Paper No. 7
Presentation Time: 3:00 PM
THERMOGRAPH RECESSIONS
LUHMANN, Andrew J.1, COVINGTON, Matthew D.
2 and ALEXANDER Jr., E. Calvin
1, (1)Department of Earth Sciences, University of Minnesota, 310 Pillsbury Dr. SE, Minneapolis, MN 55455, (2)Karst Research Institute, Titov trg 2, Postojna, SI-6230, Slovenia, luhm0031@umn.edu
The upper stratigraphic column of southeastern Minnesota consists of Paleozoic carbonate and siliciclastic rocks. Data loggers have been deployed at several springs and cave streams in the region to monitor water level, temperature, and conductivity. Recent work has classified four thermal patterns (Luhmann et al. in press). One of these thermal patterns is an event-scale thermal fluctuation that begins within hours to days following a recharge event. This pattern occurs where short duration recharge flows through an aquifer before attaining thermal equilibrium with the flow path. When this occurs, a thermograph will portray a decrease or increase in temperature, a minimum or maximum peak, and a recession. Hydrograph recessions have been analyzed for more than a century to provide aquifer characteristics or to estimate discharge through time. To the authors’ knowledge, however, no one has ever analyzed thermal recessions, and this work represents an initial attempt to understand these common features.
As event water passes through an aquifer, it may cool down or heat up the rock along a flow path. This heat exchange enables thermograph patterns to penetrate farther as water flows along the flow path. The largest contribution of event water at a monitoring location is present at or near the thermal peak. The recession shape records recharge temperature from a primary flow path where event water makes up a smaller portion of the entire flow through time following the event and/or from slower flow paths that have had more time to react with the aquifer. The thermal recovery of the flow path will also influence the recessional shape. The cooling and heating of rock during a recharge event is enhanced by convection from distributed areas at the surface and by convection from conduits into the surrounding matrix when hydraulic head gradients are reversed during recharge events. A thermal recession from a single recharge event may last several months when not compounded by subsequent recharge events.
