Storage of any waste material in a landfill poses several potential problems. One problem is the possible contamination of soil, groundwater and surface water that may occur as leachate produced by water or liquid wastes moving into, through and out of the landfill migrates into adjacent areas. With the possibility of hazardous wastes, landfills should be designed to prevent any waste or leachate from ever moving into adjacent areas. The HELP model has been developed specifically as a tool to be used by designers and regulatory reviewers for selecting practical designs that minimize potential problems.
Leachate is described as liquid that has percolated through the layers of waste material. Thus, leachate may be composed of liquids that originate from a number of sources, including precipitation, groundwater, consolidation, initial moisture storage, and reactions associated with decomposition of waste materials. The chemical quality of leachate varies as a function of a number of factors, including the quantity produced, the original nature of the buried waste materials and the various chemical and biochemical reactions that may occur as the waste materials decompose.
In absence of evidence to the contrary, most regulatory agencies prefer to assume that any leachate produced will contaminate either ground or surface waters; in the light of the potential water quality impact of leachate contamination, this assumption appears reasonable.
The quantity of leachate produced is affected to some extent by decomposition reactions and initial moisture content; however, it is largely governed by the amount of external and initial moisture content; however , it is largely governed by the amount of external water entering the landfill. Thus, a key first step in controlling leachate migration is to limit production by preventing, to the extent feasible, the entry of external water into the waste layers. A second step is to collect any leachate that is produced for subsequent treatment and disposal. Techniques are currently available to limit the amount of leachate that migrates into adjoining areas to a virtually immeasurable volume, as long as the integrity of the landfill structure and leachate control system is maintained. A schematic profile view of a typical solid waste landfill is shown in Figure 1.
In Figure 1 the bottom layer of soil may be naturally existing material or it may be hauled in, placed and compacted to specifications following excavation to a suitable subgrade. In either case, the base of the landfill should act as a liner with some minimum thickness and a very low hydraulic conductivity (or permeability) – layer 11. The barrier soil may be treated to reduce its permeability to an acceptable level. As an added factor of safety, an impermeable synthetic membrane (layer 10) is shown placed on the top of the barrier soil layer to form a composite liner.
Immediately above the bottom composite liner is a leakage detection drainage layer (layer 9) to collect leakage from the primary liner (layer 8), in this case, a geomembrane. Above the primary liner are a geosynthetic drainage net (layer 7) and a sand layer (layer 6) that serve as drainage layers for leachate collection. The drain layers composed of sandlayer that serve as drainage layers for leachate collection. The drain layers composed of sand are typically at least 1 ft thick and have suitably spaced collection pipe, avoiding a significant buildup of head and limiting leakage. The liners are sloped to prevent ponding by encouraging leachate to flow toward the drains. The net effect is that very little leachate should percolate through the primary liner and virtually no migration of leachate through the bottom composite liner to the natural formations below should occur.
Drainage layers, geomembrane liners, and barrier soil liners may be referred to as the leachate collection and removal system or a double liner system. After the landfill is closed, the leachate collection and removal system serves basically in a back-up capacity. However, while the landfill is open and waste is being added, these components constitute the principal defense against contamination of adjacent areas.
When the capacity of the landfill is reached, the waste cells may be covered with a cap or final cover, typically composed of four distinct layers (layers 1 to 4). At the base of the cap there is a drainage layer (layer 2) and a liner system (layers 3 and 4) similar to that used at the base of landfill. The top of the barrier soil layer (layer 1) is graded so that water percolating into the drainage layer will tend to move horizontally towards some removal system (drain) located at the edge of the landfill. A layer of soil suitable for vegetative growth is placed at the top of the final cover system to complete the landfill. This upper layer is about 2-ft thick having loamy and silty soil, graded so that runoff is restricted and infiltration is controlled to provide moisture for vegetation while limiting percolation through the topsoil. Runoff is promoted but is controlled to prevent excessive erosion of the cap. Vegetation such as grasses best serve this purpose.
The combination of site
selection, surface grading, transpiration from vegetation, soil evaporation,
drainage through the sand, and the low hydraulic conductivity of the barrier
soil and geomembrane liners serves effectively to minimize leachate production
from external water. The cap should be no more permeable than the leachate
collection and removal system so that the landfill will not gradually fill and
overflow into adjacent areas following abandonment of the landfill.