AREA A/B ENGINEERING REPORT - Waste Management

Geosyntec Consultants
a practical standpoint, geomembrane manufacturers, like the producers of many consumer products,
warranty geomembranes for far less than their service life. For example, a typical warranty for an
exposed LLDPE geomembrane that is not protected from the environment by an overlying soil layer
may be on the order of one year. This is intended to provide the warrantee sufficient time to install
the liner and perceive manufacturing defects; it is not intended to correspond to service life.
Therefore, the warranty period is clearly not relevant to the in-service performance of a
geomembrane barrier used in a Subtitle D final cover system.
The prescriptive low-permeability soil component of a cover system for a Subtitle-D landfill is a CCL
which, when properly installed, provides an excellent barrier to infiltration. If maintained, the CCL
should meet its hydraulic conductivity criterion for hundreds to thousands of years when used as a
barrier in a MSW landfill. The high performance of final cover systems during the PCC period is
evidenced by the low to negligible leachate generation rates observed for modern MSW landfills
currently in PCC (see, for example, previous Figure B-1).
The goals of final covers are changing to address new regulations and to optimize environmental
stewardship. In December 2003, the Interstate Technical and Regulatory Council (ITRC), which
represents a consensus of over 40 state regulatory agencies, Federal regulatory agencies, and
many other stakeholders and is supported by the USPEA, published a “Technical and Regulatory
Guidance for Design, Installation, and Monitoring of Alternative Final Landfill Covers”. The
document evaluates the range of landfill covers and the features of each that provide benefits in
terms of long-term stewardship and performance. The document concludes that alternative covers
can provide protection equivalent to prescriptive Subtitle D covers with the added benefit of
increased longevity and stability.
B4.2.2 Maintenance of Final Cover Systems
During the PCC period, the performance of the final cover system can be evaluated by monitoring
leachate generation rates over time. If rates were to unexpectedly increase, the cause would be
investigated. Although it has been speculated that final cover system failure is inevitable during the
PCC period, such findings are not being observed at modern MSW landfills that are currently in PCC
periods with properly maintained covers. The cover maintenance that is required for closed MSW
landfills has primarily been 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,
regrading the top deck to promote drainage). The effectiveness of the barrier layer in conventional
cover systems is evidenced by measured overall reduction in leachate flow rates over time from the
LCS (see Figures B-1 and B-2 and, secondarily, measurement of LFG emissions (Bonaparte, 1995;
Othman, et al, 2002).
An increasing number of landfills are being closed with an evapotranspirative (ET) final cover system
(i.e., all-soil covers) rather than a prescriptive final cover system with a CCL/geomembrane barrier.
The concern is that a CCL barrier will desiccate in arid and semi-arid climates if not protected by an
overlying geomembrane and a sufficiently thick soil erosion layer. An ET final cover typically consists
of more loosely compacted soils of sufficient thickness to optimally store and release water through ET
processes. For this reason, ET covers tend to be thicker than conventional covers, and the soils are not
MD10186.doc 131 29 March 2009