Water Behavior in Layered Porous Media with Discrete Flow Channels: Results of a Large-Scale Experiment

458 Small-scale physical models have been used for almost two centuries to investigate a wide range of large-scale subsurface processes (Hall, 1815; Daubrée, 1879; Cadell, 1888). In most cases, the experiments were not analyzed quantitatively, and the purported similarities between the model system and the fullscale system of interest are based on qualitative and/or visual characteristics alone. In many investigations, the characteristic length and time scales for the model system differ from those associated with the full-scale system by many orders of magnitude. If a model system is to accurately reproduce the behavior of the full-scale system, then the important dimensionless ratios associated with the model system must match those in the fullscale system (Buckingham, 1914; Hubbert, 1937; Langhaar, 1951; Sedov, 1993). In practice, it is not possible to construct a properly scaled model for a complex subsurface system because the limited range of material properties and practical experimental conditions do not allow all of the important dimensionless ratios to be matched. In many cases, the full-scale system is too complex for the effects of differences between the dimensionless ratios in the full-scale system and the model system to be assessed. These diffi culties can be alleviated if the scale of the experimental system is increased. A related issue is that structures spanning a very wide range of length scales and processes taking place on a wide range of time scales often contribute to the behavior of a complex system, and it is not possible to construct a small-scale (laboratory bench-top) physical model that covers a correspondingly wide range of length and time scales. Our interest in larger-scale experiments, which we refer to as “mesoscale experiments,” is motivated by the need to better understand and predict the behavior of fl uids in the subsurface and the concomitant transport of dissolved compounds and colloidal solids. In particular, we are interested in the behavior of organic, inorganic, and radioactive contaminants in the subsurface. The contamination of the subsurface by a variety of toxic substances is a widespread and important problem that threatens water quality and human health. The nature of the contaminated subsurface varies enormously from site to site, and, to a large extent, contaminated sites must be treated on an individual basis. For example, radioactive, organic (nonaqueous phase liquid—NAPL), and mixed wastes have been buried at shallow depths in surface sediments at the Idaho National Laboratory (INL) Site. There are indications that radionuclides and organic compounds from the buried wastes have penetrated through the deep (up to 200-m thick) vadose zone to the underlying Snake River aquifer (Holdren et al., 2002; Koeppen et al., 2004; Water Behavior in Layered Porous Media with Discrete Flow Channels: Results of a Large-Scale Experiment