Physical Controls on Hydrochemical Variability in the Inner Bluegrass Karst Region of Central Kentucky a

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Abstract
Areal and temporal variations in physical and chemical characteristics of ground water in the Inner Bluegrass Karst Region of central Kentucky were used to evaluate physical controls on hydrochemical variability. The karst region consists of flat‐lying interbedded limestones and shales. Physical attributes of the aquifer were assessed on the basis of spring discharges, static water levels in wells, and pumping tests. Areal and temporal variations in ground‐ water chemistry were determined from major‐ion analysis of water samples from 15 springs and 24 wells over a 17‐ month period. Water samples were divided into three chemical types on the basis of the relative concentrations of the dominant ions; Ca‐Mg‐HCO 3 , Na‐HCO 3 , and Na‐Cl water. Ca‐Mg‐HCO 3 water was divided further on the basis of Ca/Mg molar ratios into Ca subtype water (Ca/Mg > 6) and Ca‐Mg subtype water (Ca/Mg < 6). All springs are characterized by Ca subtype water. Pumping tests show that hydraulic conductivities in zones of Ca‐Mg‐HCO 3 water are significantly higher than those in zones of Na‐HCO 3 and Na‐Cl water. There is no regional trend in the distribution of these water types. Temporal fluctuations in water chemistry correlate with seasonal fluctuations in recharge. Large seasonal variations in ionic concentrations occur primarily in Ca subtype water. Hydraulic conductivities, seasonal variations in ionic concentrations, and TDS suggest that ground‐water residence time is the primary control on the occurrence of the different water types. Differences in residence time are attributed to the presence of discontinuous, horizontal shales that control the rate and depth of active groundwater ciculation.

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