SUBSURFACE GEOLOGY OF LOUISIANA HAZARDOUS-WASTE LANDFILLS - A CASE-STUDY

Many hazardous waste sites in the south Louisiana Gulf Coast have been emplaced in sediments of Plio-Pleistocene to Recent age. Because of t he fining upward nature of these regressive-transgressive fluvial-delt aic sequences and the purported confining capabilities of the shallow clay layers within them, this area would seem to be ideal for the loca tion of surface waste landfills. However, detailed geologic mapping at a site in southeastern Louisiana documents how the three-dimensional distribution of sediment types and early diagenetic features, both of which were ultimately controlled by depositional history, can increase effective vertical permeability of fine-grained sequences. Many bodie s of sand that appear to be isolated in standard geotechnical cross se ctions can be shown to be part of spatially complex three-dimensional distributary networks, with fine-grained sediments representing overba nk and backswamp deposits. Some clay layers are actually a composite o f thinner clay beds, each subjected to subaerial exposure and the deve lopment of secondary porosity related to soil formation. There has bee n documented leakage of wastes down through the clays, and a recent st udy indicates that the effective vertical hydraulic conductivity of th e clay layers exceeds 10(-5) cm s(-1), or from one to four orders of m agnitude higher than values measured on samples-from cores of the same sediment. An understanding of the depositional framework, facies arch itecture, and diagenetic history of geologic materials underlying wast e disposal sites in Louisiana is required for rational development of monitoring and remediation plans.