The Role of Depositional Facies in Contaminant Migration at Louisiana Coastal Plain Hazardous Waste Sites

Jeffrey S. Hanor

Many of the numerous surface hazardous waste sites and landfills in the south Louisiana Gulf Coast are situated on three regional sedimentary units of Plio-Pleistocene age referred to in decreasing age and outcrop distance from the present coast line as the Upland Complex (Nebraskan-Aftonian), Intermediate Complex (Kansan-Yarmouthian) and Prairie Complex (Early Wisconsin). Each of these units represents the proximal portion of a regressive-transgressive sequence which developed in response to eustatic changes in sea level, subsidence, and variations in sediment supply.

Detailed subsurface mapping of several hazardous waste sites in the region demonstrates how complex the internal distribution of depositional facies and the permeability architecture can be within these units on the 10¹ to 10³ m scale. Many sand-rich and clay-rich zones which are represented as horizontal monolithic layers on typical geotechnical diagrams can be shown to be composite bodies of sediment of complexly-varying geometry. At one such site, dense contaminated aqueous fluids are moving down dip along the base of two sands in directions opposite to those predicted by consultants from water levels in monitoring wells. The deposition of the clay-dominated transgressive upper portions of these units was typically punctuated by periods of subaerial exposure a d the formation of zones of pedogenic secondary porosity. This has increased the hydraulic conductivity of these clays and has permitted rapid downward migration of contaminants into underlying sands. Failure to understand the depositional history of these sites has lead to erroneous and costly strategies in site location and waste monitoring, containment, and remediation.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995