A THERMALLY-STABLE COLD SPRING ENVIRONMENT WITH A NEGATIVE GEOTHERMAL GRADIENT: ARBUCKLE-SIMPSON AQUIFER, SOUTHERN OKLAHOMA

High flow rates in fractured and karstic aquifers generally imply highly variable systems with strong chemical and thermal gradients. For large carbonate systems, this is not necessarily the case. The Arbuckle-Simpson aquifer system in southern Oklahoma consists of 500 square miles of fractured dolomite in the Hunton anticline with an approximate thickness of one kilometer. The system presents surface water discharge in springs that is thermally and chemically stable. Well data to a depth of 600 meters demonstrates a mildly negative geothermal gradient with no chemical or lithologic gradients. Regional model data suggest a vertically anisotropic system with flows migrating vertically through the system.

This study integrates spring and well data to model thermal variability in the system from the surface to the center of the aquifer at 600 meters. It is supported by spring and well data on chemistry, flow and electrical properties. The thermal modeling has focused on two primary factors known to contribute to perturbations in temperature gradients: groundwater movement and paleoclimatic signatures. Inverse one-dimensional models of temperature measurements were used to compute groundwater movement. Forward modeling was also used to determine a plausible velocity range for groundwater in this area. Paleoclimatic signatures were investigated using a one-dimensional transient conductive heat transfer model. To address paleoclimatic signatures, a model was selected for its strengths in incorporating data collected and its relative flexibility in assessing the parameters used in the model. Results show flow velocities present at rates high enough to significantly disturb the temperature gradient of this area. Paleoclimatic results show a defined warming in the subsurface of 0.4 °C in the past sixty years. These results demonstrate strong groundwater movement and paleoclimatic effects can result in a thermally stable cold spring environment.