Measurement of anisotropic hydraulic conductivity in unconsolidated sands: A case study from a shoreface deposit, Oyster, Virginia

In this study an integrated held and laboratory method which can be used to evaluate the magnitude and distribution of anisotropic hydraulic conductivity at the scale of bedding structures in unconsolidated sediment is demonstrated. Undisturbed sediment cores 10.16 cm in diameter and 18.25 cm long are obtained using a split-barrel core sampler. Two undisturbed 5.72 cm diameter (7.90 cm long) subcores, oriented parallel and perpendicular to the primary bedding structures, are then obtained from each field core; the hydraulic conductivities of the subcores are then measured with a constant-head permeameter. The sampling and experimental procedures are designed to minimize potential sources of error. Sixty-four 10.16 cm cores were sampled in a coastal sand outcrop located similar to 300 m from a subsurface injection experiment near Oyster, Virginia. Four distinct lithologies were sampled in horizontal transects with a spacing of 0.5 m between core locations. In addition, iron oxide bands which were prevalent throughout the exposure were sampled from both ends of the outcrop. The average hydraulic conductivity values are consistent with the observable geologic features (grain size, sorting, and iron oxide banding). Hydraulic conductivity is greater in the parallel-oriented subcore for 90% of the core pairs, and mean hydraulic conductivity ratios of parallel to perpendicular-oriented subcores (K-parallel to/K-perpendicular to) for the different lithologies ranged from 1.33 to 1.57. The major control on the anisotropy in this sediment is interpreted to be preferential grain orientations resulting from depositional processes, with additional influences from the prominence of the bedding structures and secondary features such as hydrous iron oxide banding.