THE FORGOTTEN ANISOTROPY: IS THERE SCALE-DEPENDENCY FOR PLUME MIGRATION IN THE HORIZONTAL PLANE?

Vertical anisotropy,(Kh/Kv or Kx-y/Kz), is part of the body of common wisdom in hydrogeology. The Kh/Kz defines aquifer confinement and the degree to which flow systems become differentiated into regional, intermediate and local scales. In contrast, less is known about the extent to which anisotropy in the horizontal plane (X-Y) modifies flow-path directions. Detailed local tracer tests show that at the local scale (< 200 m), horizontal anisotropy and preferential flow paths commonly refract plume paths from expected trajectories wherever aquifers do not approximate homogenous and isotropic conditions (e.g. well studied sand and gravel environments).

We propose from a combination of measured plume geometries and heuristic computer simulations that plume trajectories in the horizontal plane may be scale-dependent. Analogous to the concept of representative elemental volume (REV), there may be a representative elemental distance(RED) only after which it can be assumed that contaminant plumes travel normal to the dominant hydraulic gradient. The RED probably is larger than the length scale of the most permeable structural or depositional landforms relevant to the hydrogeologic setting, such as; point bar deposits associated with fluvial environments, ice contact deposits associated with glacial moraines, and infrastructural discontinuities associated with filled areas. Since the aspect ratio of many contaminant plumes can be large (length/width) and plumes can be very narrow, knowing the RED could be critical to determine actual plume paths in settings where heterogeneity in the X-Y plane is expected or the norm.