MODELING SEASONAL SPRING DEVELOPMENT AND HEAD REVERSALS IN AN ISOLATED RIDGETOP WETLAND IN THE DANIEL BOONE NATIONAL FOREST

The Daniel Boone National Forest, has a number of isolated ridge top wetlands that are connected to perched groundwater. In the spring and during storms the hydraulic head in the groundwater rises above the surface water in the wetland, creating a spring on the upslope side of the wetland. During the summer groundwater storage is depleted and the head in the groundwater falls below the wetland causing a head reversal and leakage from the wetland. Explaining this system with a numerical model is difficult, because ridge top isolation creates few discernable boundary conditions. Our objective was to use field investigations and LIDAR data to characterize the system and define boundary conditions in order to create a MODFLOW model that explains the development and depletion of the spring in an isolated ridge top wetland. Field investigations included installing a well field in 2017, characterizing the geology, measuring monthly hydraulic head, and measuring hydraulic conductivity. Next we analyzed LIDAR data for the wetland and characterized geomorphic features such as cliff boundaries and ephemeral channels. The final steady-state model incorporated cliff faces as no-flow boundaries, the wetland pool as a constant head, and ephemeral channels as drains. When the model was calibrated, we found that groundwater recharge and drain boundaries were able to explain most of the monthly head values with RMS error as low as 0.06. The model indicated that the groundwater spring develops due to seasonal changes between recharge and leakage through drain boundaries, while evapotranspiration was a secondary outflow.