Chemical similarities among physically distinct spring types in a karst terrain

Abstract

In karst regions where correlations between physical characteristics of springs and temporal variations in spring water chemistry have been found, spring water chemistry has been used to infer physical attributes of karst systems. Possible correlations between chemical and physical characteristics of springs were tested in the Inner Bluegrass Karst Region of central Kentucky where previous dye-tracing studies have identified two physically distinct spring types: local high-level springs discharging from shallow flow paths and major low-level springs discharging from a deep integrated conduit system. Representative high-level and major springs were sampled over a 16-month period and analyzed for major dissolved components. Both spring types showed similar variations in temperature, calcium, magnesium, bicarbonate, and hardness. No systematic differences in ionic concentrations or in saturation indices with respect to calcite and dolomite were apparent between the two spring types. Chemical similarities between high-level and major springs during low flow are attributed to recharge of major springs by percolation and by high-level springs and to the occurrence of most chemical reactions near the recharge zone rather than in the deep conduit system. During high discharge, however, most recharge to the major springs is surface runoff which produces low ionic concentrations. Similarly low ionic concentrations in the high-level springs are thought to result from rapid flow through the soil-rock zone and short flow distances. These relationships indicate that spring water chemistry is not only a function of conduit size but also an indicator of recharge type and amount and flow path length. Differing flow path lengths to major and high-level springs counteract the effect of varying conduit size between the two spring types and result in similar ionic concentrations. These data indicate that spring water chemistry cannot be used to predict physical characteristics of karst aquifers in the Inner Bluegrass Region. The physical and chemical attributes of springs in the Inner Bluegrass were compared to those of springs in the Nittany Valley of Pennsylvania. A reported high correlation between physical and chemical characteristics of springs in the Pennsylvania karst system reflects geological and structural controls not present in the Inner Bluegrass Region.