The Effects of Evapotranspiration Upon Recharge Using the Stable Isotopes of Oxygen and Hydrogen in the Carbonate Aquifers of the Cumberland Plateau in Southeast Kentucky

In this brief manuscript, we consider the variation of the stable isotopes of oxygen, δ18O, and hydrogen, δ2H, in samples collected during 2010 and 2011 from precipitation and shallow groundwater along the Cumberland Plateau of southeast Kentucky. These data from the 1,900-ha Redmond Creek karst aquifer lend insight into the source and timing of recharge to shallow groundwater in the epigenic karst of the U.S midcontinent. Specifically, we find that only 43% of precipitation remains as potential recharge after accounting for evapotranspiration, and 85% of this potential recharge occurs during only four two-week sampling periods. The isotopic composition of precipitation samples, which fluctuate seasonally according to temperature, are more variable than the composition of shallow groundwater, which remains nearly constant. The deuterium excess of these precipitation samples reflects moisture sources during the winter months more influenced by contributions from re-evaporated, continental moisture. When weighted for potential recharge, our stable isotope data in precipitation indicate a similarity with the mean values of shallow groundwater, suggesting a recharge threshold linked to evapotranspiration. Introduction and Study Area Not all precipitation recharges an aquifer. Of particular relevance to this manuscript is the concept of a ‘threshold’ for recharge to a karst aquifer, called ‘selective recharge’ by Kendall and Doctor (2011). In particular, we note that recharge in karst may involve both a discrete, ‘quick flow’ component requiring enough rainfall to generate overland flow to a karst feature (Jones et al., 2000), and diffuse recharge through the epikarst and rock matrix. Florea and Vacher (2007) noted both discrete and diffuse components during a study in Briar Cave Florida as a regional rise in the water table of the Floridan aquifer immediately following major hurricanes in 2004 followed by a delayed increase in the drip rate from the rock matrix. The component of diffuse recharge to a karst aquifer is potentially very low. For example, Jones and Banner (2003), in reference to a study in Barbados, make the following statement about tropical karst aquifers: “without such [discrete] transport, it is likely that the rainwater will be taken up by evapotranspiration” (Jones and Banner, 2003, p. 131). Using stable isotopes of oxygen and hydrogen, they conclude that, for Barbados, monthly rainfall must exceed a threshold of 19.5 cm. Florea and Vacher (2007) further generalize for places such as Florida and Barbados by stating: [I]t cannot be assumed that a rainy season. equates to a recharge season. If seasonal variation of temperature and thus potential evapotranspiration are also significant, the rainy season will coincide with the high potential evaporation season. Both derive from the high-sun season (Florea and Vacher, 2007, p. 445). This short paper explores some of these same concepts in the karst of southeast Kentucky and, like Jones and Banner (2003), uses the stable isotopes of oxygen and hydrogen to elucidate information about the sources of moisture and the nature and timing of aquifer recharge. The Cumberland Plateau The data for this study come from the Cumberland Escarpment in southeast Kentucky. The topography of the study area is split equally between uplands of the Cumberland Plateau and deeply incised valleys that drain those uplands. The terrain of the escarpment is rugged, with upper slopes lined with vertical cliffs of Pennsylvanian sandstones and conglomerates of the Lee Formation, a ‘bench’ of gentle slopes marking the exposure of transitional-marine calcareous shales that comprise the upper-Mississippian Paragon Formation, and lower slopes and sinkhole floors underlain by the relatively pure carbonates of the middle-Mississippian Slade Formation (Ettensohn et al., 1984). Relief in the study area exceeds 300 m with ridge tops above 530 m and valley floors below 300 m. Karst Geology and Hydrology The Slade Formation, regionally divided into the St. Louis, the Ste. Genevieve, and Kidder Limestone members, is significantly modified by solution activity and karst landscapes dominate the area of carbonate exposure (Simpson and Florea, 2009). Underlying, and less soluble cross-bedded siltstones of the Salem-Warsaw Formations locally act as a base for conduit development. Sinkholes and other closed drainage features prevail within incised valleys. Vertical solution shafts occur in stress release fractures along the hillsides (Ferguson, 1967) near