AREA A/B ENGINEERING REPORT - Waste Management

Geosyntec Consultants
cover surface. If leachate or gas emission rates unexpectedly increase, the cause of this
phenomenon is be investigated and addressed. Although some have posed concerns about final
cover system failure, this is unlikely as evidenced by low to negligible leachate generation rates
and continually downward trends observed at modern MSW landfills currently in PCC. 14 Further,
as discussed in Section 3.2.3, the expected service life of a GM barrier layer in a composite
cover system is of the order of 1,000 years, comparable to the long-term performance of GM
barrier materials in liner systems. Similarly, whether in combination with an upper GM component
or as a single-layer cover system, a low-permeability CCL provides an excellent long-term robust
barrier to precipitation.
Performance of Landfills during Severe Natural Events
Investigations of landfill covers and environmental protection systems following a
severe natural event suggest that landfills are highly resistant to damage from
such events. Studies performed after the Florida hurricanes of 2004 (Roberts, et
al., 2005), the Northridge and Loma Prieta earthquakes in California (Matasovic
& Kavazanjian, 1998), and the San Diego wildfires of 2003 showed that the
integrity of landfills had not been compromised. The only damage that occurred
was to surface features such as vegetation and LFG vents that were repaired at
minimal cost.
Although the final cover is far more accessible for maintenance and repair over the long term, its
design and construction is managed with the same level of care and foresight as the liner system.
If properly maintained, something relatively easily achieved, the CCL should meet its hydraulic
conductivity criterion for several thousand years. The cover maintenance required for closed
MSW landfills is primarily related to cover system vegetation (e.g., mowing, tree removal, revegetating),
and erosion and sediment control (e.g., removal of sediment from ditches and ponds,
re-grading the top deck to promote drainage). A significant portion of cover system maintenance
is related to upkeep of the stormwater management system to provide proper drainage (i.e.,
cover drainage features, sediment trapping devices, retention/sediment control ponds, diversion
channels, silt fences and other sediment control devices, and vegetation). 15
The design and use of all-soil evapotranspirative (ET) final covers was pioneered at older landfills
with pre-Subtitle D liner systems, generally at sites located in dry climates. The performance of
14 In USEPA-sponsored studies by Bonaparte (1995) and Othman, et al (2002), the observed leachate generation
rates for MSW landfills have shown continually downward trends post-closure.
15 The expected service life and long-term maintenance requirements of cover systems is discussed in a number of
seminal references, including Koerner & Hsuan (2002) and in the USPEA’s “Technical Guidance for RCRA/CERCLA Final
Covers” (Bonaparte, et al, 2002b).
MD10186.doc 48 29 March 2009