DYE TRACE RESULTS FROM TUXEKAN AND KOSCIUSKO ISLAND, SOUTHEAST ALASKA

Kosciusko and Tuxekan Island are outer islands located west of the Prince of Wales Island in southeast Alaska. These islands are part of the Alexander Terrane geologic province, accreted to the western North American craton during Jurassic to early Tertiary time (Gehrels and Berg, 1992; 1994). Silurian turbidites, carbonates, and conglomerate are the most widespread geologic units on these islands, the Heceta Limestone being the dominant bedrock type.

The potential for karst development within the Heceta Limestone is exceptional. Factors favoring this include high precipitation rates, calcium carbonate purity, strong structural fabric, and highly acidic surface waters from muskegs (Baichtal and Swanson, 1996). Repeated glaciation has significantly altered SE Alaska karst systems by eroding surface topography, covering large areas with compact till and adding to subsurface systems. Extensive past timber harvesting has also affected the karst hydrology.

Dye trace studies were conducted on the Heceta Limestone of Kosciusko and Tuxekan to assess karst hydrology. The work on Koscuisko was designed to assess hydrologic connectivity, storage capacity, flow rates, and karst system recharge from major island streams in specific areas. On Tuxekan, the objectives of this study were primarily to establish flow paths through the karst system.

Study results indicate both islands are underlain by a complex karst hydrologic system. Nearly every dye injection was recovered at one or more sampling point. Straight-line travel times indicate that movement of karst waters through the subsurface system is very rapid (up to 6,500 ft/day). Overlays of dye trace paths with mapped geology suggest that some dye paths can be explained by flow through large structural features and along geologic contacts. This was more evident on Tuxekan Island, where the majority of dye sampling points were at resurgences versus streams. Other dye paths follow more complicated paths that cannot be easily explained by dominant geologic features, at times crossing beneath topographic highs and/or dyes were detected in multiple, distal locations. Such passageways are likely to have developed through more diffuse fractures and weaknesses within the limestone. These studies demonstrate the importance of dye tracing demonstrate to understanding karst hydrology.