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Vadose Zone Journal
Vol. 4, No: xx, 2005; Pages: 694–707

Hydrologic Behavior of Unsaturated, Fractured Tuff: Interpretation and Modeling of a Wellbore Injection Test

Bruce A. Robinson,* Stephen G. McLin, and Hari S. Viswanathan

Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, NM87545.


A previously reported injection zone test in the Tshirege Memberof the Bandelier tuff has been interpreted and modeled using a varietyof conceptual models ranging from a single continuum model (SCM)to a dual permeability model (DKM). The agreement of the numericalmodels and the data was quite acceptable, both qualitatively andquantitatively. The migration of water injected in an open intervalof the wellbore exhibited flow that was controlled by gravity and, toa lesser extent, capillary forces. A sensitivity analysis showed thatthe behavior is well captured with laboratory determined hydrologicparameters and less so if rocks with higher capillary suction are simulated. More complex model formulations that explicitly account forfractures were also tested, but it was found that these models reproduce the field behavior only when parameters are selected that minimize the role of fractures. Therefore, the SCM is favored due to itssimpler formulation and ease with which it is implemented in largescale flow and transport simulations. Discrete fracture simulations areused to illustrate that fractures are predicted to play a minor role inmost subunits of the Bandelier tuff: high matrix permeability allowswater entering fractures to quickly imbibe into the matrix, therebyreducing the tendency of these rocks to exhibit preferential fractureflow. In contrast, water flow through Basalts present on the PajaritoPlateau is predicted to be dominated by fractures.

Keywords:single continuum model;dual permeability model;transport simulations;subsurface hydrology;water percolates;Hydrologic Behavior;radionuclide.

Corresponding author:

E-mail: robinson@lanl.gov


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