GEOLOGIC CONTROLS ON KARST AQUIFER FUNCTION AS REFLECTED IN HYDROGRAPHS AND CHEMOGRAPHS: RELATING RECESSION COEFFICIENTS TO FLOW REGIMES, CHEMICAL LOADS, AND FRACTAL DIMENSIONS

Geologic controls on the functioning of karst aquifers are reflected in the recessions of karst hydrographs at multiple scales. Due to the repetitive hydrograph recession behavior often observed in karst, breaks in slope of the recession curve (i.e., a change in recession coefficient) provide rough discharge limits to define flow regimes, thus allowing dynamic storage volumes for each regime to be quantified. Storage volumes placed within geologic context may aid in the delineation of contributing areas to springs. In combination with chemical data, dynamic interactions between storage compartments can be examined. For example, release of water from epiphreatic dynamic storage to springs during events is often observed ahead of allogenic or phreatic sources, reflecting multiple storage zones within a geologically compartmentalized karst aquifer. Moreover, water chemistry data grouped according to flow regime allows chemical variability to be examined in accordance with discharge, and average loads to be calculated for each flow regime of the aquifer. This method may be applied to events or to infrequent chemical sampling over long term monitoring periods, provided concurrent records of discharge are available. Multiple hydrograph recession segments can be observed at different scales within both vadose and phreatic settings (e.g., cave drips, boreholes, and springs). Prior work has shown fractal dimensions of rock fracture networks range from 0.6 to 2, while conduit (cave) dimensions range from 1-3. It is suggested here that the karstification process effectively increases the fractal dimension of the initial fracture network during the formation of conduit networks, possibly giving rise to multiple fractal dimensions within a single karst aquifer. Geochemical and hydraulic thresholds for conduit enlargement may be responsible for development of a conduit fractal dimension distinct from the original fracture set. Thus, physical interpretation of breaks in hydrograph recession slopes may benefit from the analysis of detailed datasets on conduit apertures in caves, particularly with regard to identifying multiple fractal dimensions in a single karst system and the relationship between fractal dimensions and hydrograph recession coefficients. Further work is needed to test this hypothesis.