LIDAR-DERIVED KARST FEATURE INVENTORY OF HUNTINGTON COUNTY, PENNSYLVANIA: CHALLENGES TO REMOTE IDENTIFICATION OF CLOSED DEPRESSIONS IN THE VALLEY AND RIDGE PROVINCE

Karst feature density mapping is vital for geohazard assessment, risk mitigation, and groundwater protection. Closed depressions were identified in Huntingdon County, central Pennsylvania, using a modified semi-automated method described by Doctor and Wall (2018). The study area is in an unglaciated part of the Appalachian Mountain section of the Valley and Ridge Province. Twenty percent of the area is underlain by Cambrian, Ordovician, and Devonian carbonate rocks. Closed depressions covering areas greater than 9m² and with depths over 0.2m were identified using the most recent one meter digital elevation model and individually examined using slope shade, hillshade, and aerial imagery. The depressions were categorized as karst, anthropogenic, alluvial, or natural non-karst features. Karst features were subdivided into closed depressions, springs, or swallets.

Applying this workflow in Huntingdon County resulted in challenges that were not faced in the pilot project in neighboring Mifflin County. The greatest challenge was identifying enigmatic closed depressions in the residuum- and colluvium-mantled Cambrian and Ordovician carbonates. Many of these features may be actual karst depressions, or they may be groundwater sapping structures, periglacial patterned ground, or solifluction landforms. An additional challenge was historical human impact: the population density is nearly half of the pilot project area but there is greater evidence of historical human impact, including iron mines, exploration pits, ore wash pits, and charcoal hearths. A third challenge was removing false sinks caused by natural phenomena such as landslides, breaks in slope, and solifluction. The lower resolution of geologic mapping- only 25% of the area is mapped at 1:24k, as opposed to 75% in the pilot area- and increased structural complexity did not hamper analysis due to the 1000m buffer applied.

Even in light of the challenges, Huntingdon County provided a valuable case study for this workflow. Future development, including pipeline and infrastructure work, land stability evaluation, and groundwater protection efforts need to proceed with an awareness of the risks associated with karst. The karst feature density map created through this workflow will be useful in this effort.