The Texas Disposal Systems landfill combines natural and manmade barriers to meet stringent environmental protection regulations. The base of the deep, excavated landfill floor is natural gray unweathered Taylor Clay/Shale, which is less permeable than mechanically recompacted clay. This natural base serves as the bottom liner for the landfill (except where extensive testing determines that a constructed liner of recompacted clay soils is required). This performance-based liner system also includes a leachate collection system and thick recompacted clay sidewall liners. The liner system easily meets the standards of the performance-based landfill liner system currently required by both the EPA and the TCEQ.*
The floor of the landfill is sloped toward one or more low areas within the landfill cells so the leachate (any liquid that comes from or comes into contact with the trash) can be collected and removed from the landfill. For all portions of the landfill excavated since federal EPA Subtitle D rules became effective, a leachate collection system has been, and will be, installed. The small area built prior to October 9, 1993 drains directly to the lower section of the landfill cell, which includes a leachate collection system. The leachate collection system consists of trenches filled with chipped tires or gravel around the perimeter of the landfill and spaced at regular intervals across the floor of the landfill. The design of the leachate collection system is based on time-proven empirical relationships developed for agricultural drainage. The perimeter trenches can intercept any leachate that migrates down the thick sidewall liners of the landfill installed against 1:1 slope sidewall excavations. The trenches on the floor of the landfill are designed to prevent leachate from collecting on the floor of the landfill and to channel the leachate to central collection sumps.
The landfill sidewall liners are constructed with moisture and clod sized controlled mechanically recompacted clay and are a minimum of eight feet thick. The sidewall liners separate the waste disposed at the facility from shallow groundwater, which occurs below the ground surface in the natural cracks and fissures of the weathered Taylor Clay/Shale that constitutes the surface soil and shallow subsoil at the landfill site. The design of the landfill includes excavation into the unweathered Taylor Clay/Shale so that the floor of the landfill is completely beneath the shallow groundwater. Thus, any fluids accumulating in the landfill or generated by the decomposition of the contained waste cannot escape into this shallow groundwater, even if the liner system fails, but are removed through the leachate collection system and either evaporated on-site or transported off site for treatment.
Groundwater and Methane Migration Monitoring Systems
The groundwater monitoring system is designed to intercept groundwater contaminents that might leave the site in the unlikely event of a release from the landfill. A release is unlikely because the landfill is fully lined, the leachate collection system serves the entire landfill excavation, and the floor of the landfill is below the base of the shallow groundwater, in the unweathered Taylor Clay/Shale. Additionally, the leachate collection system prevents fluid levels in the landfill from rising above the base of the shallow groundwater outside the landfill cells, the sidewall liners are a minimum of eight feet thick, and the shallow groundwater exerts an inward hydraulic pressure such that it tends to move inward toward the landfill, not outward.
The groundwater monitoring wells were sited with consideration of potential flow directions before and after landfill construction, the configuration of the floor and sidewalls of the landfill, and the configuration of the base of the shallow groundwater in relation to the weathered/unweathered Taylor Clay/Shale interface. Monitoring wells, which were installed before the federal Subtitle D rule became effective, met the new criteria. Additional groundwater monitoring wells will continue to be installed as the landfill expansion takes place and as stipulated in the TDSL permit.
Landfill gas migration monitoring wells also have been installed in accordance with the approved plan. These were installed recognizing that the potential gas migration routes will be above the shallow groundwater table. The wells are spaced at regular intervals along the property line where landfilling is occurring and has occurred, and have been installed along pipeline easements as required.
*The liner system for the TDS landfill was approved by the state, after a thorough examination and a contested case public hearing permitting process, and was designed to comply with the performance standard approach adopted by the EPA for the draft federal Subtitle D rule (Federal Register, Vol. 53, No. 168, Tuesday, August 30, 1988). The final federal rule, published on October 9, 1991 (Federal Register, Vol. 56, No.196, Wednesday, October 9, 1991), which went into effect two years later, continued to allow the performance based liner system that was an ideal match for the soil conditions at the TDS landfill and for the landfill design as permitted. The leachate collection system was added to achieve an even greater level of protection. Both the EPA and the lead state of Texas environmental agency have reviewed and approved the TDS landfill liner system. The presence of such good gray unweathered shale soil conditions in the deep excavations called for in the TDS landfill permit design requirements qualified TDS for the approval of this performance design criteria liner system approval.