pH and Major-Ion Concentrations

Values of pH in Daisy Creek and the Stillwater River changed markedly through the study reach (fig. 7, table 6). At the upstream end of Daisy Creek (site 0), the pH (7.28) was slightly basic. Immediately downstream, the pH in Daisy Creek decreased as inflows on the right bank (mined side) contributed acidic water. Two right-bank inflows (sites 114 and 161) near the injection site had pH values of 4.57 and 4.17, respectively, and caused the pH in Daisy Creek to decrease to 7.03 at site 270. Four right-bank inflows between mainstem sites 270 and 611 had pH values ranging from 2.70 to 2.96, resulting in substantial decreases of pH values in Daisy Creek to a minimum value of 3.37 at mainstem site 611. These inflows drain the manganese bog (inflow sites 292, 348, and 401) and the southern part of the McLaren Mine area (inflow site 481). Inflows on the left bank through this reach were slightly basic. Between sites 611 and 2,334, the pH in Daisy Creek increased slightly, from 3.37 to 3.87. The pH values for most right-bank inflows in this reach were low (3.28 to 4.23), but not as acidic as in the right-bank inflows in the reach immediately upstream. These right-bank inflows drain the northern part of the McLaren Mine (inflow site 691) or intercept acidic shallow ground water flowing from the mine area (inflow sites 761, 804, 1,189, and 1,700). Inflow site 1,700 was the most downstream inflow with acidic water (pH of 3.98); right-bank inflows at sites 1,865 and 1,915 had near neutral pH values (6.87 and 6.94). Between mainstem sites 2,334 and 5,475, pH in Daisy Creek increased only slightly to 4.14 because little inflow entered this reach. However, relatively large inflows at sites 5,519 and 5,671 provided enough alkaline water to increase the pH in Daisy Creek markedly from 4.14 to 6.21 between mainstem sites 5,475 and 5,839. Farther downstream, pH values changed little until the small inflow at site 7,529 caused another increase in pH to 7.72 at mainstem site 7,829. Downstream from this point, pH remained slightly basic to the mouth of Daisy Creek and in the Stillwater River.

Figure 7. Values of pH in synoptic samples collected in the Daisy Creek and Stillwater River drainage, Montana, August 26, 1999. (click here for pdf file)

Table 6. Water-quality data for synoptic samples collected in the Daisy Creek and Stillwater River drainage, Montana, August 26, 1999 (click here for pdf file) (click here for xls file)

Downstream profiles of selected major-ion concentrations are shown on figure 8. Calcium, magnesium, and sulfate concentrations in Daisy Creek increased downstream from site 0 to site 611 in response to the high concentrations of these constituents in the acidic right-bank inflows. Sulfate concentrations were particularly affected, increasing from 46.3 to 405 mg/L between site 0 and site 611. Sulfate is liberated by oxidation of sulfide minerals in the altered and mineralized bedrock in the watershed. This geochemical reaction is accelerated after the sulfide-rich rocks are disturbed and exposed to the atmosphere by mining. Just downstream from mainstem site 611, concentrations of calcium, magnesium, and sulfate decreased in response to dilution from left-bank inflows. Farther downstream, concentrations of these major ions remained stable except where the large inflows at sites 5,519, 5,671 and 11,560 (the Stillwater River) provided dilution.

Figure 8.  Concentrations of calcium, magnesium, and sulfate in synoptic samples collected in the Daisy Creek and Stillwater River drainage, Montana, August 26, 1999. (click here for pdf file)

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