TEMPORAL VARIATIONS IN DISCHARGE AND CHEMISTRY AT CAVE DRIPS IN GRAND CAVERNS, VIRGINIA

Stalagmites provide important proxies for reconstructing past climate by recording changes in isotopic and elemental values along their growth axes. In order to interpret such records, we must understand the dynamics of the drip waters from which these geochemical signals originate. Two ceiling drips within Grand Caverns, Virginia have been monitored simultaneously for changes in drip rate, electrical conductivity, and geochemistry from February, 2014 to the present. Water samples are collected on a monthly basis, and the air temperature, pressure, relative humidity and CO₂ concentration within the chamber are recorded. Both drips feed actively growing stalagmites that are located within 5 meters of each other in the same chamber. Drip rates are monitored using a tipping bucket rain gauge stationed directly below the drips, and conductivity and water temperature are continuously measured in a reservoir above the rain gauge.

Preliminary results show marked heterogeneity in both water chemistry and drip rate between the two sites. One site exhibits two different flow regimes, with a higher flow (0.29 L/h) during spring and early summer and a lower flow (0.019 L/h) during summer, winter, and fall. The other drip shows nearly constant flow of 0.018 L/h, and conductivity between 420 to 490 uS/cm. The dynamic drip shows a seasonal shift in conductivity, averaging 300 uS/cm during the winter and 414 uS/cm throughout the remainder of the year; also, an inverse relationship between cave air pressure and drip rate is observed at this site. The higher flow regime of the dynamic site may be a response to recharge during the spring, which provides enough water to the soil zone and epikarst for drainage into the cave to occur. This regime continues until the increase in evapotranspiration on the surface during the growing season creates a net water deficit above the cave. This moisture deficit is not fully recharged again until winter and early spring. Despite the observed flow variability between the two sites, the oxygen isotope compositions of both drips are nearly equal and constant at -8.1 ± 0.1 permil. This value supports the hypothesis of a cool-season bias in recharge to the drips, as it equals the amount-weighted average oxygen isotopic composition of cool-season precipitation collected in 2007 nearby at the same elevation as the cave.