EVALUATION OF THERMAL RETARDATION IN KARST AQUIFERS

To evaluate the extent to which thermal retardation is readily observed from karst aquifers, we completed analyses on spring and cave stream data from monitoring sites in Mexico, Slovenia, and the United States. We quantify the time difference between an electrical conductivity minimum and a corresponding maximum or minimum in temperature, where a positive thermal retardation occurs when the temperature maximum/minimum follows the electrical conductivity minimum. Additionally, with data collected from multiple monitoring locations along one flow path, we quantify the time it takes for a maximum/minimum in the electrical conductivity and temperature datasets to move from the upstream to the downstream monitoring location, where a positive thermal retardation occurs when the temperature travel time is longer than the electrical conductivity travel time. We find that 47% of the 620 events analyzed with data from individual monitoring stations have a positive thermal retardation, while a negative thermal retardation occurs 38% of the time. The conductivity minimum and temperature maximum/minimum occur simultaneously 15% of the time. Analysis of 24 events using the travel time method indicates a thermal retardation that is positive, zero, and negative 69%, 4%, and 27% of the time, respectively. Factors that prevent observations of thermal retardation include diurnal surface temperature variations, inputs from the rock matrix surrounding conduits, hyporheic exchange of water between conduits and the rock matrix, mixing from multiple flow paths, significant vertical relief along the flow path, and the data logging interval. These factors may minimize temperature changes or modify temperature signals beyond heat transport processes alone, and some may impact temperature while causing minimal modification of other more conservative parameters that facilitate determination of thermal retardation. While these factors highlight complex processes in karst aquifers, thermal retardation analysis informs our understanding of flow and transport dynamics in the aquifer and has practical application, such as in the characterization of flow path geometry.