The role of soil in landfills is to:
a) Provide Cover. Table 13 presents a system for ranking soil types
according to the United Soil Classification System (USCS) in relation to
landfill cover factor. Cover soil serves several functions including:
1) Control of water infiltration which in turn effects leachate
2) Support of vegetative growth.
3) Encapsulation of the deposited waste to isolate it from the
local environment, and to reduce odor emissions and aesthetic impacts.
4) Impedance of fire propagation from cell to cell.
b) Attenuate Potential contaminants. Soil pH and cation exchange
capacity (CEC) influence the ability of a soil to attenuate cations in
leachate that may form. CEC and pH are influenced by clay content, free
iron oxide content, organic matter, the lime concentration of a soil, and
soil permeability. In general, as CEC and pH increase, heavy metals are
more readily retained. Similarly, as the clay content, free iron oxide
content, and lime concentration of a soil increase, its pH and CEC (i.e.,
attenuation capacity) generally increase. Table 14 shows typical ranges
CEC values in various soils.
Soils can be modified to enhance their attenuation capacity. The
easiest approach is to apply lime to the soil surface, which increases pH
The clay and free iron oxide content can also be modified, but with great
difficulty and questionable success. Recent studies show that metal
concentrations in leachate decrease when flow rates (or flux rates)
decrease; however, further research is needed to determine the mechanisms
associated with this phenomena and the others discussed above.
7.2.6 Geology. The geology of a site is an important consideration.
Formations that have faults, major fractures, joint systems, and other
discontinuities or are soluble should be avoided or provisions made in th
landfill design to protect against groundwater contamination, if required
In general, limestone, dolomite, and heavily fractured crystalline rock a
less desirable than siliceous sandstone, siltstone, and other consolidate
conditions of a site should include:
a) Depth to groundwater from the bottom of the fill (including
historical highs and lows), and knowledge of the properties of subgrade
b) Direction of groundwater movement. By knowing the direction of
groundwater movement under the site, potential impacted areas can be
identified, and locations for onsite groundwater monitoring wells can be
determined. subsequently, when more specific site studies are conducted,
hydraulic gradients can be computed.
c) Quality and availability of groundwater, its current and
projected use, and the location of primary recharge and discharge zones.
example, a good landfill location may be a site overlying poor-quality
low-yielding groundwater where the groundwater basin does not discharge t
nearby water course.