The Reklaw Formation is the upper bounding unit for the Carrizo-Wilcox Aquifer throughout the Gulf Coastal Plain of East Texas and consists of low permeability, glauconite-rich strata that isolate semi-confined portions of the aquifer system from potential contaminants. Electrical resistivity methods were employed within a forested watershed in Nacogdoches County, Texas to characterize solute transport. 2-D and 3-D temporal resistivity data collected with an AGI SuperSting (R8/IP) were processed with AGI Earthimager 2D/3D software for inversion modeling. Data were collected over 135 days within a 14 X 26 meter (46 X 85 feet) gridded survey at 15-day intervals after initiation of a NaCl solute plume; numerical modeling was developed from physical site characterizations.
Resistivity analyses and numerical modeling demonstrated solute migration is extremely slow within the Reklaw Formation, confirming strata effectiveness for preventing contaminant migration into the Carrizo-Wilcox Aquifer. Numerical modeling indicated rapid solute dilution with migration dominated by diffusion. 2-D inversion modeling confirmed dominance of solute diffusion, but clearly identified macropore heterogeneity that increased advection transport; 3-D inversion modeling proved relatively ineffective. This study demonstrates the effectiveness of electrical resistivity characterization for delineating heterogenic and anisotropic controls on solute migration that are often poorly defined in simple numerical modeling
