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Long-term transient regional groundwater flow in a heterogeneous mature basin with large hydraulic conductivity contrasts

University of British Columbia

A finite element model is developed to solve the two-dimensional, transient, groundwater flow equation for the purpose of describing the development of regional flow over geologic time, in response to erosional modifications of the land surface, in a mature basin with large hydraulic conductivity contrasts. A simplified model of the Red Earth region, Alberta, Canada, referred to as the 'standard' model, is the working example. There are several numerical problems inherent in transient flow models. Oscillations occur at nodes at early time as they adjust to new, stressful boundary conditions; these oscillations die out at later times. Oscillations also occur at certain nodes, as head values reach steady state, if the length to depth ratios of the elements are too large; a refinement of the mesh reduces these oscillations to insignificant amounts. To simulate a model in which several million years are required to reach steady state, it is necessary to use a continuously varying time step such that the percent change at any node within the flow region does not exceed a specified tolerance level; three percent is used for most cases in this study. The finite element model is verified by comparing solutions to several problems with those of both 1-D and 2-D steady state analytical solutions and 1-D transient analytical solutions. Comparisons are excellent in all cases. A sensitivity analysis is performed on a layered system in which the values of five parameters, hydraulic conductivity, specific storage, anisotropy, basin size, and water table configuration, are slightly perturbed. Results indicate that the model is most sensitive to changes in: a) values of hydraulic conductivity of low-K layers, b) the z-direction component of hydraulic conductivity in anisotropic low-K layers, c) the geometry of the basin by changing the layer thicknesses, and d) values of specific storage in high-K layers. Hypothetical basins with a variety of stratigraphic and structural configurations and a realistic model of the Red Earth basin are simulated. Flow patterns are analyzed and compared in order to develop an understanding of the time-dependent nature of hydraulic head adjustments within a heterogeneous, mature basin. The significance of making potential errors in assuming steady state flow in a mature basin when transient conditions actually exist is discussed. Applications of this type of transient analysis include: a) exploring for hydrocarbons or mineral deposits, b) isolating radioactive wastes, c) determining safe development rates of groundwater resources, and d) dating geological events that result in changes in boundary conditions of regional groundwater flow systems.

Text

2010-06-20

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http://hdl.handle.net/2429/25879

Geological Sciences

University of British Columbia

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