FIELD ANALYSIS OF THERMAL RETARDATION IN KARST AQUIFERS

Theoretical considerations suggest that water temperature will generally be modified during flow through karst aquifers, producing a damping and retardation of thermal signals. This point has been corroborated by limited data, but a much more extensive analysis is needed to test for field support of the phenomenon. Thus, we analyzed cave stream and spring monitoring data from Arkansas, Kentucky, Minnesota, Pennsylvania, Texas, and Virginia in the United States and from one flow system in Slovenia. Data employed include electrical conductivity and temperature at the various monitoring sites, and analyses were limited to recharge events that produce an electrical conductivity minimum and a corresponding temperature minimum or maximum. Complex events without a clear relationship between individual electrical conductivity and temperature changes were excluded from the analysis to limit uncertainty, as recharge events or periods may produce multiple electrical conductivity and temperature changes due to the dynamics involved with recharge and flow through karst aquifers. Analyses are ongoing, but we have found that temperature minima or maxima most frequently occur after electrical conductivity minima, indicating thermal retardation. The temperature minimum or maximum occurs before the electrical conductivity minimum from 0% to 40% of the time at the various sites, averaging 20% of the time across all sites. It is expected that temperature minima/maxima may occur before electrical conductivity minima at times due to the dynamics and timing of recharge events in the midst of daily or seasonal temperature variations at the surface. Still, the results are promising, as thermal retardation can be used to estimate flow path diameter to facilitate aquifer characterization.