CHARACTERIZING CARBON TRANSPORT AND DISSOLUTION PROCESSES IN A COMPLEX KARST GROUNDWATER REVERSAL SYSTEM AT MAMMOTH CAVE, KENTUCKY, USA

In karst landscapes, the source, transport, and fate of carbon is of interest for determining carbon storage and release rates, geochemical evolution of karst aquifers, global carbon budgeting, and conduit formation and evolution. Karst systems with springs within close spatial proximity to surface rivers may evolve to be complex reversal systems when flooding occurs and the river backfloods into the cave. This can introduce additional sources of CO₂ into the system with varying residence times, thereby influencing the rate of dissolution and conduit formation within the cave system at a point where water is typically discharging saturated with respect to calcite. Two major spring sites, River Styx Spring and Echo River Spring, at Mammoth Cave, flow into the Green River, which serves as baselevel for the system; under elevated river conditions, the springs reverse and backflood into the cave. During this process, enhanced dissolution may occur within the system and the water may undergo rapid geochemical evolution toward undersaturation and penetrate several kilometers into the conduits, with residence times of hours to days. Over a five-month period, four sites were sampled, including the two major springs (River Styx Spring and Echo River Spring), downstream of where the springs exit in the Green River, and River Styx in cave. Weekly samples are being collected and analyzed for, pH, SpC, temp, and DO, along with carbon isotopes, alkalinity, anions, and cations. Calculated dissolved inorganic carbon (DIC), pCO₂ concentrations, and d¹³C_DIC are being analyzed during baseflow and during spring reversal events to better understand dissolution dynamics and carbon sourcing, interaction, and fate. Sondes are being be installed at River Styx spring and in the cave to collect 10-minute resolution data at River Styx, which responds rapidly to reversals. Preliminary data indicate large shifts in the groundwater geochemistry during spring reversals, which happen under varying precipitation and antecedent moisture conditions, often rapidly, with variability in temperature, SpC, and pH easily discernable. Continued sampling and higher-resolution data will allow for improved understanding of the timing and contribution from various sources of carbon dioxide within the system and its influence during reversals.