INFLUENCE OF KARST HYDROLOGY ON GEOCHEMISTRY OF MUSKEG DRAINAGE AND SPRING RESURGENCES, TONGASS NATIONAL FOREST, ALASKA

Southeast Alaska’s Tongass National Forest covers 69,000 km² of remote, highly complex accretionary terrane that has developed into a complex and heterogeneous geologic landscape which includes numerous blocks of carbonate rock. In the early 1990’s it was recognized that spectacular karst was developed within those blocks, impacting geology, geomorphology, ecology, and archeology of those areas and beyond. During the Wisconsinan glaciation, there were areas of compacted glacial sediments and silts deposited over the bedrock. Muskeg peat lands developed over these poorly drained areas. The dominant plants of the muskeg ecosystem are Sphagnum mosses, whose decomposition leads to highly acidic waters with pH as low as 2.4. In the proximity of the carbonate blocks, these waters drain off the muskegs into cave systems, eventually running to the ocean. The setting provides a unique environment for monitoring the impact of the karst systems on water quality and biological productivity. In accordance with the Tongass Land Management Plan, one of the research priorities of the National Forest is to determine the contributions of karst groundwater systems to productivity of aquatic communities. On Northern Prince of Wales Island, the Conk Canyon Cave insurgence and the Mop Spring resurgence were continuously monitored from June to August 2006 to understand the buffering capacity of the cave system. Over the length of the system, the pH increases from an average 3.89 to 7.22. The insurgence water temperature, during the summer months, ranged from between 10^oC to 17^oC. After residence in the cave system, the resurgence water had been lowered to 6^oC to 9^oC. Over the continuum from insurgence to resurgence, the specific conductance had increased by an order of magnitude with the resurgence waters having a higher ionic strength. The cave environment acts as a buffer on the incoming acidic muskeg water to yield resurgence water chemistry of a buffered karst system. These buffered waters contribute to the productivity in aquatic environments downstream. The waters from this system drain into Whale Pass, an important location for the salmon industry. The cool, stable temperatures, as well as buffered flow rates and pH delivered by the karst systems are associated with higher productivity of juvenile coho salmon.