SANITARY LANDFILL OPERATIONS - Environmentally Conscious Materials Handling - page 211

3. Long-term movements of the landfill cover
4. Quality of groundwater at and around the fill areas
5. Air quality above and at the perimeter of the landfill
6. Structural integrity of the landfill cap and liner system

5.5 Site Closure and End Use

A cover or cap is an umbrella over the landfill to keep water out (to prevent
leachate formation). It will generally consist of several sloped layers: clay or
membrane liner (to prevent rain from intruding), overlain by a very permeable
layer of sandy or gravelly soil (to promote rain runoff), overlain by topsoil in
which vegetation can root (to stabilize the underlying layers of the cover). If the
cover (cap) is not maintained, rain will enter the landfill, resulting in buildup of
leachate to the point where the bathtub overflows its sides and wastes enter the
environment.
Methodology for closing landfills can include performance-based factors as
well as end-use considerations for potential threats to human health and the
environment. The performance-based decision-making factors for ending PCC at
landfills include (Morris et al. 2003):
• Quantification of landfill source characteristics (i.e., leachate and landfill
gas)
• Definition of trends in concentrations and quantities at the source
• Evaluation and prediction of the release of constituents for potential
impacts to human health and the environment
• Monitoring to confirm evaluations or predictions
In a landfill, when the moisture content of the waste is reduced to about 20 per-
cent, rate of gas production significantly slows. With a good-quality cap design,
it is possible to limit the moisture supply to a landfill, but over time, the cap may
lose its integrity due to environmental and geotechnical stresses. Consequently,
the cap requires periodic maintenance to prevent excessive amounts of moisture
from entering the waste. The implication that monitoring will be discontinued
after 30 years because the landfill is stable and no longer represents a threat to
the environment requires a scientific and systematic approach for monitoring per-
formance of closed landfills (Barlaz 2004). By implementing proper engineering
measures during the operation of a landfill, the stabilization period of the MSW
can be reduced significantly.
There are two types of end uses for landfills (Tansel 1998):
1. Passive use (i.e., such as green space, wildlife or nature conservancy, and
hiking trails)
2. Active use (i.e., such as sports fields, golf courses, industrial uses, and
transfer stations)