southwestern portion of the INEEL where, because of potential contaminants in this area, some of the most Questions of major importance for subsurface contaminant transintensive subsurface characterization efforts have been port at the Idaho National Engineering and Environmental Laboratory (INEEL) include (i) travel times to the aquifer, both average or done (Fig. 1). A major source of these contaminants is typical values and the range of values to be expected, and (ii) modes the INEEL facility known as the Subsurface Disposal of contaminant transport, especially sorption processes. The hydraulic Area (SDA) for radioactive and other hazardous waste. and geochemical framework within which these questions are ad-As a result, the SDA and its nearby surroundings are dressed is dominated by extreme heterogeneity in a vadose zone and the portion of the INEEL that has been most intensively aquifer consisting of interbedded basalts and sediments. Hydraulically, studied with respect to hydraulic and geochemical issues. major issues include diverse possible types of flow pathways, extreme This paper reflects this emphasis, though where possible anisotropy, preferential flow, combined vertical and horizontal flow, we have formulated generalizations to apply to most or and temporary saturation or perching. Geochemically, major issues all of the INEEL. This review is designed to complement include contaminant mobility as influenced by redox conditions, the that of Smith (2004). In terms of local-scale geologic concentration of organic and inorganic complexing solutes and other local variables, the interaction with infiltrating waters and with the framework, it picks up from Smith's presentation of teccontaminant source environment, and the aqueous speciation of con-tonics, volcanism, and basalt characteristics; it emphataminants such as actinides. Another major issue is the possibility of sizes relatively small-scale features and their role in colloid transport, which inverts some of the traditional concepts of transport processes. We cover basic issues of transportmobility, as sorbed contaminants on mobile colloids may be transrelevant characteristics as known mainly from investigaported with ease compared with contaminants that are not sorbed.tions since the 1950s by relatively standard techniques. With respect to the goal of minimizing aquifer concentrations of con-Much material in this paper derives from a 1999 taminants, some characteristics of the vadose zone are essentially USGS panel review of DOE-contractor investigations completely favorable. Examples include the great thickness (200 m) related to existing and potential contaminant transport of the vadose zone, and the presence of substantial quantities of fine (Rousseau et al., 2004). It follows the series of USGS sediments that can retard contaminant transport both hydraulically and chemically. Most characteristics, however, have both favorable Reports on the INEEL subsurface by Barraclough and and unfavorable aspects. For example, preferential flow, as promoted ...