Magnitude and variations of groundwater seepage along a Florida marine shoreline

Direct groundwater inputs are receiving increasing attention as a potential source of nutrients and other dissolved constituents to the coastal ocean. Seepage into St. George Sound, Florida was measured extensively from 1992 to 1994 using seepage meters. Spatial and temporal variations were documented along a 7-km stretch of coastline and up to 1 km from shore. Measurements were made at 3 transects perpendicular to shore and 1 transect parallel to shore. The general results indicated that seepage decreased with distance from shore (2 of 3 transects), and substantial temporal and spatial variability was observed in seepage flow from nearshore sediments. In addition, trends in mean monthly integrated seepage rates were similar to precipitation patterns measured at a nearby coastal weather station. Based on these measurements, we estimate that the magnitude of groundwater seepage into the study area is substantial, representing from 0.23 to 4.4 m(3) . sec(-1) of flow through the sediments, approximately equivalent to a first magnitude spring. Although it is unknown how representative this region is with respect to global groundwater discharge, demonstrates that groundwater flow can be as important as riverine and spring discharge in some cases. Our subsurface discharge rates suggest groundwater is an important hydrologic source term for this region and may be important to the coastal biogeochemistry as well.