IWRA Proceedings

AbstractThe U.S. Geological Survey conducted a pilot geophysical survey at the Texas A&M University Brazos River Hydrologic Field Research Site (BRHFRS) near College Station, Texas, to characterize the electrical stratigraphy and hydrostratigraphy of the Brazos River alluvial aquifer to a high precision to aid in generating an improved ground-water availability model. Three non-invasive surface geophysical techniques were used to characterize the hydrostratigraphy of the Brazos River alluvial aquifer. Two electrical methods, time-domain electromagnetic (TDEM) soundings and two-dimensional direct-current (2D–DC) resistivity imaging, were used to define the lateral and vertical extent of the Brazos River alluvial aquifer (broken down into a sand and gravel unit and the overlying Ships Clay) and the underlying Yegua Formation–a thick shale unit. Magnetic resonance sounding (MRS), a recently developed geophysical technique, was used to derive estimates of the hydrogeologic properties including percent water content, transmissivity, and hydraulic conductivity. Results from the geophysics study demonstrated the usefulness of combining TDEM, 2D–DC resistivity, and MRS methods to reduce the need for drilling additional boreholes in areas with data gaps and create more accurate ground-water models. TDEM data defined a three- layer electrical stratigraphy corresponding to a conductor-resistor-conductor which represents the Ships clay, Brazos River alluvial aquifer, and Yegua formation. Based on the TDEM data, there are sharp electrical boundaries at about 4-to-6 m below land surface and at 20-to-22 m below land surface which define the geometry of the more resistive alluvial aquifer. These abrupt changes define the more resistive Brazos River alluvial aquifer. The thickest portion of the more resistive alluvial aquifer is interpreted to be an ancestral channel deposit of the Brazos River that has not previously been identified. This determination is based on correlating lithology to resistivity and comparisons to the 2D–DC and MRS soundings. The higher resistivities indicate coarse sediments. According to the 2D–DC resistivity profile, variations in resistivity within the Brazos River alluvial aquifer range from 10 to more than 175 ohm -m over a distance of 200 m. The lowest zones of resistivity occur from land surface down to about 7 m below land surface correlating to the Ships clay. Based on the MRS soundings data, the most productive parts of the Brazos River alluvial aquifer occur between 12 to 15 m below land surface in a buried channel deposit towards the western part of BRHFFRS. The higher recorded values of water content and hydraulic conductivity collected with MRS correlate to the greatest saturated thickness derived from the TDEM, 2D–DC resistivity data, and borehole logs. As the water content and hydraulic conductivity increase further below land surface, coarser material such as sand and gravel increase. By combining both TDEM and 2D–DC resistivity data, information on the aquifer geometry and lateral variations in resistivity can be determined and used to build the framework for a geologic conceptual model and integrating MRS data to create the hydrostratigraphy and subsequently a more accurate ground-water model