20 Hydrogeology and Marine/Coastal Geoscience

This session covers aspects related to SGDs & their associated aquifers, including but not limited to: water–rock interaction, coastal aquifer contamination, geochemical transformation, hydrological flows, saline intrusion, coastal estuaries, and environmental, economic, and social SGD-related issues.

Thursday, 23 April 2026
8:00 AM - 10:00 AM
LMH, Meeting Room

Fluorescent Dye Tracing to Delineate Karst Watersheds in the Prince of Wales Island Group, Tongass National Forest, Southeast Alaska

Paper number: 20-4
Program: Technical Sessions
Time: 9:00 AM – 9:15 AM

Authors

Abstract

Karst aquifers within the Tongass National Forest exert strong controls on groundwater flow, surface-water connectivity, and downstream aquatic ecosystems, yet remain poorly characterized in some recently identified carbonate terrains. Because karst systems rapidly transmit water, sediment, and contaminants, accurate delineation of subsurface drainage is essential for forest management decisions, particularly where timber harvest and road construction may impact water resources.

Over the past two field seasons, the Crawford Hydrology Laboratory (CHL), in collaboration with the Tongass National Forest (TNF), designed and implemented multi-fluorescent tracer tests to delineate karst watersheds and address data gaps relevant to forest management planning. These efforts were focused on newly identified carbonate bedrock areas on Kosciusko Island as well as a known cave system on Heceta Island. These tracer tests were made possible by TNF geologic mapping, field surveys, and inventories of previously undocumented caves, gravity-fed springs, and coastal submarine springs.

Qualitative point-to-point dye tracing methods were employed using activated charcoal receptors deployed in surface streams, springs and submarine ocean boils. Background monitoring was conducted for both rounds of tracing. Fluorescent dyes were injected into karst features with active flow ranging from shallow coastal cave systems to deeper inland pits fed by muskeg runoff. As the field season is short, weather is unpredictable, and receptors being a favorite wildlife snack, post-injection monitoring was limited to just a few weeks post injection.

All dyes were recovered at monitored springs, confirming subsurface karst flow paths. While additional tracing and geologic characterization are needed to fully delineate basin extents, these results provide the first direct evidence of groundwater flow directions in the newly identified karst area on Kosciusko Island and resulted in the re-discovery of a large submarine ocean boil which answered a more than decade long question about the Mint Lake Cave system on Heceta Island.  Future work will continue to implement dye tracing to support management strategies for groundwater quality, springs, and aquatic resources within the Tongass National Forest.

Geological Society of America Abstracts with Programs. Vol. 58, No. 3

doi: 10.1130/abs/2026CD-410828

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