5-9 Englacial Mixing and the Preservation of the Isotopic Fingerprint of Glacial Meltwater in Mountain Groundwater Systems

Session: Advances in Mountain Hydrology: Connecting Cryosphere, Surface, and Subsurface Processes

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Authors:

Abstract:

Recent research shows that glacial meltwater is an important source of recharge to mountain groundwater systems including bedrock aquifers. It’s highly likely that glacial meltwater has been an important source of recharge for as long as alpine glaciers have been present in a watershed. However, we’re only beginning to appreciate the role of glacial meltwater in hydrogeological, hydrogeochemical, and ecological processes in the mountain block. This emerging field of study faces many challenges including a lack of data on groundwater processes in the high mountains, limited data on alpine glaciers (volume of ice, areal coverage, melt rates), and limited isotopic data from ice cores. Here, we address the following question, is glacial meltwater important to mountain groundwater across spatial scales? We collected samples of water from springs, seeps, glacial ice, glacial meltwater, seasonal snow, and rain in Glacier National Park (GNP), Montana and Mount Hood (MH), Oregon over four years. Samples of water were analyzed for stable isotopes of water (¹⁸O and ²H), tritium (³H), chlorine-36 ratios (³⁶Cl/Cl), and standard suite of cations and anions. The isotopic data shows more complexity than can be explained by simple ice-cube models. Results from a Bayesian stable isotope mixing model indicate that nearly all of the springs in GNP and many of the springs in MH are supported by recharge from glacial meltwater. However, ³H data are too ambiguous to address the overarching question. In fact, ³H does not seem to be a robust indicator of glacial meltwater in either study site. For example, glacial meltwater sampled at the terminus of Eliott Glacier in MH was ³H-dead, as expected, one year but had elevated ³H activities in subsequent years. Chlorine-36 ratios show more utility in capturing the isotopic fingerprint of glacial meltwater, preserving relationships with elevation, and identifying components of bomb-pulse fallout preserved in the ice. However, inconsistencies are also present in the ³⁶Cl data. To explain these discrepancies, we developed an englacial mixing model which suggests that the stage of glacier melt combined with englacial mixing processes can substantially alter the isotopic composition of subglacial flow. In other words, subglacial flow is not a “clean” isotopic signal of melting glacial ice. Englacial mixing must be considered when evaluating isotopic data of groundwater in deglaciating watersheds.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10574

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