Electrical Resistivity in the Bull River and Surrounding Creeks, Savannah, GA

We used electrical resistivity as a tracer of groundwater discharge to the Bull River, Lazaretto Creek and Oyster Creek, near Savannah, Georgia. The Bull river and nearby tidal creeks are brackish with diurnal tides. The brackish groundwater discharge to these creeks will have lower resistance to electrical conductance compared to seawater. Thus, the electrical resistivity method is an ideal method for groundwater discharge studies in tidal areas. We used the Advanced Geosciences, Inc. R8/IP Super Sting Marine to perform electrical resistivity surveys from June 10 to June 12, 2014. All surveys were made within three hours of low tide. The surveys involved towing ~60 m of cable behind a slow moving boat (average speed= 3.1 kph). These surveys use a dipole-dipole configuration to maximize horizontal viewing. We typically imaged the water column and substrate to a depth of 13 m. We used three different models during post-processing (data inversion). One model had a fixed water resistivity, a second unchanged model, and a third model used continuous measurements of the conductivity of the water column collected with an EXO 1 (YSI Inc.). During data processing we accepted model inversions with root mean squares below 10%, and L2s under 2.00. Our resistivity values ranged from 0.1 to 11.2 ohm-meters. The electrical resistivity data typically suggest the water column is well-mixed in the smaller tidal creeks. There is evidence for a stratified water column in some parts of Bull River, Lazaretto Creek, and Oyster Creek. There are also spots of high resistivity on the surface and long stretches of high resistivity on the bottom of the channel, which we tentatively interpret as scours. Furthermore, in some locations, substrate materials with relatively high resistance correspond to rapid depth increases in the river’s channel. Future research will involve making a geographic information system to examine spatial relationships of these features.