Installation of surface levees and subsurface slurry walls around a 1600-m by 950-m hazardous waste landfill in southeastern Louisiana has inadvertently converted the site into a large-scale permeameter. Differences in water levels in wells screened above and below a 15-m-thick clay ''confining layer'' define a vertical hydraulic gradient of +0.1. Basic climatological data permit calculation of a complete water budget for the site, including vertical recharge qz down through the clay. Cumulative precipitation over a 44-month period was 5.5 m, and recharge 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 magnitude higher than laboratory values for the same sediment. Intercalated sands and zones of pedogenic secondary porosity and fracturing developed during periods of subaerial weathering are apparently the dominant controls on vertical permeability, not the matrix properties of the clay. ¿ American Geophysical Union 1993