BASELINE WATER-QUALITY CHARACTERIZATION OF AN ALPINE WATERSHED, SOUTH-CENTRAL MONTANA

Emigrant Creek is an alpine watershed located approximately 20 miles north of Yellowstone National Park, along the western edge of the Absaroka-Beartooth Wilderness, within the Emigrant—Mill Creek mining district, and adjacent to a ‘hot springs’ resort. In June of 2015, an exploratory drilling plan was submitted to assess the presence of base and precious metals in the watershed on US Forest Service lands and on private patented claims. Prior to the proposed drilling, the Montana Bureau of Mines and Geology characterized baseline water-quality in the Emigrant Creek and East Fork of Emigrant Creek drainages, and assessed the potential connection to the geothermal system feeding the hot springs. Water samples were collected from 17 groundwater and surface-water sites, and analyzed for major ions; trace metals; stable-water isotopes of oxygen and hydrogen; and tritium.

Although all the sampled water had <300 mg/L total dissolved solids (TDS), dissolved constituents concentrations and the pH varied. The TDS for the groundwater and surface water samples ranged from 59 to 271 mg/L. Acidic (field pH < 3.0), metal-rich, SO4-type water was observed in the sample from an abandoned mine-adit discharge and in samples from two nearby springs. Moderately acidic (pH 5.8 – 6.1) Ca-Mg-SO4-HCO3-type water was observed in two samples from the East Fork of Emigrant Creek, and in samples from two springs in the East Fork drainage near the patented mining claims. Surface-water samples from Emigrant Creek and groundwater samples from the Emigrant Creek drainage, had neutral pH’s (7.2 – 7.8) and were of a Ca-Mg-HCO3-SO4-type.

The proposed exploratory drilling is about 6 miles east of the hot springs resort and about 2,500 feet higher in elevation. High elevation catchments can be important sources of groundwater recharge to adjacent lowland areas. However, there is no clear hydrogeologic or geochemical evidence to suggest that water from the Emigrant Creek watershed is connected to the geothermal system that feeds the hot springs.