Characterizing Epikarst Hydrogeology Using Integrated Hydrologic, Geochemical and Biological Approaches

Recent interdisciplinary research in a shallow valley-floor cave in southwestern Virginia shows that hydrologic, geochemical and biological data can be integrated to improve characterization of epikarst hydrologic properties. Three in-cave sites (two <20m and one <30m below the surface) are instrumented for high-frequency data collection (hydrologic, meteorological, and geochemical), as well as periodic sampling for cave drip biota, stable isotopes, nutrients, cations and anions. The surface overlying the sites is pastured grassland with thin clay-rich soils over Cambrian Elbrook limestone/dolostone bedrock with scattered outcrops.

Preliminary results indicate that recharge through the epikarst is highly dependent on sufficient precipitation and infiltration over the winter months (when ET is low), followed by continued precipitation in the spring. Drip rates over late summer and fall consistently decrease, indicating that little recharge is occurring. Drips show depth-dependent delays in the springtime responses to precipitation, as well as apparent temporal variation in the number of flow paths which are hydraulically conductive. Evidence of this is spatial and temporal variability in drip discharge rates after similar precipitation events during the spring. Our working hypothesis is that unsaturated flow paths become increasingly saturated during winter months until a hydraulic threshold is reached in early spring, hydraulic conductivity is achieved along a flow path, and recharge occurs. As more flow paths become hydraulically conductive they are able to transmit more water to the drip site.

Drip-discharge hydrograph analysis reveals that multiple flow pathways with different hydraulic properties exist in the epikarst at these sites. Hydrograph components likely represent flow through fractures and bedding-planes, and flow through the soil matrix. Drip-water fauna can also provide useful information regarding epikarst characteristics. Copepods, amphipods, and syncarid have been identified in drip samples and their presence indicates that portions of the epikarst are sufficiently saturated year-round to sustain these crustaceans.