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U.S. Geological Survey Open-File Report 2009–1175

Prepared in cooperation with the U.S. Department of Energy, National Nuclear Security Administration, Nevada Site Office under Interagency Agreement DE-AI52-07NV28100.

Estimation of Unsaturated Zone Traveltimes for Rainier Mesa and Shoshone Mountain, Nevada Test Site, Nevada, Using a Source-Responsive Preferential-Flow Model

By Brian A. Ebel and John R. Nimmo

Abstract

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Traveltimes for contaminant transport by water from a point in the unsaturated zone to the saturated zone are a concern at Rainier Mesa and Shoshone Mountain in the Nevada Test Site, Nevada. Where nuclear tests were conducted in the unsaturated zone, contaminants must traverse hundreds of meters of variably saturated rock before they enter the saturated zone in the carbonate rock, where the regional groundwater system has the potential to carry them substantial distances to a location of concern. The unsaturated-zone portion of the contaminant transport path may cause a significant delay, in addition to the time required to travel within the saturated zone, and thus may be important in the overall evaluation of the potential hazard from contamination.

Downward contaminant transport through the unsaturated zone occurs through various processes and pathways; this can lead to a broad distribution of contaminant traveltimes, including exceedingly slow and unexpectedly fast extremes. Though the bulk of mobile contaminant arrives between the time-scale end members, the fastest contaminant transport speed, in other words the speed determined by the combination of possible processes and pathways that would bring a measureable quantity of contaminant to the aquifer in the shortest time, carries particular regulatory significance because of its relevance in formulating the most conservative hazard-prevention scenarios.

Unsaturated-zone flow is usually modeled as a diffusive process responding to gravity and pressure gradients as mediated by the unsaturated hydraulic properties of the materials traversed. The mathematical formulation of the diffuse-flow concept is known as Richards' equation, which when coupled to a solute transport equation, such as the advection-dispersion equation, provides a framework to simulate contaminant migration in the unsaturated zone. In recent decades awareness has increased that much fluid flow and contaminant transport within the unsaturated zone takes place as preferential flow, faster than would be predicted by the coupled Richards' and advection-dispersion equations with hydraulic properties estimated by traditional means. At present the hydrologic community has not achieved consensus as to whether a modification of Richards' equation, or a fundamentally different formulation, would best quantify preferential flow.

Where the fastest contaminant transport speed is what needs to be estimated, there is the possibility of simplification of the evaluation process. One way of doing so is by a two-step process in which the first step is to evaluate whether significant preferential flow and solute transport is possible for the media and conditions of concern. The second step is to carry out (a) a basic Richards' and advection-dispersion equation analysis if it is concluded that preferential flow is not possible or (b) an analysis that considers only the fastest possible preferential-flow processes, if preferential flow is possible. For the preferential-flow situation, a recently published model describable as a Source-Responsive Preferential-Flow (SRPF) model is an easily applied option. This report documents the application of this two-step process to flow through the thick unsaturated zones of Rainier Mesa and Shoshone Mountain in the Nevada Test Site.

Application of the SRPF model involves distinguishing between continuous and intermittent water supply to preferential flow paths. At Rainier Mesa and Shoshone Mountain this issue is complicated by the fact that contaminant travel begins at a location deep in the subsurface, where there may be perched water that may or may not act like a continuous supply, depending on such features as the connectedness of fractures and the nature of impeding layers. We have treated this situation by hypothesizing both continuous and intermittent scenarios for contaminant transport to the carbonate aquifer and reporting estimation of the fastest speed for both of these end members.

For Rainier Mesa our analysis suggests that preferential flow is possible through the entire unsaturated zone, which implies that the SRPF model can be applied to estimate the fastest radionuclide traveltimes down to the carbonate aquifer. SRFP model estimates for fastest travel times at Rainier Mesa are tens to hundreds of years for intermittently supplied preferential paths, as may be likely for contamination from those working points and tunnel inverts where there is no continuous discharge of water in bulk. The estimates at Rainier Mesa are approximately one month for continuously supplied preferential paths, as may be likely for contamination from working points and tunnel inverts with continuous discharge, tunnel effluent ponds, or sealed tunnel inverts. The SRPF model traveltimes at Shoshone Mountain for intermittently supplied preferential paths, considered likely for all working points and tunnel inverts at that site, are hundreds of years. The presence of a thick layer of siliceous rock under Shoshone Mountain may interrupt all preferential flow paths before they reach the carbonate aquifer, in which case it would increase estimated traveltimes to more than a thousand years. Even the fastest of these SRPF contaminant transport times may not imply serious potential for radionuclide contamination of the regional flow system beneath Rainier Mesa. This is because of the potential hydraulic disconnect between the saturated zone in the upper carbonate aquifer and the lower carbonate aquifer that forms the regional groundwater flow system. The application of the SRPF model to Rainier Mesa and Shoshone Mountain emphasizes the importance of radionuclide sources that are located in the vicinity of both continuously supplied water and preferential flow paths.

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Suggested citation:

Ebel, B.A., and Nimmo, J.R., 2009, Estimation of unsaturated zone traveltimes for Rainier Mesa and Shoshone Mountain, Nevada Test Site, Nevada, using a source-responsive preferential-flow model: U.S. Geological Survey Open-File Report 2009–1175, 74 p.



Contents

Abstract

Introduction

Rainier Mesa and Shoshone Mountain

Conceptual Flow Model for Rainier Mesa and Shoshone Mountain

Potential Sources for Ground-Water Contamination by Radionuclides at Rainier Mesa and Shoshone Mountain

Traveltime Estimation Method

Preferential Flow Traveltime Estimates

Discussion

Summary

Acknowledgements

Appendix A. Estimated Traveltimes for All Working Points at Rainier Mesa and Shoshone Mountain.

References Cited


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