Locating Previously Unmapped Water-Filled Cave Passages Feeding the Echo River Accessory Spring Using Electrical Resistivity Tomography at Mammoth Cave National Park, KY

Mammoth Cave is the longest cave system in the world and records the eastern United States' drainage history over the past 10 million years. Much of the water flowing through the Mammoth Cave system emerges from springs along the Green River—two notable examples being the Echo River Spring and Echo River Accessory Spring. While the source of water to the Echo River Spring is well understood through dye studies, scuba exploration, and remotely operated vehicles, the same methods have been unsuccessful in identifying the independent water source supplying the Echo River Accessory Spring. We use 2.5D electrical resistivity tomography (ERT) to locate water-filled cave passages feeding the Echo River Accessory Spring. The ERT lines are roughly 400 m in length and have a maximum depth penetration of approximately 100 m. We image a low resistivity layer near the surface ranging from 5-15 m thick of approximately 30-100 Ohm*m that represents river alluvium. Below this low resistivity layer, we image a high resistivity layer of 10,000 Ohm*m or greater that represents the limestone bedrock. Within this layer, we image low resistivity anomalies of approximately 300 Ohm*m with diameters of 20 m at a depth of 20-30 m surrounded by the highly resistive limestone bedrock. We interpret the limited low resistivity anomalies as previously unmapped water-filled cave passages that supply water to the Echo River Accessory Spring. These results are crucial in advancing our current understanding of water flow in the Mammoth Cave system.