WATER TEMPERATURES AND HEAT TRANSPORT IN KARST: A REVIEW OF RECENT ADVANCES

Water temperature is a crucial water quality parameter. Additionally, a karst spring thermograph provides useful information about an aquifer. In recent years, the equipment needed for continuous monitoring of water temperature has become quite inexpensive, resulting in a boom of temperature data collected in karst settings. Here we review a number of recent advances that have improved our understanding of thermographs at karst springs and heat exchange mechanisms in karst conduits. Water temperatures at 25 springs and cave streams in Minnesota were monitored over periods typically greater than a year. The thermal signals observed at these springs fall into four basic patterns that can be interpreted in terms of the effectiveness of heat exchange in the aquifer and the nature of the recharge. The patterns provide important information about flow path size, recharge type and duration, and aquifer depth. Additionally, we have conducted a theoretical study of the relative importance of heat exchange mechanisms in karst, delineating proper and improper assumptions for use in modeling of karst conduit water temperatures, and deriving the approximate length scales that are associated with the damping of thermal signals. The results of this theoretical study were used to construct models of heat transport in two cave streams that display qualitatively different thermal patterns. These models are able to reproduce the thermal behavior of the cave streams using conduit geometries that are similar to the true geometries. Finally, modeling of a multi-tracer experiment at Freiheit Spring in Minnesota suggests that temperature provides a means of constraining conduit hydraulic diameters that is complementary to other estimates using spring responses.