The number and types of data required to support decisions concerning waste disposal in desert environments depends in part on answers to three interrelated questions. First, how well are the relevant processes and their governing equations understood? Second, how well are the non-linear parameters appearing in the equations represented by measurements? And third, how well can the complexity characteristic of field settings be treated? These questions are illustrated by results from process-oriented studies of transport through a 110-m thick unsaturated zone near disposal facilities for radioactive and chemical wastes, about 16 km east of Death Valley National Park. Sponsored by the USGS Toxic Substances Hydrology Program, the studies have focused on how interacting processes of barometric pumping, evapotranspiration, chemical kinetics, and biochemical transformations shape the transport of tritium, radiocarbon, and volatile-organic compounds in an arid alluvial system.