ISOTOPIC EVOLUTION OF GROUNDWATER IN A TELOGENETIC KARST AQUIFER: A METHOD TO STUDY RECHARGE AND CONTAMINANT TRANSPORT

There exists a limited understanding of hydrogeologic flow and contaminant transport within karst aquifers, particularly in the epikarst zone, which are highly susceptible to natural and anthropogenic contamination, such as agricultural runoff, due to the interconnected nature of the surface and subsurface. Understanding the storage, flow, and transport dynamics through the epikarst is fundamental to protecting the water quality of karst aquifers, as it is often where contaminants are located and also where mixing can occur between meteoric and groundwaters. While few tools exist to study the “black box” nature of epikarst dynamics, stable isotope hydrology offers an effective alternative to traditional karst investigative techniques, such as dye tracing, to evaluate flow dynamics and water-soil-rock interactions across a range of spatial and temporal resolutions. A secondary method performing base flow separation of multiple discrete storms was used to quantify epikarst storage by comparing recharge volumes with in-cave waterfall discharges. Using stable isotopes of water (¹⁸O/¹⁶O and ²H/¹H) recharge can be traced from meteoric sources through the epikarst, providing insight to the pathways of recharge, flow, and transport dynamics across multiple spatial and temporal scales in telogenetic karst systems. During 2011-2013, weekly isotope samples were collected from rainfall and an epikarstic in-cave waterfall in Crumps Cave in south-central Kentucky, USA. Using the isotope data, along with 10-min in-cave waterfall discharge, precipitation, and geochemical data, the storm, seasonal, and annual transport dynamics of water through the epikarst were evaluated. While seasonal and storm pulses showed significant variation associated with changing atmospheric sources, the epikarst waterfall isotope signal remained constant across the dataset. This homogenizing effect implies mixing occurs in the shallow epikarst zone across both short and long time intervals. Previous estimates of baseflow separation of the waterfall indicated that the epikarst storage is potentially two to three orders of magnitude higher than at regional springs, revealing that storage in the epikarst can play a significant role in recharge and as a mixing zone for contaminant transport.