Metals

Metal-load profiles display the net effect of metal inputs from surface and subsurface sources as well as load changes resulting from geochemical reactions in the mainstem. Load profiles for aluminum, cadmium, copper, and zinc exhibit many of the same patterns, while profiles for iron and lead each display different patterns. Aluminum and iron loads are discussed first because of the importance of these metals, as well as pH, in controlling partitioning of the other metals.

Load profiles for aluminum and iron are shown in figure 14. For both metals, loading to Daisy Creek was negligible between sites 0 and 270. The instream loads increased substantially between sites 270 and 611. At site 611, surface inflows accounted for 62 to 77 percent of the dissolved and total-recoverable instream loads, whereas 23 to 38 percent came from subsurface inflow. Right-bank inflow sites 401, 411, and 481 contributed 59 to 73 percent of the instream load. Left-bank inflows contributed little aluminum or iron.

Figure 14. Downstream profiles of aluminum (top) and iron (bottom) loads in Daisy Creek and the Stillwater River, Montana, August 26, 1999. (click here for pdf file)

Downstream from site 611, the instream load profiles for aluminum and iron differ, particularly for the dissolved loads. These differences reflect the important role pH plays in partitioning these metals between dissolved and colloidal phases in streams affected by acid rock drainage. Between sites 611 and 4,283, instream dissolved-aluminum loads continued to increase downstream, essentially in step with total-recoverable loads. This increase was derived primarily from subsurface inflow, as indicated by a flat profile for cumulative surface-inflow load. Downstream from mainstem site 5,475, the instream dissolved-aluminum load decreased by almost two orders of magnitude, from 161 to 2.52 mg/s in less than 400 ft (between mainstem sites 5,475 and 5,839). In this short reach, the pH in Daisy Creek increased from 4.14 to 6.21 (table 6) owing to tributary inflow, resulting in rapid conversion of dissolved aluminum to colloidal aluminum. The total-recoverable aluminum load decreased through much of the rest of the study reach as this colloidal material settled to the streambed in the lower-gradient reaches of Daisy Creek, especially downstream from site 7,324 and in the Stillwater River.

Instream dissolved-iron loads decreased sharply downstream from site 611 (fig. 14), and most of the instream iron load was colloidal, as indicated by the much larger total-recoverable instream load. The decrease in dissolved load was most rapid between sites 611 and 1,082, where pH in Daisy Creek increased from 3.37 to 3.81 (table 6). The dissolved load decreased at a more gradual rate to virtually nothing at mainstem site 7,829. Unlike aluminum, the instream dissolved-iron load decreased gradually between sites 1,082 and 7,829, perhaps because photoreduction of iron oxyhydroxides maintained the dissolved-ferrous iron concentration in the stream (McKnight and others, 1988). Total-recoverable instream loads for iron decreased steadily downstream from site 7,324 through the rest of the study reach, similar to aluminum, suggesting that colloids settled on the streambed.

Load profiles for copper (fig. 15) follow a downstream pattern similar to that for aluminum. The instream load started near zero at site 0 and increased substantially between sites 270 and 611 in response to surface and subsurface inflows. Dissolved and total-recoverable copper loads continued to increase at a more gradual rate from site 611 downstream to site 4,283. The gradual increase in instream load was derived from subsurface inflow because surface-inflow loads were negligible in this reach, as indicated by the flat cumulative surface-inflow load profile. From site 0 downstream to site 5,475, almost all of the instream copper load was dissolved as opposed to colloidal. However, this pattern changed abruptly downstream from site 5,661 as dissolved copper was removed in response to increased pH, which caused dissolved copper to sorb to aluminum and iron colloids. The decrease in instream dissolved-copper load was more gradual than for aluminum (fig. 14), but almost all dissolved copper was converted to the colloidal phase upstream from the mouth of Daisy Creek.

Figure 15. Downstream profiles of cadmium (top) and copper (bottom) loads in Daisy Creek and the Stillwater River, Montana, August 26, 1999. (click here for pdf file)

The load profiles for cadmium (fig. 15) and zinc (fig. 16) follow a pattern generally similar to that for copper (fig. 15). Instream loads increased dramatically between sites 270 and 611 and more gradually between sites 611 and 4,283. The loads were almost entirely dissolved in these two reaches. Downstream from mainstem site 5,475, the decreases in dissolved cadmium and zinc instream loads were not as sharp as for dissolved aluminum or copper. Rather, the decreases were more gradual, and a substantial dissolved load persisted to the mouth of Daisy Creek and into the Still­water River. Similar to copper and aluminum, total-recoverable instream loads for cadmium and zinc decreased in the downstream half of Daisy Creek, as the colloidal load settled on the streambed.

Figure 16. Downstream profiles of lead (top) and zinc (bottom) loads in Daisy Creek and the Stillwater River, Montana, August 26, 1999. (click here for pdf file)

The load profiles for lead (fig. 16) are similar to those for aluminum (fig. 14) except for two unusual features, both of which are not well understood. First, lead is the only metal that exhibited any significant increase in cumulative surface-inflow load downstream from mainstem site 611 [at inflow site 1,700 (table 7)] . The dissolved lead concentration (76.4 μg/L) is anomalously high, and the source of the lead is unknown. Historical water-quality data for samples collected during 1994-98 at sites DCT-10 and DCT-14, which are in the same drainage and upstream from site 1,700, had dissolved lead concentrations near or less than the minimum reporting level (2-3 μg/L) (Michael Cormier, Maxim Technologies, Inc., written commun., 1999). The second unusual feature of the lead profiles is the large increase in total-recoverable instream load between mainstem sites 5,475 and 7,324. Increases in total-recoverable loads for iron and aluminum were also noted in this reach, but were not proportionally as large as that for lead. The cause of the sharp increase and the subsequent large decrease in total-recoverable load between sites 7,324 and 7,829 is not apparent. The increase in instream total-recoverable lead load downstream from mainstem site 5,475 corresponds to the concurrent sharp decrease in instream dissolved-lead load. However, these shifts may be relatively unimportant because the total-recoverable lead concentrations through this entire reach are relatively low (2.3-5.4 μg/L, table 6) in comparison to copper and zinc concentrations.

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