Hydrolo•7y Unit, Idaho National En•7ineerin•7 Laboratory, Idaho FallsA series of laboratory tests was conducted to determine routing criteria for streamlines through fracture junctions. These tests showed that two criteria are all that is necessary to route streamlines through any two-dimensional junction under laminar flow conditions. These criteria are (1) that streamlines do not cross and (2) that flow along adjacent streamlines must be in the same direction. Using these two criteria, a unique distribution of streamlines can be determined for both continuous and discontinuous fracture junctions.

Physical and numerical fracture network models have been used to analyze the transport of conservative solutes in systems of parallel‐sided fractures. The processes controlling dispersion in fracture systems that are explicitly simulated by the numerical model are (1) development of a velocity profile within individual fractures, (2) transverse molecular diffusion between streamlines, both within fractures and at fracture junctions, and (3) advection with the bulk fluid through a system of fractures with a range of hydraulic gradients and apertures. The first two processes, referred to as microdispersion, are often assumed to be secondary to the third, referred to as macrodispersion. The validity of this assumption is, however, highly dependent on the hydrodynamics of the system under consideration. Data collected from a physical model of a fracture network are used to validate a numerical model that explicitly simulates all three transport processes. The numerical model is then used to evaluate the relevance of microdispersion processes in a system where macrodispersion is significant.

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The Raft River Geothermal Site has been evaluated over the past eight years by the United States Geological Survey and the Idaho National Engineering Laboratory as a moderate-temperature geothermal resource. The geoscience data gathered in the drilling and testing of seven geothermal wells suggest that the Raft River thermal reservoir is: (a) produced from fractures found at the contact metamorphic zone, apparently the base of detached normal faulting from the Bridge and Horse We11 Fault zones of the Jim Sage Mountains; (b) anisotropic, with the major axis of hydraulic conductivity coincident to the Bridge Fault Zone; (c) hydraulically connected to the shallow thermal fluid of the Crook and BLM wells based upon both geochemistry and pressure response; (d) controlled by a mixture of diluted meteoric water recharging from the northwest and a saline sodium chloride water entering from the southwest. Although the hydrogeologic environment of the Raft River geothermal area is very complex and unique, it is typical of many Basin and Range systems. 1. bj ii ACKNOWLEDGMENTS This report contains information collected over seven years. Although many people contributed to making this effort successful, it is impossible to acknowledge everyone. We do wish to thank the Depart

The objective of this work was to investigate flow and transport in a layered, variably-saturated system consisting of both fractured rock and sedimentary material during focused infiltration from the surface. Two tracer tests were carried out using the Vadose Zone Research Park (VZRP) at Idaho National Laboratory (INL). The first test was conducted under quasi-steady-state conditions and the second was initiated in a much drier system and thus provided information regarding flow and transport under transient conditions. A one-dimensional analytical model was used to fit breakthrough curves resulting from the two tracer tests. The results of this modeling provide insight into the nature of flow in the fractured basalt, surficial alluvium, and sedimentary interbeds that comprise the vadose zone of the eastern Snake River Plain. Flow through the fractured basalt is focused and preferential in nature and multiple flow paths arise due to numerous fractures functioning as transmissive pathways in addition to flow splitting along geologic contacts. Flow velocities were significantly higher during the test with the wetter flow domain, presumably due to increases in hydraulic conductivity associated with higher water contents of the geologic materials. Perching was observed above the alluvium/basalt contact and above the lower boundary of a locally continuous sedimentary interbed. The perching behavior between the two contacts was fundamentally different; the perched layer above the alluvium/basalt contact was neither laterally extensive nor temporally persistent in the absence of infiltration from the surface. In contrast, the perched layer along the interbed was significantly thicker and gave rise to lateral flow over distances on the order of 100s of meters. Vertical transport is shown to occur predominantly through the main bulk of the sedimentary material of the interbed rather than through preferential pathways; lateral flow appears to occur primarily in the fractured basalt directly above the interbed.ii ACKNOWLEDGEMENTS