PHYSICAL HYDROGEOLOGIC AND GEOCHEMICAL FRAMEWORK OF THE SILURIAN-DEVONIAN CARBONATE AQUIFER IN SOUTH-CENTRAL LOUISVILLE, KENTUCKY

A multidisciplinary investigation conducted by the U.S. Geological Survey in 1999-2001 characterized the physical hydrogeologic and geochemical framework of the Silurian-Devonian carbonate aquifer in south-central Louisville, Kentucky. The northern part of the aquifer is unconfined and exhibits moderately well developed karst, while the southern part of the aquifer is confined by carbonaceous shale and exhibits little or no karst. Water-level measurements, hydraulic-conductivity measurements, and borehole geophysical and flowmeter logs were used to delineate the physical hydrogeologic framework of the aquifer—including the distribution of permeability, water-bearing zones, aquifer thickness, and hydraulic head. Major-ion composition, tritium, and stable isotopes of hydrogen and oxygen were used to investigate the geochemical framework—factors affecting the ground-water chemistry—at a larger scale than that of individual boreholes.

Ground-water flow in the aquifer is structurally controlled and mostly occurs through a few transmissive horizontal fractures within a narrow stratigraphic interval. Below this interval, the number and hydraulic conductivity of fractures decrease, and the carbonate rocks act as a lower confining unit. Spatial trends in the major ion, tritium, and stable isotope composition of ground water delineate distinct zones where transmissivity and the infiltration and circulation of fresh meteoric recharge are relatively enhanced or restricted within the aquifer. Younger, bicarbonate-dominated water occurs where the infiltration and circulation of fresh meteoric recharge is greatest. Older, chloride-dominated water occurs where infiltration and circulation of fresh meteoric recharge is restricted, and residual saline water has not been flushed from the aquifer. Mixed bicarbonate and chloride water occurs where the transmissivity is sufficient to allow some circulation of fresh meteoric recharge, but the saline water has not been completely flushed from the aquifer.