37-4 Not so Impermeable: Quantifying Okefenokee Swamp’s Contribution to the Upper Floridan Aquifer

Session: Innovations in Research of Groundwater-Surface Water Interactions over Multiple Spatio-Temporal Scales

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Abstract:

The Okefenokee Swamp has long been assumed to be hydraulically isolated from the underlying Upper Floridan Aquifer (UFA) due to separation by the Hawthorn layer, presumed an impermeable confining layer. Under this traditional view, vertical exchange is negligible. We challenge this paradigm using two lines of evidence: (1) stable isotopic signatures (δ²H and δ¹⁸O) of water from the swamp and aquifer, comparing historical data from 1997 to new samples from 2025, and (2) time-series analysis of swamp and UFA water levels. The 2025 isotope data reveal that a large fraction (approximately 62%–92%) of swamp water recharges into the UFA, indicating strong vertical hydraulic exchange. Concurrently, there is a strong downward hydraulic gradient from the swamp to the Floridan, and swamp stage and aquifer head fluctuate synchronously with a lag of about one month, suggesting dynamic hydraulic connectivity. These findings refute the notion of an impermeable separation; instead, it appears that the Okefenokee and the UFA behave as a connected system, typically with downward flow from the swamp to the Floridan. This has significant implications for regional water budgets and resource management, as vertical losses from the swamp and aquifer drawdowns are likely greater than previously recognized.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-5548

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