<p>This study advances a methodology for estimating both nitrate and technetium-99 (Tc-99) vadose zone concentrations using electrical resistivity tomography (ERT) data. The objective is to demonstrate how geophysical data can be quantitatively transformed into contaminant concentration distributions to support long-term contaminant transport assessments. ERT-derived bulk electrical conductivity data were converted to three-dimensional contaminant concentration distributions through empirically derived relationships based on estimated disposed inventories and borehole characterization data. The approach was applied to a legacy waste site to generate multiple nitrate and Tc-99 distributions, capturing uncertainty in contaminant extent and magnitude. These contaminant distributions were used as initial conditions for vadose zone flow and transport simulations evaluating 1,000 years into the future. Contaminant distribution results estimated total nitrate mass from 9.76 × 10⁶ to 1.59 × 10⁷ kg and Tc-99 activity from 87.8 to 338.3 Ci. Simulations suggest peak contaminant arrival to groundwater between 2125 and 2145. For the simulation with the greatest cumulative release to groundwater, 78.3 Ci of Tc-99, representing 23% of the initial imposed distribution activity, remained in the vadose zone at calendar year 3070. This work extends the use of ERT beyond conventional geophysical interpretation to include contaminant concentration estimates. The methodology provides alternatives for areas where direct sampling is limited and offers a quantitative framework for evaluating uncertainty in subsurface contaminant inventories and transport behavior.</p>