Page 324 - Materials Chemistry, Second Edition

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The Sanitary Landfill 295

Geomembrane
Leachate
collection system

Sump
Compacted clay
−7
K <1x10 cm/s
s
FIGURE 10.2 The layers beneath a sanitary landfill as required under RCRA (U.S. EPA, 1994). Note: Not
to scale.

than 1x10 7 cm/sec (Figure 10.2). The geomembrane liner must measure at least 30 mil thick (1 mil
0.001 in.) except for high-density polyethylene (HDPE) geomembranes, which must be at least 60 mil
thick. The geomembrane liner minimizes the exposure of the compacted soil liner to leachate. A
leachate collection and removal (LCR) system is situated above the composite liner to collect, divert,
and remove liquids during landfill operation and well after closure. The LCR must be designed to limit
the depth (hydraulic head) of the leachate above the liner to less than 30 cm (U.S. EPA, 1993b).
Second, in states with EPA-approved programs, landfills may be constructed to comply with a
design approved by the state regulatory agency. In approving the design, the agency must ensure that
maximum contaminant levels (MCLs) will not be exceeded in the uppermost aquifer at a ‘relevant
point of compliance.’ This point is determined by the agency, but it must be located not further than
150 m from the landfill boundary. The U.S. EPA MCLs for a number of solid waste constituents are
shown in Table 10.1. In planning such performance-based landfill designs, other factors must also be
considered, such as the hydrogeologic characteristics of the facility and surrounding land, the local
climate, and the amount and composition of the leachate (U.S. EPA, 1993b).
It must be emphasized that, in the design of Subtitle D landfills, the U.S. EPA provides mini-
mum design standards only. Several states require double-composite liner systems in the design of
Subtitle D landfills.

10.4.5 CLAY LINERS
Clay is an extremely important component of soil liners because it tends to be relatively available
and amenable of mechanical and other stresses. Clay materials, being natural, incorporate readily
with native soil materials and are obviously very durable. Additionally, the clay fraction of the soil
ensures low hydraulic conductivity. The U.S. EPA requires that soil liners be constructed so that
hydraulic conductivity is less than 1 10 7 cm/sec (Figure 10.3). To meet this requirement, certain
characteristics of the soil materials must be met. First, the soil should contain at least 20% fines (i.e.,
fine silt and clay-sized particles). Second, the plasticity index (PI) must be greater than 10%. Third,
coarse fragments should be screened to no more than about 10% gravel-size particles. Soils with a
greater percentage of coarse fragments can contain areas that have high hydraulic conductivities.
Finally, the material should not contain rocks larger than 2.5 to 5 cm (1 to 2 in.) in diameter (U.S.
EPA, 1989).
Many different clay types exist with variations in surface area, external and internal charge, and
interlayer cations. These differences in chemical and physical properties influence swelling behav-
ior, potential for cracking and liquid transmission, and ultimately determine their possible utility in
landfill liners.