Abstract: Subsurface Geology of a Hazardous Waste Landfill, Louisiana Gulf Coast

Jeffrey S. Hanor

A detailed study of the subsurface geology of a 1600 m by 950 m hazardous-waste landfill site in southeast Louisiana, based on 425 borehole logs, documents the degree of geologic complexity that can exist at such sites and how geologic heterogeneity enhances effective vertical permeability and rates of solute transport through clay-dominated confining layers. Instead of being situated on a clay island, as described in the permit application, the site has actually been emplaced in two vertically stacked fluvial units. The lower unit consists of a 25 m-thick, fining-upward, regressive-transgressive sequence of sands and gravels, silts, and clays. Elongate sand bodies extend offsite at all depths. The deposition of the clay-dominated upper part of this sequence was punctuated by episodes of subaerial exposure and the formation of zones of pedogenic secondary porosity, including fracturing. These sediments were exposed and eroded to depths of up to 5 m during the last eustatic low stand of sea level, and a younger 5 to 10 m thick sand- and silt-dominated fluvial sequence was deposited on top.

It is possible to determine from hydrologic data for the site that the clay-dominated layer has an effective vertical permeability of approximately 10^-14 sq m, or from one to four orders of magnitude higher than laboratory permeabilities measured on cores of the same sediment. Intercalated sands and zones of secondary porosity are apparently the dominant controls on vertical permeability, not the matrix properties of the clay.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994