HR: 08:30h
AN: H21A-03    [PDF]
TI: Hydrogeophysics in the Coastal Zone: Spatial and Temporal Constraints on Groundwater Processes from Coincident Wells and Geophysical Surveys
AU: * Ruppel, C
EM: cdr@piedmont.eas.gatech.edu
AF: Georgia Tech, Earth and Atmos. Sciences, 311 Ferst Drive, Atlanta, GA 30332-0340 United States
AU: Schultz, G
EM: gschultz@ara.com
AF: Applied Research Associates, 415 Waterman Rd, South Royalton, VT 05068
AB: The electrical conductivity contrast between fresh and saline groundwaters renders DC resistivity imaging and inductive EM methods particularly useful for studying hydrologic problems in the coastal zone. However, such settings also pose unique logistical challenges, ranging from difficulty acquiring geophysical data in salt marsh or intertidal zones to complications in the analysis of geophysical data that are strongly affected by high conductivity salt water. In this study, we first describe a new inversion algorithm for inductive EM data collected in high conductivity settings where the linear relationship between induced magnetic field strength and integrated terrain conductivity breaks down. The inversion, which is based on a robust, convergent, regularized least squares approach that accounts for the nonlinearity in mutual dipole induction, produces an "image" of subsurface fresh and saline pore water distributions that closely matches the actual distribution inferred from geochemical analyses of groundwater recovered from coincident shallow monitoring wells at a permeable upland site. In the second part of the study, we report on key results from 5 years of electrical and inductive EM surveys conducted at intratidal to interannual time scales coincident with $>$ 50 shallow monitoring wells in uplands and salt marshes of the Georgia Bight. Combining inversions of EM data, the results of extensive DC resistivity surveys, and geochemical analyses of groundwaters from our well networks, we delineate submarsh freshwater discharge pathways to adjacent estuaries, describe the dynamics of fresh and saline groundwater interactions and exchange during a tidal cycle, and constrain possible Hele-Shaw convection cells within salt marshes.
DE: 0925 Magnetic and electrical methods
DE: 1829 Groundwater hydrology
DE: 3260 Inverse theory
DE: 4235 Estuarine processes
SC: Hydrology [H]
MN: 2003 Fall Meeting