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

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 (K_eff) 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 K_eff direction that is consistent with the dominant fracture strike observed from outcrop and borehole televiewer data. However, principal direction and magnitude of this K_eff 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, K_eff 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.