Continuous Electrical Resistivity Survey of Bull River, Lazaretto Creek, and Oyster Creek Near Savannah, Georgia
The purpose of our research is to use electrical resistivity as a tracer of groundwater discharge to the Bull River, Lazaretto Creek and Oyster Creek, near Savannah, Georgia. Electrical resistivity is a relatively new tool for groundwater discharge studies. The Bull river and nearby tidal creeks are brackish with diurnal tides. 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 streams. We used the Advanced Geosciences, Incorporated R8/IP SuperSting Marine to perform a continuous electrical resistivity survey from June 10 to June 12, 2014. All surveys were performed within three hours of low tide. The surveys involved towing ~60 meters of cable behind a slow moving boat (average speed= 3.1 kph). The cable was configured to collect data using a dipole-dipole configuration. We typically imaged the water column and substrate to a depth of 13 meters. We used three different models during post-processing (data inversion). One model had a fixed water resistivity, a second model unchanged, and a third model used continuous measurements of the conductivity of the water column collected with an EXO 1 (YSI Incorporated). During data processing we accepted model inversions with root mean squares below ten percent, and L2s under 2.00. Our resistivity values ranged from 0.1 to 2.0 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. Furthermore, in some locations, substrate materials with relatively high resistance correspond to rapid depth increases in the river’s channel. Future research will involve ground-truthing the study area by performing at least two shore perpendicular electrical resistivity surveys on land. These surveys will use a dipole-dipole configuration to maximize horizontal viewing and an inverse Schlumberger configuration to maximize vertical viewing. These surveys will help us investigate the electrical resistance of the substrate materials, giving us vital information for correctly interpreting our marine surveys. To our knowledge, this is the first survey of its kind in Georgia’s estuaries. Our findings can be used by local water and ecosystem managers to better understand how groundwater discharge and even estuary circulation impact the local fauna and flora of the area. We hope that our findings will also be useful for understanding similar estuary systems in the southeast.
Georgia Undergraduate Research Conference (GURC)
Swanson, Jake P., Cody Mahaffey, Jacque L. Kelly.
"Continuous Electrical Resistivity Survey of Bull River, Lazaretto Creek, and Oyster Creek Near Savannah, Georgia."
Geology and Geography Faculty Presentations.