MAGNITUDE AND VARIATIONS OF GROUNDWATER SEEPAGE ALONG A FLORIDA MARINE SHORELINE

Direct groundwater inputs are receiving increasing attention as a pote ntial source of nutrients and other dissolved constituents to the coas tal ocean. Seepage into St. George Sound, Florida was measured extensi vely from 1992 to 1994 using seepage meters. Spatial and temporal vari ations were documented along a 7-km stretch of coastline and up to 1 k m from shore. Measurements were made at 3 transects perpendicular to s hore and 1 transect parallel to shore. The general results indicated t hat 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 int egrated seepage rates were similar to precipitation patterns measured at a nearby coastal weather station. Based on these measurements, we e stimate 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 t hrough the sediments, approximately equivalent to a first magnitude sp ring. Although it is unknown how representative this region is with re spect to global groundwater discharge, demonstrates that groundwater f low can be as important as riverine and spring discharge in some cases . Our subsurface discharge rates suggest groundwater is an important h ydrologic source term for this region and may be important to the coas tal biogeochemistry as well.