T129 Flow in Fractured and Karstic Aquifers: Models and Methods

Monday, 1 November 2010
1:30 PM - 5:30 PM
Mile High Ballroom 1AB

HYDROLOGIC RESPONSES IN EPIKARST: A COMPARATIVE STUDY BETWEEN VIRGINIA AND TEXAS

Paper number: T129-6
Program: Technical Sessions
Time: 2:45 PM – 3:00 PM

Authors

Abstract

The epikarst regulates both the quantity and quality of recharge to karst aquifers and, as a result, is a “critical zone” in karst systems. Over the past three years, we have collected continuous hydrologic data from two caves in different hydrogeologic regimes to examine relationships between precipitation, drip response, climate, vegetation, and epikarst properties. At James Cave (VA), bedrock has low matrix storage and epikarst is covered by 1-2m of residuum. Permeability and storage are primarily in solutionally enlarged fractures and bedding planes, with soils and residuum contributing to baseflow. At Cave Without A Name (CWAN; TX), bedrock is covered by thin or no soil, and permeability and storage are primarily found in matrix porosity and enlarged fractures and bedding planes.

Discharge data from drip sites in each cave reveal differences and similarities in the hydrologic functioning of epikarst. In James Cave, drips at three sites respond to precipitation events only after sufficient precipitation during late winter and early spring, with subsequent rapid responses in quickflow (peaking at 5 to 30 L/hr), and corresponding increases in baseflow (~0 to 1 L/hr). Baseflow recession analyses indicate that epikarst contributions support baseflow in the nearby cave stream during the summer months. Baseflow levels depend on receiving sufficient precipitation during the previous fall and winter. At CWAN, however, drip responses at five sites are primarily related to periods with above average precipitation, with less connection to season and more dependence on El Nino and La Nina climate patterns. Three drip sites responded quickly to repeated rain events after a 2-year drought when most drip sites were essentially dry. After a large rain event and rapid drip initiation, two of these (>10 L/hr) cease to drip after 2-4 weeks, while the third site increased from ~80 ml/hr to ~1500 ml/hr and now maintains this drip rate. The other two sites showed no response to precipitation and continue to drip at essentially the same rate (~5-10 ml/hr). Results thus far support a complex conceptual model of flow and storage in the epikarst where physical and hydrologic properties, in combination with climatic conditions, determine how epikarst regulates recharge to the underlying aquifer.