HR: 1340h
AN: H43F-0547    [Abstracts]
TI: Joint Geophysical and Hydrologic Constraints on Shallow Groundwater Flow Systems in Clastic Salt Marshes of the South Atlantic Bight
AU: * Ruppel, C
EM: cdr@eas.gatech.edu
AF: Georgia Tech, Earth and Atmos. Sciences, Atlanta, 30332
AU: Fulton, P
H43F-0547 AF: now at: Penn. State University, Geoscience Dept., University Park, 16802
AU: Schultz, G M
H43F-0547 AF: now at: Applied Research Assoc., 415 Waterman Rd, South Royalton, VT 05068
AU: Castillo, L
H43F-0547 AF: now in El Salvador, unknown, San Salvador, 00000
AU: Bartlett, J
H43F-0547 AF: now at: Desert Research Institute, 2215 Raggio Parkway, Reno, 89512
AU: Sibley, S
H43F-0547 AF: now at: University of Wisconsin, Environmental Chemistry and Technology, Madison, 53706
AB: Salt marsh systems play a critical role in buffering upland coastal areas from the influence of open saltwater bodies and in filtering contaminants that originate offshore or are flushed from uplands. For these reasons, it is important to understand the salt marsh hydrologic cycle, especially the interaction of groundwater and surface water across low-lying coastal fringes and the changes in physical, chemical, and ecological parameters across salinity gradients extending from upland to tidal creek to open water. For the past 5 years, we have conducted hydrogeophysical surveys (inductive EM and DC resistivity) and collected limited, coincident groundwater hydrologic data in clastic salt marshes throughout the South Atlantic Bight (SAB), stretching from South Carolina on the north to the Georgia-Florida border on the south. All of the marshes are dominated by Spartina and Juncus grasses and are cut by tidally-influenced creeks, but both the lithology and age of the marshes vary widely. For example, one highly homogeneous marsh study site has formed only within the past century, while most sites have existed for thousands of years and have laterally and vertically heterogeneous lithology. Geophysical images of the marsh subsurface and coincident monitoring of groundwater temperature, water level, and/or chemistry consistently show that marshes in the mixed energy environment of the middle part of the SAB (GCE LTER) tend to be dominated by submarsh discharge of freshwater to adjacent tidal creeks. In the South Carolina part of the SAB, we have greater evidence for seepage, particularly through biologically-created macropore networks and permeable sediment bodies that intersect tidal creeks. It is possible though that the South Carolina results are not so much 'universal' as reflective of local lithology. In a very young marsh near the Florida border, geophysical imaging implies a mixture of seepage and submarsh flow, and hydrologic data provide unequivocal proof that the near-surface marsh muds act as a low permeability barrier to downward penetration of tidal creek surface waters during periodic inundation of the marsh. Taken together, the results imply that subsurface freshwater bodies flowing beneath some salt marshes act as extensions of the classic freshwater lens that develops beneath uplands and help to resist saline intrusion toward uplands. Certain factors allow us to predict the occurrence of seepage, instead of submarsh flow, in SAB salt marshes with some degree of confidence. Where we have acquired time series, both the hydrogeophysical and hydrologic data suggest that groundwater transport processes are at approximate steady-state at the length scales (vertical and horizontal) and over the duration of our measurements.
DE: 0442 Estuarine and nearshore processes (4235)
DE: 0925 Magnetic and electrical methods (5109)
DE: 1830 Groundwater/surface water interaction
DE: 1835 Hydrogeophysics
DE: 4235 Estuarine processes (0442)
SC: Hydrology [H]
MN: Fall Meeting 2005