FIELD PERFORMANCE OF THREE COMPACTED CLAY LANDFILL COVERS

A study was conducted at sites in sub-tropical Georgia, seasonal and humid Iowa, and arid southeastern California to evaluate the field hydrology of compacted clay covers for final closure of landfills. Water balance of the covers was monitored with large (10 m x 20 m), instrumented drainage lysimeters for 2-4 yr. Initial drainage at the Iowa and California sites was less than that corresponding to the regulatory standard of saturated hydraulic conductivity for clay barriers (10^-7 cm/s); initial drainage rate at the Georgia site was more than twice the regulatory standard. At all sites the drainage exceeded the regulatory standard by the end of the monitoring period and increased by factors ranging from 100-750. The drainage rates developed a rapid response to precipitation events, suggesting that increases in drainage rate were the result of preferential flow. Field measurements of water content and temperature at all three sites suggested that desiccation and/or freeze-thaw cycling probably resulted in formation of preferential flow paths through the barrier layers. After four years, the clay barrier at the Georgia site was excavated and examined for changes in soil structure and hydraulic conductivity. In situ and laboratory tests indicated that the hydraulic conductivity increased approximately three orders of magnitude (from ~10^-7 to ~10^-4 cm/s) during the service life. A dye tracer test confirmed the presence of preferential flow paths and soil structure analysis showed that extensive cracking and root development occurred throughout the entire depth of the barrier layer. Data from all three sites showed the effectiveness of all three covers as hydraulic barriers diminished during the 2-4 yr monitoring period, which was short compared to the required design life (often 30 yr) of most waste containment facilities.