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

water flow for the Bandelier tuff. These tuffs are multiple-flow ash-flow sheets that are typically the first A previously reported injection zone test in the Tshirege Member unit exposed on the mesas and most canyons of the of the Bandelier tuff has been interpreted and modeled using a variety of conceptual models ranging from a single continuum model (SCM) Pajarito Plateau. There are various degrees of welding to a dualpermeability model (DKM). The agreementof the numerical within the tuff units, and many of the subunits contain models and the data was quite acceptable, both qualitatively and fractures that must be considered as possible fast pathquantitatively. The migration of water injected in an open interval ways.Ifwaterpercolatesthroughtheporousandpermeof the wellbore exhibited flow that was controlled by gravity and, to ability rock matrix, then the Bandelier tuff may provide a lesser extent, capillary forces. A sensitivity analysis showed that a significant barrier to contaminant migration. Percolathe behavior is well captured with laboratory determined hydrologic tion rates on the Pajarito Plateau have been estimated parameters and less so if rocks with higher capillary suction are simuto be in the range of several hundred millimeters per lated. More complex model formulations that explicitly account for year beneath wet canyons such as Los Alamos Canyon fractures were also tested, but it was found that these models reproduce the field behavior only when parameters are selected that mini- (Robinson et al., 2005). Under these high-infiltration mize the role of fractures. Therefore, the SCM is favored due to its conditions, if fractures flow, transport velocities are simpler formulation and ease with which it is implemented in large- likely to be much more rapid in the vadose zone than scale flow and transport simulations. Discrete fracture simulations are if the water percolates through the rock matrix. For used to illustrate that fractures are predicted to play a minor role in this rock to attenuate the infiltration of contaminated most subunits of the Bandelier tuff: high matrix permeability allows groundwater, it must be demonstrated that water percowater entering fractures to quickly imbibe into the matrix, thereby lates through the porous rock matrix rather than chanreducing the tendency of these rocks to exhibit preferential fracture neling through fractures. flow. In contrast, water flow through Basalts present on the Pajarito Plateau is predicted to be dominated by fractures. The issue of flow in unsaturated, fractured tuff has been explored both theoretically and experimentally. Nitao and Buscheck (1991) examined an idealized system of parallel, equally spaced fractures with water enR isk investigations, development of monitoring protering at the top and migrating downward under gravity. grams, or strategies for remediation of contaminatedgroundwaterrequireaquantitativeunderstanding They showed that for rocks with large porosities and of the relevant groundwater flow and transport pro- permeabilities, there is a strong driving force for water cesses. This understanding is usually obtained through to quickly imbibe into the rock matrix. Under these numerical models of fluid flow and contaminant trans- conditions, the water front is quickly attenuated, and in port. At Los Alamos National Laboratory (LANL) the limit, the downward-migrating wetting front essenmany released contaminants are present in fluids still tially behaves as though the fractures are not present. residing in the unsaturated zone. Therefore, attaining a Experimentally, Robinson and Bussod (2000) prepredictive capability for flow and transport behavior in sentedunsaturatedfluidflowandtracertestresultsfrom the unsaturated zone is critical to the success of models. an experimental facility in tuffs with similar hydrologic Characterization of the subsurface hydrology and trans- properties as the Bandelier tuff. These injection tests port in the unsaturated zone to the degree needed to indicated that the presence of a fracture in the nondeveloprealisticflowandtransportmodelsisachalleng- welded, permeable Calico Hills tuff near Yucca Mouning activity, given the wide range of recharge rates, the tain, Nebraska, appearedto have no effect onthe migrapresence of perched water, and the introduction of a tion of water and tracer away from the injection point. host of contaminants of different chemical properties. Tseng et al. (2003) demonstrated that a SCM formulaAs a step toward developing a predictive tool to simu- tion could provide a good match to this field data, late contaminant migration at LANL, this study exam- though scale issues were apparent that could not be inesdifferentconceptual modelsofunsaturatedground- readily resolved. In contrast, fast flow through fractures has also been clearly demonstrated in unsaturated, fractured tuffs (e.g., Birdsell et al., 2005). For example,