39-9 Hydrologic Buffering in Mountain Headwaters: Geochemical Evidence from the Silvertip Mountain Karst, Northern Rockies, Montana

Session: Understanding Karst Hydrology and Karst Aquifers Using Innovative Tracers and Other Technologies

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Mountain headwaters are vital freshwater sources, particularly in snowmelt-dominated regions where sustained baseflow depends heavily on groundwater storage. In alpine environments, both unconsolidated sediments and karst aquifers store and transmit water, but their relative contributions to spring discharge—and how those contributions vary through time—remain poorly understood. These systems are dynamic, responding to seasonal melt, meteorological variability, and structural controls on flow. This study investigates the spatial and temporal variability of groundwater storage and quantifies the relative contributions of meteoric water, unconsolidated sediment storage, and karst conduit flow to spring discharges in the Silvertip System, located in the Bob Marshall Wilderness of Montana.

Fluorescent dye tracer tests conducted during the July–August field seasons of 2022 and 2023 delineated at least five groundwater basins within the Silvertip Mountain karst area (~7 km²). Geochemical and isotopic samples were collected across three field seasons (2021–2023). δ²H, δ¹⁸O, and d-excess values revealed interannual differences in recharge conditions. In 2021, intermediate values reflected later-season melt and progressive snowpack enrichment. In 2022, isotopically light values with high d-excess indicated cold-season melt with minimal evaporation. In 2023, samples plotted below the local meteoric water line, with low or negative d-excess, consistent with evaporative enrichment and non-equilibrium melt under warm, dry El Niño conditions.

End-member mixing analysis (EMMA) was used to quantify storage contributions to the main spring. Sulfate distinguishes meteoric recharge from evolved groundwater, while dissolved silica separates flow through unconsolidated sediments from deeper karst conduits. Relative contributions of each storage component reflect annual recharge patterns inferred from isotope values. EMMA results show that 2021 and 2023 shared a broadly similar storage pattern, though 2021 retained a higher meteoric signal under low-flow conditions. In 2022, persistent snowmelt largely overwhelmed subsurface storage signals. Spatially, karstic baseflow dominated in the north, whereas all southern springs but one reflected greater input from unconsolidated sediment storage. 2023 exhibited the most spatial heterogeneity.

These findings highlight the hydrologic complexity and buffering capacity of small alpine karst systems, which have been shown in similar settings to play an outsized role in sustaining regional baseflow under increasingly variable climatic conditions. This work helps quantify the temporal evolution of storage contributions and assess the long-term resilience of alpine karst aquifers in a changing hydrologic regime.

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

doi: 10.1130/abs/2025AM-10082

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