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Geographically isolated wetlands (GIWs) are those considered to be completely surrounded by uplands and with little or no surface water connections to other wetlands or waters. Numerous studies of GIWs suggest controls on their hydrology are largely local, being driven by a surficial aquifer or under perched conditions. A group of GIWs in west central Florida, USA (locally referred to as “sandhill” wetlands and waters) were examined to determine the degree of hydrologic control by a large, regional source (i.e., the Upper Floridan aquifer). To make this determination, the chemistry, elevation and level variation of surface water and shallow groundwater from these GIWs and waters were evaluated relative to water from surficial aquifer piezometers and from shallow and deep regional aquifer piezometers.
Results from hydrographic analyses indicate close and even coincident water level elevations between wetlands and the nearest regional aquifer monitor wells. Regression analysis of wetland-well levels show high to extremely high correlation coefficients (R2 values between 0.79 and 0.99), with among-wetland variations due likely to wetland-well distance and differences in land surface elevations and site-specific hydrogeomorphology. Geochemical and isotopic analyses of wetland water samples indicate a mixing of surface and groundwater end-members due to the direct or near-direct connectivity between the wetlands and the Upper Floridan aquifer. The mixing is due to the unconfined, thinly confined or karstic-breached nature of this aquifer. Collectively, this evidence suggests sandhill wetlands and waters are largely under the control of the potentiometric surface of the regional Upper Floridan aquifer and thus represent a variant of GIWs situated at the extreme end of a groundwater-to-surface water or regional-to-local hydrologic source continuum not previously documented. These findings are important not only for their contribution to the current body of GIW knowledge, but also for the information they provide to natural resource managers, particularly in evaluating potential impacts to wetlands and waters of this kind from groundwater withdrawal and climate change. |