AREA A/B ENGINEERING REPORT - Waste Management

Geosyntec Consultants
ET covers at these sites has been studied for decades. ET covers have proven to be effective at
alleviating potential problems with subsurface gas migration. These successes, coupled with
improved availability of reliable design tools for ET final cover systems and a 2004 revision to
the USEPA’s Subtitle D rules which permits the wider use of alternative covers, have given rise to
an increasing number of newer landfills across a variety of climatic zones closing with all-soil ET
covers rather than a prescriptive RCRA Subtitle D final cover system.
To determine the properties that are effective in a
given environment and understand how to
address a possible upset, a study of the
cover’s compatibility with the local
ecosystem can be performed. This kind
of study involves evaluating a natural,
and sometimes archeological, material or
setting that is analogous to a proposed
cover system material or setting. For
example, there is ample archaeological
evidence to show the very long-term integrity of
Natural Analog
Evapotranspirative (ET)
Cover Systems
Soil covers that are compatible with the
surrounding ecosystem provide a similar
function to prescriptive covers and are
expected to have a service life of a thousand
years or more (Bonaparte, et al, 2002a).
certain manmade earthen structures under a wide range of climatic conditions, such as Native
American burial mounds or the prehistoric earth enclosures (henges) of the British Isles.
Alternatively, when ET covers are constructed with surficial site soils, their long-term performance
can be inferred by observation of vegetation and precipitation recharge conditions at the site. In
this context, natural analog studies have been used to demonstrate the design of ET covers for
critical structures (e.g., radioactive waste depositories) to support service lives of thousands of
years and to predict the effects of long-term climate change, ecological change, and soil
development on these cover systems. Studies at MSW landfills have shown that ET covers can
likely have service lives in excess of a thousand years with minimal maintenance and still satisfy
performance criteria for infiltration control. 16
Earthen cover systems have many potential benefits for system maintenance
over time. Because these covers consist of soils that are designed to store and
release water rather than provide a barrier to infiltration, they are an
excellent counter-balance to increasing gas pressure within the landfill. In the
event that a repair is needed, it is a relatively simple task (i.e., adding
appropriate soil material) that immediately improves the performance of the system by adding
storage capacity. Moreover, the properties desirable in an earthen final cover are also
desirable properties to promote the oxidation of methane, since the extent of methane oxidation
is influenced heavily by the availability of oxygen that diffuses vertically from the ground
surface. Earthen cover systems have thus been demonstrated to effectively reduce methane
emissions through oxidation (see discussion on passive gas management in Section 3.5.2), thus
reducing the greenhouse gas emission potential of a MSW landfill, which is critical to future
sustainable landfill management strategies.
16 As discussed by Gee & Ward (1997), Gee, et al (1997), Waugh (1997), ITRC (2003), Scanlon, et al (2005), and
Dwyer & Bull (2008).
MD10186.doc 49 29 March 2009