USING GEOCHEMICAL MODELING TO UNDERSTAND THE SOURCES OF DISSOLVED IONS AND FLOW PATTERNS AT MILL SPRING IN SOUTHEAST KENTUCKY

Many efforts in past decades attempted to define conceptual models to understand speleogenesis and patterns of water flow in karst aquifers. Two basic approaches have been widely applied to mathematical and numerical models: the equivalent porous medium approach and the coupled continuum-pipe flow approach. While extensive geochemical methods have been applied to porous media models, fewer examples by comparison have modeled geochemical behavior in a dual continuum more representative of karst.

In this study, we use quarterly water quality data collected from 2000 to 2013 in the Mill Spring karst aquifer, located on the sinkhole plain along the Cumberland River of Southeast Kentucky, to develop a geochemical mixing model in PHREEQC Version 2. Our approach is based upon the chemistry of two primary end members converging upon conduits feeding the spring, discrete allogenic recharge from sinking streams and diffuse matrix flow entraining shallow brines. These end members were defined by precipitation chemistry data and published data on oilfield brines in the region. Results validate a key feature of epigenic karst aquifers; carbonate equilibrium reactions dominate the chemistry, particularly during periods of greater meteoric recharge. Additionally, the results also suggest that geochemical reactions involving brines may contribute to carbonate dissolution during periods of base flow.