systematic field-based testing program of hydraulic conductivity and dispersivity over a range in scale

In a small watershed underlain by relatively homogeneous (nonlayered), sandy, glacial outwash, hydraulic conductivity (K) and longitudinal dispersivity (α L) were systematically measured over a range in scale. Test methods employed in the investigation are conventional and/or based on accepted principles, measurement scales are those typically encountered in applied field problems, and the hydrogeologic setting is typical of large areas within the northern hemisphere and similar to many alluvial environments worldwide. The results show that K measured under radial flow is scale-dependent up to a radius of influence (r i) of approximately 15 m but reaches an asymptotic value above this threshold. This asymptotic value of K is consistent with results obtained at the regional scale (~10⁴ m) for the same aquifer using non-radial three-dimensional techniques; the initial increase appears to reflect increasing dimensionality of the radial tests with larger r i. Although α L was evaluated over a much smaller interval relative to K, α L proved to be independent of scale over the full range tested, from 2 to 29 m, an unexpected result. The findings support the idea that there is, as yet, no predictable relationship between scale effects in K and α L at scales normally encountered in applied field hydrogeology.