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DISTRIBUTION CATEGORY UC-70Preliminary hydrologic calculations were performed to determine if choice of drift backfill could influence water flow past waste packages adjacent to a repository drift in unsaturated volcanic tuff. Additional calculations were performed to determine if water would flow into a shaft that penetrates a nonwelded-welded tuff interface. These hydrologic calculations consisted of numerical simulations using the computer code TRUST. Idealized configurations of a vertical shaft extending to the repository and a repository drift with horizontal and vertical emplacement of waste packages were evaluated. Both fine-grained and coarse-grained materials were considered as backfill to the drift and shaft. In the numerical simulations, coarse-grained backfill material drained more completely than fine-grained material and formed a more effective capillary barrier to water flow. Although the magnitude of flow in the modeled regions is small, selection of backfill was shown to influence flow through the repository drift and vertical shaft. However, it is concluded that selection of backfill does not significantly influence water flow past vertically or horizontally emplaced waste packages.

Distribution Category UC-70The uncertainty in travel tiMe for water through the unsaturated and saturated zones of Yucca Mountain and vicinity was determined by considering uncertainty associated with input raraMeters to the hydrologic MOdels of these zones. A first-order analysis was used to investigate uncertainty in water travel time throu~h the unsaturated zone at Yucca Mountain, based on an analytic solution for water flow. Results of the investigation of uncertainty for the unsaturated zone indicated that uncertainty in the percolation estimate contributed significantly more to uncertainty in travel time than uncertainty in estimates of hydraulic conductivity. Monte Carlo and first-order approaches ~ere used to investigate uncertainty in ground-water travel tiMe for different cases that varied in the treatment of the input oarameters to the hydrologic model of the unsaturated zone. Comparison of the Monte Carlo and first-order estimates of mean ground-water travel time and travel time uncertainty in the saturated zone demonstrates that the first-order aporoach underestimated both the mean and variance of travel time for all cases considered. This underestiMation suggests that the Monte Carlo approach should be used to estiMate mean travel time and its variance as well as the probability density distribution of travel time. A sensitivity study of ground-water travel time in the saturated zone indicated that the zones with smaller transmissivity produced the larqest influence in travel time for most cases. INTRODUCTIONPacific Northwest Laboratory (PNL) has been supporting Sandia National Laboratories during performance assessment activities for the Nevada Nuclear Waste Storage Investigations (NNWSI) project. Performance assessment contributions from PNL have included computer simulations of water flow and radionuclide transport at Yucca Mountain, which is located on and adjacent to the southwest corner of the Nevada Test Site. These simulations were designed to help supply technical insight into programmatic issues such as repository horizon selection, ground-water percolation, ground-water travel times, radionuclide migration projections, biosphere discharge points, repository stability, and suggested data-gathering programs.One guideline prescribed by the Department of Energy (DOE) for repository siting is the 1000-year minimum ground-water travel time determined for the site prior to waste emplacement (DOE 1984). The time is to be evaluated for any potential path of likely radionuclide travel from the disturbed zone to the accessible environment. The accessible environment may be defined as 10 kilometers (10,000 meters) from the repository boundaries or possibly the nearest potable water supply. The DOE guidelines disqualify a site if the expected ground-water travel time along any path of likely and significant radionuclide travel from the disturbed zone to the accessible environment is less than 1000 years. Determination of uncertainty in travel time will be necessary to establish the credibility of ...

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A visual selection technique can be utilized to locate potential well field sites prior to on‐site testing. The technique is based upon an understanding of the regional hydrology and an appreciation for development goals such as the identification of high‐yield and low‐impact locations. Although the uniqueness of Pasco, Pinellas, and Hillsborough Counties in Western Florida has been incorporated into the current application, the methodology, with modifications, is transferable to other geographic regions. Overlays of each criterion used in site selection are combined to form a regional composite showing favorable site locations and further serve a useful communications role as visual aids during public presentation. Implications of direct and indirect cost savings to the public are obtained when the methodology is applied to the location of municipal well fields.