Multiscale Hydraulic Conductivity Characterization in a Fractured Granitic Aquifer: The Evaluation of Scale Effect

Abstract

We characterized horizontal hydraulic conductivity (K) of a fractured granitic aquifer using single- and cross-hole hydraulic tests to evaluate “scale effect.” For selected boreholes, K estimates were obtained using single-hole FLUTe liner and slug tests. Several cross-hole pumping tests were carried out at various durations. Drawdown responses were first interpreted using analytical well-test solutions to obtain an effective horizontal conductivity (Keff) assuming a homogeneous and infinite aquifer. The same drawdowns were then numerically inverted using transient hydraulic tomography (THT) to delineate spatial distributions of K and storativity in the area encompassing the boreholes. Papadopulos (1965) and a nonlinear least squares minimization method produced a similar principal Keff direction that is consistent with the dominant fracture strike observed from outcrop and borehole televiewer data. However, principal direction and magnitude of this Keff depend on the pumping test duration and the number of monitoring boreholes used in the interpretation. As a group, K obtained from cross-hole tests is larger than that obtained from single-hole tests. However, because cross-hole tests stressed the aquifer at both interwell and larger scales, Keff obtained from interpreting cross-hole data is observed to decrease with pumping time, likely due to the dominance of less permeable fractures at larger scale. This lateral reduction of mean K is also revealed by THT as low K zones surrounding the test boreholes. Overall, K is found to increase from single-hole to the interwell scale and then decrease at larger scale, exhibiting a non-monotonic scale effect. © 2021. American Geophysical Union. All Rights Reserved.