ISOTOPIC CHARACTERIZATION OF NATURAL WATERS AT MT. RAINIER

The glacio-volcanic hydrologic system at Mt. Rainier, a stratovolcano in the Cascade Volcanic Arc, comprises a suite of geochemical interactions between rising volcanic gases, circulating groundwater, bedrock, and an overlying mantle of glacial ice. Both authors, first independently and now together, seek to expand upon earlier datasets to provide a blueprint for thermal and chemical monitoring of volcano activity. For example, PI Todd focuses on hydrothermal weathering on the flank of the volcanic edifice that mobilizes sulfate and chloride ions and how monitoring of those ions may lead to better prediction of locations and times of higher risk of glacial outburst floods. PI Florea, in contrast, focuses on the summit fumarole caves and how the water, fumarole gas, and ice chemistry in the crater are a product of water circulation in the upper edifice and crater.

In the August 2017 campaign, both authors collected samples from a suite of thermal springs, talus springs, end-glacial streams, fumaroles, drip waters, and glacial ice for geochemical and isotopic analysis. From all sample sites, the isotopic ratios of δ²H and δ¹⁸O were analyzed and compared to data from earlier studies. The δ²H and δ¹⁸O values of talus springs largely align with recharge originating from localized flow systems. Thermal springs and end-glacial streams are more depleted than their elevation would predict; a portion of their recharge originates from higher elevations. In the crater, fumarole moisture and drip waters in the glacial caves overlap and are narrower in range than glacial ice; in other words, from locally recycled moisture.

Three thermal springs (Paradise 1 & 2, Longmire) and one end-glacial stream (Winthrop) were additionally analyzed for dissolved ions, δ³⁴S, and ³⁶Cl/Cl and ¹²⁹I/I. The Paradise thermal springs (7-70 mg/L Cl; 18-200 mg/L SO₄) and the end-glacial stream (3 mg/L Cl; 13 mg/L SO₄) returned similar δ³⁴S values of +4‰, compared to the ~0.5‰ in summit fumaroles and the +15‰ from Longmire hot spring (200 mg/L Cl; 18 mg/L SO₄). Ratios of ¹²⁹I/I follow the same trend as δ³⁴S, suggesting older waters contribute more to the thermal springs at Paradise. The ³⁶Cl/Cl data are the inverse of ¹²⁹I/I. This may seem contradictory, but it is important to note that, unlike ¹²⁹I/I, modern ³⁶Cl/Cl is lower today than during peak nuclear testing.