Not so isolated: isotopic and hydraulic evidence of vertical connectivity between the Okefenokee Swamp and Floridan Aquifer

The Okefenokee Swamp has long been assumed to be hydraulically isolated from the Upper Floridan Aquifer (UFA) due to the intervening Hawthorne Formation, a Miocene-age confining unit. Here we test that assumption using stable water isotopes and multiyear time series of swamp and aquifer water levels. Isotope mixing model using δ ^18 O indicates that 27%–95% of groundwater beneath the swamp is swamp-derived, with the remainder resembling regional UFA water. Coherence analysis and impulse-response modeling show that water level fluctuations propagate across the Hawthorne layer with a lag of ∼30 d, yielding a vertical hydraulic diffusivity of ∼291 m ^2 d ^−1 . These independent lines of evidence confirm that the swamp and aquifer are hydraulically connected through a semi-confining system. Vertical leakage from the swamp may represent 5%–15% of annual rainfall and contributes to the isotopic enrichment of UFA waters downgradient. This overturns prevailing hydrogeologic conceptual models and suggests that wetland–aquifer interactions are more dynamic and consequential than previously recognized.