Water balance measurements and computer simulations of landfill covers.

摘要

Six large-scale landfill test covers were constructed and monitored for water balance from May 1997 through June 2002. Two of the covers were used as U.S. EPA standard baseline prototypes for comparison: one that met minimum requirements set forth for municipal landfills (RCRA Subtitle D Cover) and the other meeting minimum requirements set forth for hazardous waste landfills (RCRA Subtitle C Cover). Four alternative covers were then constructed side-by-side with the baseline covers to enable direct comparison under the same ambient conditions. The first alternative cover featured a geosynthetic clay liner (GCL) designed for low saturated hydraulic conductivity. The remaining three covers were designed specifically for optimal performance in dry environments; specifically, they were designed to take advantage of the storage capacity of the cover and maximize removal of water via evapotranspiration (ET). Two of the dry environment alternative landfill covers featured capillary barriers within their profiles while the last cover consisted of a simple monolithic soil cover, referred to as an ET Cover.; The covers' water balance and vegetation aspects were measured from May 1, 1997 through June 30, 2002. Using flux to compare performance among the six covers, the ET Cover and capillary barriers performed very well, as did the Subtitle C Cover. The Subtitle D Cover was the worst performing cover (i.e. experienced the greatest water flux), while the GCL Cover had a flux less than the Subtitle D Cover but greater than the other covers. Field measurements revealed that all subsurface water flow occurred as unsaturated flow with preferential flow contributing significantly to each cover's total flux.; Two common water balance computer programs (Hydrologic Evaluation of Landfill Performance and UNSAT-H) were evaluated for their applicability as a design tool for landfill covers and for their accurate prediction of a cover's flux. The first evaluation involved using input parameters that simulated a typical design process; the second involved using soil hydraulic properties measured on the covers in their initial or as-built condition along with actual vegetation and weather measurements made on-site; and the final evaluation involved the use of soil hydraulic properties measured on the covers at the end of the monitoring period along with actual vegetation and weather data as input parameters. Neither program predicted flux through these landfill cover profiles with the accuracy desired by regulators and design engineers.