μg/L, an increase of about 150 percent from the minimum value. These variations do not correlate with streamflow and corresponding dilution effects interpreted from the chloride data (fig. 5). Rather, the variations are thought to be related to diel cycles in other stream-water characteristics, such as pH or temperature, that can affect the partitioning of metals between the dissolved and adsorbed phases. Upstream from mainstem site 5,475, similar diel variations in concentration are not expected to occur because the low pH (<4.14) does not favor sorption of metals to colloids and the streambed. This hypothesis was supported by five samples collected at site T-2 over a 36-hr period on August 25-26. Zinc concentrations in these samples ranged from 501 to 543 μg/L but displayed no diel pattern.

Data for site T-4 show diel variations similar to those at site T-3.  Concentrations generally were higher in the morning and lower in the afternoon.  The sampling interval (3-6 hr) was too long to precisely determine the timing of the minimum and maximum  concentrations.  The magnitude of variation for each metal at site T-4 was somewhat different from the vari­ations at site T-3.  Dissolved copper concentrations varied between 17 and 27 mg/L, an increase of about 59 percent from the minimum value.  Dissolved man­ganese concentrations varied between 60 and 93 mg/L, an increase of about 55 percent from the minimum value.  Dissolved zinc concentrations ranged from 2.2 to 9.7 mg/L, an increase of about 340 percent from the minimum value.

Because the synoptic samples were collected sequentially in the upstream direction from early morn­ing to mid-afternoon, concentrations of copper, manganese, and zinc would have decreased during the sampling period.  Concentrations in the samples col­lected near the downstream end of the study reach (at the beginning of the day) would have had higher concentrations relative to the samples taken upstream at a later time.  However, because dissolved  concentrations of these metals were higher upstream in the mainstem owing to metal-enriched inflows, the diel effect appears to be somewhat offset by the effect of inflows.

Concentrations of aluminum and iron (fig. 11) increased sharply between sites 0 and 611, where max­imum total-recoverable concentrations were 18,400 mg/L and 23,900 mg/L, respectively.  Total-recoverable concentrations decreased gradually throughout the rest of the study reach, whereas dissolved concentrations decreased sharply downstream from site 5,475.  Partitioning of aluminum and iron between dissolved and colloidal phases is controlled by pH.  Upstream of site 481, where pH in Daisy Creek varied from near neutral to 4.5 (fig. 7), concentrations of total-recoverable aluminum were higher than dissolved concentrations (fig. 11).  The dissolved aluminum contributed by acidic right-bank inflows was converted to colloidal aluminum, which coated the streambed (fig. 12). Downstream from site 481, the pH of Daisy Creek decreased to less than 4.5, and most aluminum was dissolved. The pH in Daisy Creek was low downstream to sites 5,519 and 5,671, the first large inflows downstream from the reach of Daisy Creek affected by drainage from the McLaren Mine area. Neutralization provided by these slightly basic inflows resulted in massive precipitation of aluminum, which thickly coated the streambed and caused a sharp decrease in dissolved aluminum concentrations.

Figure 11. Concentrations of aluminum, iron, and manganese in synoptic samples collected in the Daisy Creek and Stillwater River drainage, Montana, August 26, 1999. (click here for pdf file)

Figure 12. Aluminum and iron colloids in Daisy Creek.  View is upstream with surface-inflow site 481 on left side of stream.  Upstream of site 481, the pH of Daisy Creek is greater than about 4.5, and white aluminum colloids form.  Inflow (pH of 2.85) from site 481 decreases pH in Daisy Creek to less than 4.5, and only orange iron-oxyhydroxide colloids form. (Photograph by J.H. Lambing, U.S. Geological Survey). (click here for jpg file)

Iron partitioning followed the same pattern as aluminum (fig. 11). However, the pH value below which iron was entirely dissolved was 3.5. Water this acidic was found in the short reach of Daisy Creek that extends from mainstem site 819 upstream to at least mainstem site 611 and probably as far upstream as inflow site 481. Left-bank inflows downstream from site 819 caused a sufficient increase in pH in Daisy Creek to allow formation of iron-oxyhydroxide colloids. Field observations indicated a visible change in the water in Daisy Creek from clear at site 761 to cloudy at site 1,082 (table 4). Stream water remained cloudy with iron and aluminum colloids from this point downstream to the end of the study reach.

Concentrations of manganese (fig. 11) increased sharply between sites 0 and 611, similar to aluminum and iron. Downstream from site 611, concentrations decreased. However, unlike aluminum and iron, dissolved concentrations were the same as total-recoverable concentrations, indicating that manganese-oxyhydroxide colloids did not form.

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