587-601 GW S-O 03 color

The generalized conceptual model for a fractured bedrock aquifer consists of low-permeability matrix blocks separated by highly permeable fractures that act as preferential pathways for ground water flow. The National Research Council (NRC 1996) summarizes three approaches for simulating ground water flow in fractured aquifers. The most common approach is to represent the system with an equivalent continuum model, often referred to as an equivalent porous media (EPM) model. With this approach, equivalent continuum properties assigned to model cells represent the combined effects of individual fractures and the rock matrix. The discrete fracture model, or discrete fracture network model, is the second approach used to simulate ground water flow in fractured rocks. With this approach, flow is explicitly simulated in each fracture using, for example, solutions to the Navier-Stokes equation (Bear 1993), Kirchoff’s laws for electrical circuits (Kraemer and Haitjema 1989), or hydraulically connected circular discs (Cacas et al. 1990a, 1990b). The third approach is a hybrid method that uses discrete fracture models to estimate effective properties for continuum approximations. Regardless of the approach, accurate representation of dominant fractures probably is more important than model selection (NRC 1996; Selroos et al. 2002). The use of continuum models to directly simulate flow within individual fractures may be considered as another approach for representing fracture flow. While these continuum models are not considered true discrete fracture flow models, they can be used to explicitly simulate flow within individual fractures. Fractures and densely fractured flow zones can be represented in continuum models by adding zones of increased hydraulic conductivity in the appropriate orientation. Rayne et al. (2001) demonstrate that subhorizontal fracture zones can be represented with a continuum model by using thin, highly permeable layers that follow the depths of the mapped fracture zones. Eaton Abstract A method is presented for incorporating the hydraulic effects of vertical fracture zones into two-dimensional cellbased continuum models of ground water flow and particle tracking. High hydraulic conductivity features are used in the model to represent fracture zones. For fracture zones that are not coincident with model rows or columns, an adjustment is required for the hydraulic conductivity value entered into the model cells to compensate for the longer flowpath through the model grid. A similar adjustment is also required for simulated travel times through model cells. A travel time error of less than 8% can occur for particles moving through fractures with certain orientations. The fracture zone continuum model uses stochastically generated fracture zone networks and Monte Carlo analysis to quantify uncertainties with simulated advective travel times. An approach is also presented for converting an equivalent continuum model into a fracture zone continuum model by establishing the contribution of matrix block transmissivity to the bulk transmissivity of the aquifer. The methods are used for a case study in west-central Florida to quantify advective travel times from a potential wetland rehydration site to a municipal supply wellfield. Uncertainties in advective travel times are assumed to result from the presence of vertical fracture zones, commonly observed on aerial photographs as photolineaments.