Stable Isotopes as a Tool to Characterize Carbon Cycling and Develop Hydrologic Budgets in Mantled Karst Settings

Isotopes of water (δ2H/δ18O), carbon dioxide (δ13C-CO2), and dissolved inorganic carbon (δ13C-DIC) were used to explore water quality, trace carbon cycling, and quantify recharge sources through mantled karst and into Blowing Spring Cave (BSC). Of the possible sources of contamination in the BSC recharge area, septic-tank effluent was hypothesized to degrade water quality at the spring outlet of BSC because of the dominance of septic tanks for waste treatment, unsuitable topography and soil for septic-tank absorption fields, increased nitrate and chloride concentrations concomitant with increased urbanization, and increased Escherichia coli with discharge. Carbon cycling between the soil and BSC was constrained by (1) mixing of gaseous soil CO2 and surface-atmosphere CO2 to produce cave-air CO2 concentrations and isotopic compositions, (2) kinetic degassing of cave drip-water, causing greater δ13C-DIC values than expected during equilibrium carbon isotopic fractionation between CO2 and DIC, and (3) exchange of soil-gas CO2 with groundwater prior to entering the cave, providing evidence that cave-stream water was characterized by open-system conditions with regards to soil CO2. This conceptual model of carbon cycling in BSC--where cave-air CO2 is partially sourced from soil CO2 and aqueous and gaseous carbon reservoirs within the cave are relatively decoupled--provides evidence that the majority of soil CO2 enters the cave as a gas. A three-component hydrograph separation was completed to quantify precipitation (QR), soil water (QS), and bedrock-matrix water (QB) contributions to BSC during storm events. Antecedent moisture conditions changed throughout the sampling period because as rainfall and base-flow discharge increased, δ18O values of cave water increased, chloride concentration in soil and cave water decreased, and DIC in cave water decreased. Combined QS and QB accounted for 36 to 119% of total discharge during storm events, depending on time after the onset of precipitation and antecedent moisture conditions. QR was greatest during a wet-season storm and rapid dilution of major cations and anions occurred with increased discharge. In contrast, a dry-season storm had the lowest QR and anion concentration peaked with maximum QS. Even during extremely dry periods, pre-event water stored in the unsaturated zone can contribute to groundwater flow during storms