THERMAL PATTERNS OF KARST SPRINGS AND CAVE STREAMS IN SOUTHEASTERN MINNESOTA

Karst aquifers dominate the local and regional hydrogeology in southeastern Minnesota where a thick sequence of Paleozoic carbonate and siliciclastic rocks are found. These aquifers are triple permeability systems, where flow occurs through the matrix of the rock, fractures and bedding planes, and conduits. Data loggers have been deployed in several springs and cave streams throughout the region to monitor basic physical and chemical parameters to better understand the relative influence of each permeability component for each flow system. The thermographs from these sites are the focus of this study.

The thermographs record at least four distinct patterns, and all of our sites portray one or more of these temperature patterns. Pattern 1 is an event-scale temperature fluctuation that occurs over hours or days. Pattern 2 is a seasonal fluctuation in temperature that is in phase with surface temperatures. Pattern 3 is a seasonal fluctuation in temperature that is out of phase with surface temperatures. Pattern 4 is long-term temperature stability, over time scales of weeks to months.

Pattern 1 and 2 are products of flow paths with inefficient heat exchange between recharging water and the karst aquifers. Pattern 1 results from short duration, episodic recharge events, while Pattern 2 results from seasonal temperature changes in perennial surface recharge. Patterns 3 and 4 occur due to efficient exchange of heat with the aquifers. Pattern 3 is displayed when the aquifer temperature changes due to recharge or to surface temperature fluctuations. Pattern 4 occurs when the aquifer temperature is constant. An understanding of these characteristic thermal patterns and the heat transport processes that are responsible for producing them provides insight into the characterization of karst aquifers.