THE SPATIAL DISTRIBUTION OF PERMEABILITY IN DEPOSITIONAL SEDIMENTARY STRUCTURES

Ground water flow and contaminant transport can be controlled by spatial variations in permeability over distances of cms to dms. In granular sediments, such variations are often associated with depositional sedimentary structures, which cause slight variations in grain size, size distribution, shape, and alignment. The goal of this study was to explore the relationship between the spatial distribution of permeability and sedimentary structures. Four different types of sedimentary structures (dunes, plane beds, ripples, and a delta) were created at Binghamton University using medium sand in separate runs of a recirculating flume with a constant sediment feed. Box cores were extracted from the sediments. Once dried, permeability was measured on a 2 cm grid over the exposed faces of the cores using a compressed-air permeameter. The spatial distribution of permeability was described for different planes (vertical, parallel to flow; horizontal, parallel to flow; and vertical, perpendicular to flow) by the mean length of zones of high and low permeability and by computing semivariograms. The sedimentary structures all have different patterns of permeability variations. The ranges of semivariograms vary in different directions for a given type of sedimentary structure, showing that permeability within sedimentary structures is anisotropic. For ripples and dunes, the mean length of cross strata sets does not compare well with the range of semivariograms or the wavelength of periodicity in the semivariograms. Therefore, it is not clear what features of the sedimentary structures control the spatial distribution of permeability.