EFFECTIVE HYDRAULIC CONDUCTIVITY OF FRACTURED CLAY BEDS AT A HAZARDOUS-WASTE LANDFILL, LOUISIANA GULF-COAST

Installation of surface levees and subsurface slurry walls around a 16 00-m by 950-m hazardous waste landfill in southeastern Louisiana has i nadvertently converted the site into a large-scale permeameter. Differ ences in water levels in wells screened above and below a 15-m-thick c lay ''confining layer'' define a vertical hydraulic gradient of +0.1. Basic climatological data permit calculation of a complete water budge t for the site, including vertical recharge q(z) down through the clay . Cumulative precipitation over a 44-month period was 5.5 m, and recha rge was over 1.0 m. The calculated vertical hydraulic conductivity of the clay sequence is approximately 10(-5) cm s-1, up to 4 orders of ma gnitude higher than laboratory values for the same sediment. Intercala ted sands and zones of pedogenic secondary porosity and fracturing dev eloped during periods of subaerial weathering are apparently the domin ant controls on vertical permeability, not the matrix properties of th e clay.