Major Ions

Load profiles for calcium and sulfate are shown in figure 13. The shapes of the load profiles are differ­ent because the sources of these elements are different, with calcium derived by weathering of the rocks prevalent in much of the watershed while sulfate is produced by weathering of the altered and mineralized rocks rich in sulfide minerals. Calcium concentrations varied relatively little (24.5-60.6 mg/L, table 6) in the mainstem, especially compared to the range in concentrations of metals, and therefore, instream calcium loads reflect, to a large extent, the streamflow profile (fig. 6). Large increases in calcium load occurred at the large tributaries (inflow sites 5,519, 5,671, 11,560, and 11,644). The difference between the instream load profile and the cumulative surface-inflow load profile indicates the magnitude of calcium loading from subsurface inflow (fig. 13). At the end of the study reach (mainstem site 12,410), surface inflows contributed 74 percent of the calcium load while subsurface inflow contributed 26 percent of the load.

Figure 13. Downstream profiles of calcium and sulfate loads in Daisy Creek and the Stillwater River, Montana, August 26, 1999. (click here for pdf file)

In contrast to calcium, the instream load profile for sulfate (fig. 13) was dominated by load increases upstream of mainstem site 5,475, where surface and subsurface drainage from the McLaren Mine area enters Daisy Creek. Upstream of mainstem site 611, about 65 percent of the sulfate load came from surface inflows and about 35 percent came from subsurface inflows. Because almost all of the sulfate load in surface inflows along Daisy Creek came from right-bank inflows, the subsurface inflow also is assumed to have come mostly from the right bank. Between mainstem sites 611 and 5,475, the increase in instream sulfate load was derived almost entirely from subsurface inflow, as indicated by a nearly flat profile for cumulative surface-inflow load. Downstream from mainstem site 5,839, the instream sulfate load increased at about the same rate as calcium, primarily from inflow of ground water from unaltered and unmineralized areas. At the end of the study reach, surface inflows contributed only 51 percent of the sulfate load.

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