We have applied traveltime tomography to 3D seismic refraction data collected at Hill Air Force Base, Utah, in an approximately 95 ϫ 40-m area over a shallow ͑Ͻ 20 m͒ groundwater contamination site. The purpose of this study is to test the ability of 3D first-arrival-time data to characterize the shallow environment and aid remediation efforts. The aquifer is bounded below by a clay aquiclude, into which a paleochannel has been incised and acts as a trap for dense nonaqueous phase liquid ͑DNAPL͒ contaminants. A regularized nonlinear tomographic approach was applied to 187,877 first-arrival traveltimes to obtain the smoothest minimumstructure 3D velocity model. The resulting velocity model contains a velocity increase from less than 300 to 1500 m/s in the upper 15 m. The model also contains a north-southtrending low-velocity feature interpreted to be the paleochannel, based on more than 100 wells in the area. Checkerboard tests show 7.5-10 m lateral resolution throughout most of the model. The preferred final model was chosen after a systematic test of the free parameters involved in the tomographic approach, including the starting model. The final velocity model compares favorably with a 3D poststack depth migration and 2D waveform inversion of coincident reflection data. While the long-wavelength features of the model reveal the primary target of the survey, the paleochannel, the velocity model is likely a very smooth characterization of the true velocity structure, particularly in the vertical direction, given the size of the first Fresnel zone for these data.

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