Rock-Water Interactions in Tumbling Rock Cave, Alabama, USA: Geochemical Analysis of an Underground Stream System

Sustainable water management in karst landscapes remains a global challenge due to extreme local and regional heterogeneities (e.g. complex conduit and fracture networks, mixing of various water sources, and interactions between groundwater and matrix material). Our understanding of these factors, however, is key to natural resource usage and conservation. In partnership with the Southeastern Cave Conservancy, our reconnaissance study focused on the hydrogeologic characterization of Tumbling Rock Cave in northeastern Alabama, USA. The rock-water interactions component aimed to link our knowledge of local stratigraphy with modern groundwater chemistry to understand the impact of geology on the cave's stream system, since matrix mineralogy is the major control on groundwater composition. With ten kilometers of surveyed passageways spanning the Monteagle Limestone and Hartselle Sandstone formations, Tumbling Rock is a well-explored but under-studied cave system. One major stream flows through the cave; collects water from drips, conduits, and waterfalls; and exits as a spring, but its inflows and composition were previously unknown. The stream, inflow locations, and other pertinent features were mapped with a traditional survey assisted by terrestrial LiDAR and preexisting maps, and a conceptual model of the stream system was developed. Water samples were collected at times of low and moderate discharge from multiple reaches of the stream and various inflows. Suites of chemical and physical measurements were recorded, including pH, electrical conductivity, temperature, and concentrations of dissolved oxygen, nitrates, and phosphates. Twelve samples were analyzed for their elemental composition, specifically common ions in natural waters: Na+, K+, Ca2+, Mg2+, Cl-, HCO3-, CO32- and SO42-. These were compared by stratigraphic location, proximity and type of nearest inflow, and depth in the cave; plotted as Piper diagrams; and modeled with Visual MINTEQ. Our analysis provided information about the state of rock-water interactions and potential for dissolution and precipitation throughout the cave. This research improved our understanding of Tumbling Rock Cave as a case study in karst hydrogeology and informed the natural resource management practices of the Southeastern Cave Conservancy.