Surface Water–Groundwater Connection at the Los Alamos Canyon Weir Site: Part 2. Modeling of Tracer Test Results

Abstract

Field observations of bromide transport in the unsaturated zone are used to constrain simulations that provide estimates of bulk porosity and permeability for the Cerros del Rio. The Cerros del Rio basalt is of particular interest because it underlies many of the potential waste sites at the Los Alamos National Laboratory. A highly simplified model is able to capture the general behavior of the breakthrough data. The simplifying assumption is that the basalt can be modeled as a homogeneous continuum with high permeability and low porosity. We estimate that the permeability of the bulk rock is 10−11 to 10−12 m2, whereas the porosity is estimated to lie between 0.001 and 0.01. The porosity estimates from this study are particularly useful for kilometer‐scale simulations that include flow and transport through the Cerros del Rio basalt because estimates based on other methods, such as core testing, are highly scale dependent and should not be extrapolated to larger scales. Although this model does not include the complex physics of flow in the fractured basalt, it is useful for simulations on the kilometer scale that require averaging of rock properties and optimization of computational speed. The porosity and permeability values obtained from this analysis will help to weight probability distributions used in the kilometer‐scale simulations of contaminant transport.