SEASONAL INFLUENCE OF PRECIPITATION, SOIL MOISTURE, AND CAVE AIR CO2 ON THE STABLE OXYGEN AND CARBON ISOTOPIC COMPOSITION OF DRIP WATER IN GRAND CAVERNS, VIRGINIA

In Grand Caverns, Virginia, two perennially dripping stalactite sites provided an opportunity to examine the modern transfer of vadose zone recharge into the cave environment. Precipitation samples and soil moisture measurements were collected above the cave for over two years (2016 – 2018), along with measurements of the drip rate, major ion chemistry, the stable isotopic compositions of drip water (δ¹⁸O, δ²H) and δ¹³C of dissolved inorganic carbon (δ¹³C_DIC). A volumetric soil moisture threshold of ~30% was observed to permit recharge pulses into the cave that occurred during winter or spring (Feb – May), with soil moisture recharge occurring only in the cool-season months (Nov-Mar). Cave recharge events were identified as abrupt increases in drip rate corresponding to shifts from a lower flow regime (4 mL/h – 8 mL/h) unresponsive to precipitation to a flashier, higher flow regime (40 – 330 mL/h) that appears to be hydraulically connected to the surface. The mean annual amount-weighted oxygen isotopic composition of precipitation (-5.9‰) was more positive than the near-constant oxygen isotope composition of the drip waters (-8.0‰) and the drip water instead closely matched the cool-season amount-weighted average precipitation value (-8.2‰), suggesting a bias toward cool-season recharge. The δ¹³C_DIC composition of drip waters that had equilibrated with the cave atmosphere shifted seasonally by more than 3‰ (-15.3 ‰ to -11.8 ‰) and appeared to be controlled by the CO₂ concentrations in the cave atmosphere (960 ppm to 11600 ppm). More positive δ¹³C -DIC values were observed in winter than in summer, related to the observed lower CO₂ concentrations in the cave atmosphere during the winter months. Results from this study provide direct insights into vadose zone mechanisms governing groundwater recharge events in Appalachian karst terrains, and cave atmospheric processes that influence inorganic carbon dynamics within drip waters.