tems is the gas flow rate. Flow rates should be at least
(1) Specific regulations concerning final cover
equal to the rate of gas production but low enough to
are summarized in table 6-6. The prime function of
prevent excess oxygen from being drawn into the
finmal cover is to minimize infiltration of precipitation.
system. Details concerning proper design of pipe vent
systems are contained in Methane Generation and
surface water run off, wind dispersion of hazardous
Recovery from Landfills, EMCON Associates (1980).
waste, and direct contact with hazardous waste by
(1) Vent trenches are constructed by excavating
animals or humans. To prevent liquid accumulation
a deep trench which is backfilled with gravel to provide a
within closed disposal units, the regulations specify final
path of least resistance through which gases can migrate
cover must have a permeability less than or equal to the
vertically. Design considerations in constructing vent
permeability of any bottom liner system or natural
trenches include ensuring proper ventilation by backfilling
with sufficiently permeable material and avoiding
(2) For long-term performance with minimum
maintenance, the final cover must be designed to
passive closed vent trenches, ventilation can be
enhanced by proper design of laterals and risers.
accumulation of gas pressures, and accommodate
(2) In active vent trenches with forced ventilation,
settling and subsidence.
the equations and design criteria for active control wells
b. Elements of the cover system. Design
apply, with allowances for the smaller area and greater
features and criteria recommended for final cover in the
permeability of the trench backfill. The key design
EPA guidance documents are shown in figure 6-10. The
consideration for vent trenches is that the depth of the
recommended three-layered final cover includes:
trench extend to the ground-water table or an
* A soil layer for vegetation
unfractured impervious stratum to prevent gas from
* A drainage layer
migrating under the trench.
* A low permeability layer
(1) The upper soil layer is to sustain vegetation
and minimize erosion of the cover; the middle drainage
6-7. Final cover
layer is to carry infiltrating water from sustained
a. Regulatory requirements. Final cover is
low-permeability layer is to prevent fluid inflow and
required for closure of all hazardous waste landfills,
ensure that infiltrating water is carried by the drainage
surface impoundments developed for waste disposal,
and those surface impoundments and waste piles at
which all contaminated subsoils cannot be removed or
Table 6-6. Requirements for Surface Water Run-on and Run-off Control Systems
Section of 40 CFR 264 Describing Requirements
Cover the unit with a final cover designed and
constructed to: '
Provide long-term minimization of the
migration of liquids through the closed unit.
Function with minimum maintenance Promote
drainage and minimize erosion or abrasion of
the final cover Accommodate settling and
subsidence so that the cover's integrity is
maintained; and Have a permeability less than
or equal to the permeability of any bottom liner
system or natural subsoils present.
Maintain the integrity and effectiveness of the final
cover, including making repairs to the cap as
necessary to correct the effects of settling,
subsidence, erosion, or other events.
Prevent run-on and run-off from eroding or
otherwise damaging the final cover.
*If not all contaminated subsoils can be practicably removed or decontaminated, the unit must be closed in accordance
with requirements that apply to landfills.
Adapted from 40 CFR 264