ADVANCES IN EPIKARST ZONE HYDROGEOLOGY IN THE PENNYROYAL PLATEAU OF SOUTH-CENTRAL KENTUCKY: IMPLICATIONS FOR CONTAMINANT TRANSPORT

The movement of autogenic recharge through the shallow epikarstic zone in soil-mantled karst aquifers is important in understanding recharge areas and rates, storage, and contaminant transport processes. To understand the storage and flow of autogenic recharge flowing to a single epikarst drain in Crump’s Cave on Kentucky’s Mississippian Plateau, we employed two methods. First, we performed base flow separation of eleven discrete storms, where stormflow was integrated to measure individual storm volumes, and a nominal recharge area parameter ζ was determined for each by setting the storm surface recharge and drainage volumes equal and then dividing the resulting recharge volume by the presumably uniform measured rainfall depth.

Values of ζ range from 843 m² to 11,200 m², with lower values likely resulting from water entering epikarst storage that does not reach the drain during that storm response. Values of ζ may thus provide way to quantify varying epikarst storage input. Data also suggest that the actual recharge area is on the order of 100x100 m. Using between-storm baseflow discharges and individual values of ζ , we calculated unit-baseflow (UBF--baseflow discharge per unit recharge area), and show that calculated values for the epikarst drain are 2-3 orders of magnitude higher than published UBF values for regional springs. This gives quantitative evidence that storage is more concentrated in the epikarst than the regional aquifer as a whole.

A second independent proxy of recharge and epikarst storage conditions from isotopic analysis of precipitation and epikarst water supports this finding. Rainfall amounts above the cave and discharge of the water flowing from the drain below were measured every ten minutes in 2010-11. Weekly precipitation and cave waterfall samples were collected for stable isotope (O and H) analysis, with higher resolution sampling of the waterfall during storm events. Precipitation isotope δ¹⁸O values ranged from -5.7 to -16‰, while the cave waterfall δ¹⁸O values averaged -6.3‰ (ranging between -5.2 to -7.3‰), indicating that despite intense storm events with highly variable δ¹⁸O values, a rapid homogenization of meteoric recharge water with epikarst storage water occurs in the system, further supporting that substantial storage occurs in the epikarst zone through diffuse flowpaths.