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Karst springs of Cumberland County, Pennsylvania, are important water resources, but their sources and flow paths are unknown. We traced flow in a mantledkarst groundwater system in the Great Valley section of the Valley and Ridge Physiographic Province using fluorescent dyes, with focus on Big Spring Creek. Upper Big Spring Creek is assigned High Quality/Exceptional Value status by Pennsylvania based on its high water quality and value as a multi-use resource with exceptional recreational or ecological significance. Subsurface flow followed the geologic strike after Sulpho Rhodamine B (Acid Red 52) dye was released on exposed carbonates. Subsurface flow had a maximum effective linear velocity of 2.5 km d 21 , which is 8 times greater than sodium fluorescein (Acid Yellow 73) dye released separately into a losing stream over colluvium (0.3 km d 21 ). Sulpho Rhodamine B was detected strongly 8.9 km away at Big Spring Creek's largest source spring (,250 ppt water; 50 ppb eluate), but weakly in an east source (2.5 ppb eluate). Sodium fluorescein was detected after four weeks at 0.07 to 0.15 ppb in eluate at springs at Huntsdale Hatchery, 9.5 km from release atop the colluvial mantle. Slow flow derived from losing streams on the colluvial mantle likely maintains water quality of Big Spring Creek and similar systems. However, this recharge is distant, and the flow passes below karst recharge features in the valley center, creating many opportunities for contamination. Future studies of contaminant and sediment loadings to subterranean basins and of source-water protection strategies that recognize these patterns are necessary to protect these streams.

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