The subsurface distribution of a nuclear waste tank leak on the USDOE's Hanford Site was sampled by slant drilling techniques to characterize the chemical and radiological characteristics of the leaked material and assess geochemical transport properties of hazardous constituents. Sediment core samples recovered from the borehole were subjected to distilled water and acid leaching procedures with the resulting leachates analyzed for isotopic and chemical signatures. Inductively coupled plasma–mass spectrometry (ICP–MS) techniques were used for determination of isotopic ratios for Cs, and Mo. Analysis of the isotopic patterns of Mo, combined with associated chemical data, showed evidence for at least two separate intrusions of nuclear waste into the subsurface. Isotopic data for Cs was inconclusive with respect to a source attribution signature.