Considerable research has shown that the major control on the transport and fate of a pollutant as it moves through an aquifer is the spatial distribution of hydraulic conductivity. Although chemical and microbial processes play important roles, their influence cannot be understood without a detailed knowledge of the subsurface variations in hydraulic conductivity at a site. Many theories have been developed to quantify, in a generic sense, the influence of these variations using stochastic processes or fractal representations. It is increasingly apparent, however, that site-specific features of the hydraulic conductivity distribution (such as high conductivity zones) need to be quantified to reliably predict contaminant movement. Conventional hydraulic field techniques only provide information of a highly averaged nature or information restricted to the immediate vicinity of the test well. The objective of this project was to develop and improve field techniques for defining the three-dimensional (3D) spatial distribution of hydraulic conductivity by using hydraulic tomography coupled with high-resolution slug testing.
High-resolution slug testing allows the vertical distribution of hydraulic conductivity near an observation well to be delineated. This method was combined with another innovative method for investigating the hydraulic conductivity distribution between wells, called hydraulic tomography. The phase and amplitude through space of an oscillating signal is measured in order to estimate the hydraulic conductivity distribution of the material through which it traveled. Slug test data are most accurate near the tested well and should probably not be extrapolated blindly between wells. Together, slug testing and hydraulic tomography should be more powerful than either one used in isolation and should provide the best opportunity to characterize the hydraulic conductivity in situ by a direct measure of water flow, as an alternative to indirect methods using geophysical techniques.
A theoretical and numerical framework was developed to describe the propagation of a sinusoidal signal. The distinct advantage of using a sinusoidally varying source is that the intrinsic response of the geologic material is measured directly by the phase and the amplitude of the received sine wave. Two types of source geometries were used: the whole well line source and the isolated point source. The line source introduces a greater amount of energy and therefore has a greater propagation distance than the point source. However, the point source geometry allows for a better vertical resolution of the aquifer characteristics. Applying the theory to a real study site required the development of new equipment and field procedures. To validate the processing procedures, the finalized data was compared to other geophysical measurements (electrical conductivity and high-resolution slug testing) at the test site.
This project developed techniques to map 3D hydraulic conductivity distributions, eliminating considerable computer resources and time required by traditional approaches. The incorporation of a more realistic representation of the hydraulic conductivity distribution into a site model will lead to more reliable risk assessment and remediation plans.