Greg Ruskauff, PHg, and Tiraz BIrdie, PhD, both Senior Hydrogeologists in INTERA's Las Vegas, Nevada project office supporting environmental restoration of the Nevada Test Site, will speak at the 2009 Devils Hole Workshop being held May 6th and 7th at the Death Valley National Park Visitor Center in Devils Hole, Nevada. Additional details on their individual presentations are provided below. Greg Ruskauff, PHg, will speak on Model Uncertainty Approaches—Frenchman Flat, Nevada. Underground nuclear testing via deep vertical shafts was conducted at the Nevada Test Site (NTS) from 1951 until 1992. The Frenchman Flat area of the NTS was used for seven years. The Undergound Testing Area (UGTA) Project is currently conducting correction action investigations to ensure the protection of the public and environment. Traditionally, the focus of uncertainty analysis in groundwater modeling has been on model parameters. However, significant uncertainties may also arise due to incomplete understanding of the subsurface geologic framework and other conceptual uncertainties. There is growing understanding that the modeling paradigm should be expanded to include more than one plausible conceptual model of the system (e.g., Neuman and Wierenga, 2003). In Frenchman Flat, this issue was directly addressed by developing multiple possible models of the geologic framework that are then tested for their ability to match observed hydrologic conditions. These different framework models were combined with transport parameter uncertainty to produce one aspect of the uncertainty analysis. Another component considered the flow model parameter uncertainty for a given framework using calibration-constrained null-space Monte Carlo analysis. This suite of flow models was combined with transport parameter uncertainty to provide a contrasting view of uncertainty. Initial resuls suggest that the uncertainties are comparable. Tiraz Birdie, PhD will speak on the Use of Geothermal Data to Identify Groundwater Flow Paths at the Nevada Test Site. Yucca Flat is one of five Corrective Action Units (CAU) at the Nevada Test Site. Beteween 1957 and 1992, a total of 747 underground nuclear detonations were conducted at this CAU. The U.S. Department of Energy is currently developing a computer simulation model in order to establish the 1,000-yr contaminant boundary at the site. Due to limited water level data, it is difficult to precisely determine the direction and rate of groundwater flow at the site. An in depth analysis of geothermal data suggests that temperature profiles in wells have the potential to enhance our understanding of groundwater flow patterns (and rates) in the subsurface, which could be used to reduce predicitive uncertanties in the computer models. Yucca Flat is within the Basin and Range physiographic province of southern Nevada. Three normal faults, which entend to the top of the basement rock (at a depth of approximately five km), influence the direction of groundwater flow in the area. Any potential contamination at the site is expected to exit Yucca Flat via these faults. Groundwater movement in the faults, capture and eventually transport geothermal heat out of Yucca Flat. As a consequence, a cool thermal trough has developed in the vicinity of the faults. Computer simulation with heat conduction models indicate that the observed temperatures in the vicinity of the major faults are 25oC cooler than would occur without groundwater flow. This information is being used to constrain the Yucca Flat groundwater model, and estimated flow rates in the subsurface.