Steady vs Dynamic Stream-Aquifer Interactions: Lower Flint River Basin, Southwest Georgia, USA

Abstract:

Abstract. Surface-groundwater interactions in karst systems are confounded by the lack of data to characterize the temporal variation and spatial heterogeneity in these systems. This study presents a year-long monitoring campaign investigating steady and dynamic stream-aquifer exchanges and fluxes on Ichawaynochaway Creek, a fifth-order tributary to the Flint River in the karstic Dougherty Plain in southwestern Georgia. We compare net change in discharge between two USGS gaging stations separated by 24-km on a reach with no intervening surface-water inflows. Approximately 150 gigaliters of net channel gains were observed during the study period from July 2018 to July 2019. Most gains were observed between October 2018 and March 2019, largely contributed by Hurricane Michael in October 2018 (the first Category 5 hurricane to strike the contiguous United States since Hurricane Andrew in 1992). Channel and bank storage increased during storm events but then decreased during the recession limb. Negligible gains and some losses occurred during the Spring and Summer months. Stream-aquifer interactions were monitored using six piezometers installed to a depth of approximately 5 m in surficial sediments at distances of 4 to 120 m from the bank. Unlike reach-scale data indicating net groundwater discharge, hydraulic gradients at the piezometer site indicate net groundwater recharge. We note that the anomalous hydraulic gradient at the piezometer site is similar in magnitude and direction to the regional groundwater gradient, suggesting reach orientation influences stream-aquifer interactions. Stream-aquifer flux was then calculated using a variation of Darcy’s Law with a maximum value of approximately 300 m/d at a distance of 120 meters from the Creek bank. Dynamic responses to Hurricane Michael in October 2018 are used to estimate aquifer properties; diffusivity and transmissivity increased along the transect (9 to 420) and (3 to 150), respectively. This study elucidates how stream orientation with respect to the regional hydraulic gradient affects groundwater discharge and recharge in a karstic system during steady flow conditions.