Publication Date
4-1-2018
Abstract
Caves provide useful access points for sampling and monitoring of vadose groundwater to better understand the hydrologic conditions controlling the timing and magnitude of critical recharge events. In this study, two cave ceiling drips within a single room at Grand Caverns, Virginia have been monitored simultaneously for changes in drip rate, electrical conductivity, and geochemistry from February, 2014 to July, 2017. The results show distinctly different behavior in the discharge between the two drip sites. One site exhibits seasonal changes in flow rate while the other site has remained nearly constant at 0.018 L/h. The dynamic site has a lower flow regime during the summer, fall, and winter (0.004 L/h - 0.008 L/h) until precipitation or snow melt events in the early spring triggered a sudden increase in discharge up to 0.328 L/h. There is an apparent volumetric soil moisture threshold of approximately 30% that must be exceeded before there is a response in the drip rate within the cave. Both the peak discharge rate and duration of the annual recharge period depends upon what time of year it occurs, with larger recharge events beginning in February and March and smaller recharge events occurring as late as April and May. This higher flow regime continues until the summer when an increase in evapotranspiration on the surface creates a soil moisture deficit that corresponds to a drop in discharge within the cave. This moisture deficit is usually not fully recharged again until winter and early spring when surface precipitation continues and evapotranspiration is low. Stable oxygen and hydrogen isotopes of the drip water and of precipitation above the cave were measured as potential tracers for determining the seasonality of recharge to the cave drips. Despite there being large seasonal shifts in the isotopic composition of rain waters collected above the cave, the oxygen isotope composition for both drip sites have remained nearly equal and constant at -8.0 ± 0.1 permil. This value is similar to the cool-season amount-weighted average of precipitation collected at the surface, and thus supports the hypothesis of a cool-season recharge bias to the drips.
Rights Information
DOI
https://doi.org/10.5038/9780991000982.1030
Investigating Vadose Zone Hydrology in a Karst Terrain Through Hydrograph and Chemical Time-Series of Cave Drips at Grand Caverns, Virginia
Caves provide useful access points for sampling and monitoring of vadose groundwater to better understand the hydrologic conditions controlling the timing and magnitude of critical recharge events. In this study, two cave ceiling drips within a single room at Grand Caverns, Virginia have been monitored simultaneously for changes in drip rate, electrical conductivity, and geochemistry from February, 2014 to July, 2017. The results show distinctly different behavior in the discharge between the two drip sites. One site exhibits seasonal changes in flow rate while the other site has remained nearly constant at 0.018 L/h. The dynamic site has a lower flow regime during the summer, fall, and winter (0.004 L/h - 0.008 L/h) until precipitation or snow melt events in the early spring triggered a sudden increase in discharge up to 0.328 L/h. There is an apparent volumetric soil moisture threshold of approximately 30% that must be exceeded before there is a response in the drip rate within the cave. Both the peak discharge rate and duration of the annual recharge period depends upon what time of year it occurs, with larger recharge events beginning in February and March and smaller recharge events occurring as late as April and May. This higher flow regime continues until the summer when an increase in evapotranspiration on the surface creates a soil moisture deficit that corresponds to a drop in discharge within the cave. This moisture deficit is usually not fully recharged again until winter and early spring when surface precipitation continues and evapotranspiration is low. Stable oxygen and hydrogen isotopes of the drip water and of precipitation above the cave were measured as potential tracers for determining the seasonality of recharge to the cave drips. Despite there being large seasonal shifts in the isotopic composition of rain waters collected above the cave, the oxygen isotope composition for both drip sites have remained nearly equal and constant at -8.0 ± 0.1 permil. This value is similar to the cool-season amount-weighted average of precipitation collected at the surface, and thus supports the hypothesis of a cool-season recharge bias to the drips.
