Water allocation in the Apalachicola-Chattahoochee-Flint River Basin (ACF) has been the subject of litigation for three decades. Agricultural water use in the Lower Flint watershed has contributed to reduced flows, especially during droughts, stressing downstream ecosystems. While agricultural producers have adopted many water conservation strategies, assessing the effects of water conservation on streamflow is difficult because of complex groundwater-surface water interactions. Our objectives were twofold. First, we characterized spatial and temporal trends in shallow groundwater levels from a creek bank, a geographically isolated wetland, and eight wells accessing the Upper Floridan Aquifer in Baker County, GA, USA. Second, we created a finite element model, ICHAFE, to calculate the local potentiometric surface of the Upper Floridan Aquifer, which is currently unavailable in present models. A well transect perpendicular to Ichawaynochaway Creek, a major tributary of the Flint River, and two transects in a geographically isolated wetland were outfitted with water level recorders and dataloggers. Well sites included two USGS wells and six abandoned household wells, which were checked bi-weekly. Site specific information and existing USGS data were incorporated into ICHAFE to simulate the effects of local groundwater extraction and identify areas of hydraulic entry and exit over time.
Results/Conclusions
The study period has been exceptionally wet, with ~41% more precipitation than normal since June 2018, and these conditions have provided new insights into surface water-groundwater interactions. Water surface elevation data along the creek transect showed water was leaving Ichawaynochaway Creek laterally and travelling into the bank during both high and low flows suggesting frequent aquifer recharge. At the wetland site, hydraulic heads of the aquifer exceeded surface water levels indicating potential surface water groundwater mixing caused by internal hydrostatic pressure. Fluctuations in the aquifer water levels showed high spatial variability across the study area, indicating areas of higher and lower sensitivity to local recharge, forest transpiration, and/or pumping for irrigation. Model development is ongoing and future work will include a sensitivity analysis of the aquifer to simulate local groundwater extraction using the ICHAFE model to identify regions more susceptible to irrigation-induced water level fluctuations. Knowledge of local aquifer recharge, subsurface flow direction, and areas of high sensitivity to irrigation withdrawals will help inform land conservation decisions by highlighting areas more susceptible to change and aid future efforts to maintain streamflow during drought.